Atmel C51 User manual
Atmel 8051 Microcontrollers
Hardware Manual
Section 1
The C51 Instruction Set ........................................................................ 1-1
1.1
1.2
Program Status Word................................................................................1-1
Addressing Modes ....................................................................................1-2
1.2.1
Direct Addressing ..............................................................................1-2
1.2.2
Indirect Addressing............................................................................1-2
1.2.3
Register Instructions ...........................................................................1-2
1.2.4
Register-specific Instructions..............................................................1-2
1.2.5
Immediate Constants..........................................................................1-3
1.2.6
Indexed Addressing...........................................................................1-3
1.3
1.4
1.5
Arithmetic Instructions...............................................................................1-4
Logical Instructions ...................................................................................1-5
Data Transfers .........................................................................................1-6
1.5.1
1.6
1.7
1.8
Internal RAM......................................................................................1-6
External RAM ...........................................................................................1-9
Lookup Tables .........................................................................................1-9
Boolean Instructions ...............................................................................1-10
1.8.1
Relative Offset ..................................................................................1-11
1.9 Jump Instructions ....................................................................................1-11
1.10 Instruction Table .....................................................................................1-13
Section 2
Common Features Description ........................................................... 2-16
2.1
2.2
Introduction ............................................................................................2-16
Special Function Registers .....................................................................2-17
2.2.1
Accumulator......................................................................................2-18
2.2.2
B Register.........................................................................................2-18
2.2.3
Program Status Word .......................................................................2-18
2.2.4
Stack Pointer ....................................................................................2-19
2.2.5
Data Pointer......................................................................................2-19
2.2.6
Ports 0 to 3 .......................................................................................2-19
2.2.7
Serial Data Buffer .............................................................................2-19
2.2.8
Timer Registers ................................................................................2-19
2.2.9
Capture Registers.............................................................................2-19
2.2.10
Control Registers.............................................................................2-20
2.3
Oscillator and Clock Circuit .....................................................................2-20
2.3.1
2.4
More about the On-chip Oscillator ....................................................2-20
CPU Timing.............................................................................................2-21
2.4.1
X1 Mode (Standard Mode) ...............................................................2-21
2.4.2
X2 Mode ...........................................................................................2-22
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Table of Contents
2.5
Port Structures and Operation ................................................................2-23
2.5.1
I/O Configurations............................................................................2-23
2.5.2
Writing to a Port...............................................................................2-24
2.5.3
Port Loading and Interfacing ............................................................2-26
2.5.4
Read-Modify-Write Feature ..............................................................2-26
2.6
2.7
2.8
Accessing External Memory ...................................................................2-27
PSEN ......................................................................................................2-28
ALE .........................................................................................................2-28
2.8.1
Overlapping External Program and Data Memory Spaces...............2-29
2.8.2
ALE Disable Mode............................................................................2-29
2.9
Timer/Counters .......................................................................................2-30
2.9.1
Timer/Counter Operations ................................................................2-30
2.10 Timer 0....................................................................................................2-30
2.10.1 Mode 0 (13-bit Timer) .......................................................................2-31
2.10.2 Mode 1 (16-bit Timer) .......................................................................2-31
2.10.3 Mode 2 (8-bit Timer with Auto-Reload).............................................2-32
2.10.4 Mode 3 (Two 8-bit Timers) ...............................................................2-32
2.11 Timer 1....................................................................................................2-32
2.11.1 Mode 0 (13-bit Timer) .......................................................................2-33
2.11.2 Mode 1 (16-bit Timer) .......................................................................2-34
2.11.3 Mode 2 (8-bit Timer with Auto Reload).............................................2-34
2.11.4 Mode 3 (Halt)....................................................................................2-34
2.11.5 Interrupt ............................................................................................2-34
2.11.6 Timer Registers ................................................................................2-35
2.12 Timer 2....................................................................................................2-38
2.12.1 Introduction.......................................................................................2-38
2.12.2 Auto-reload Mode .............................................................................2-39
2.12.3 Programmable Clock-output.............................................................2-39
2.12.4 Timer Registers ................................................................................2-41
2.13 Serial Interface........................................................................................2-43
2.13.1 Baud Rates.......................................................................................2-45
2.13.2 Baud Rate Selection Table for UART...............................................2-46
2.13.3 Internal Baud Rate Generator (BRG) ...............................................2-46
2.13.4 Using Timer 1 to Generate Baud Rates ...........................................2-48
2.13.5 Using Timer 2 to Generate Baud Rates ...........................................2-48
2.13.6 More About Mode 0 ..........................................................................2-50
2.13.7 More About Mode 1 ..........................................................................2-51
2.14 Framing Error Detection..........................................................................2-53
2.15 Automatic Address Recognition ..............................................................2-54
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Table of Contents
2.15.1 Multiprocessor Communications ......................................................2-54
2.15.2 Given Address ..................................................................................2-54
2.15.3 Broadcast Address ...........................................................................2-55
2.15.4 Reset Addresses ..............................................................................2-55
2.16 Interrupts .................................................................................................2-61
2.16.1 How Interrupts Are Handled .............................................................2-62
2.16.2 External Interrupts ............................................................................2-63
2.16.3 Response Time ................................................................................2-63
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Section 1
The 8051 Instruction Set
The 8051 instruction set is optimized for 8-bit control applications. It provides a variety of
fast addressing modes for accessing the internal RAM to facilitate byte operations on
small data structures. The instruction set provides extensive support for one-bit variables as a separate data type, allowing direct bit manipulation in control and logic
systems that require Boolean processing.
An overview of the 8051 instruction set is presented below, with a brief description of
how certain instructions might be used.
1.1
Program Status
Word
The Program Status Word (PSW) contains several status bits that reflect the current
state of the CPU. The PSW, shown in Table 1-1 on page 2, resides in SFR space. It
contains the Carry bit, the Auxiliary Carry (for BCD operations), the two register bank
select bits, the Overflow flag, a parity bit, and two user-definable status flags.
The Carry bit, other than serving the functions of a Carry bit in arithmetic operations,
also serves as the “Accumulator” for a number of Boolean operations.
The bits RS0 and RS1 are used to select one of the four register banks shown below.
A number of instructions refer to these RAM locations as R0 through R7. The selection
of which of the four banks is being referred to is made on the basis of the bits RS0 and
RS1 at execution time.
The parity bit reflects the number of 1’s in the Accumulator: P = 1 if the Accumulator
contains an odd number of 1’s, and P = 0 if the Accumulator contains an even number of
1’s. Thus the number of 1’s in the Accumulator plus P is always even.
Two bits in the PSW are uncommitted and may be used as general purpose status flags.
The PSW register contains program status information as detailed in Table 1-1.
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The 8051 Instruction Set
Table 1-1. PSW: Program Status Word Register
(MSB)
(LSB)
CY
AC
F0
RS1
RS0
OV
-
Symbol
Position
CY
PSW.7
Carry flag
AC
PSW.6
Auxiliary Carry flag.
(For BCD operations.)
F0
PSW.5
Flag 0
(Available to the user for general purposes.)
P
Name and Significance
Register bank Select control bits 1 & 0. Set/cleared
by software to determine working register bank (see
Note).
RS1
PSW.4
RS0
PSW.3
OV
PSW.2
Overflow flag.
-
PSW.1
(reserved)
P
PSW.0
Parity flag.
Set/cleared by hardware each instruction cycle to
indicate and odd/even number of “one” bits in the
accumulator, i.e., even parity.
Note:
The contents of (RS1, RS0) enable the working register banks as follows:
(0.0)-Bank 0(00H-07H)
(0.1)-Bank 1(08H-0FH)
(1.0)-Bank 2(10H-17H)
(1.1)-Bank 3(18H-1FH)
1.2
Addressing
Modes
The addressing modes in the 8051 instruction set are as follows:
1.2.1
Direct Addressing
In direct addressing the operand is specified by an 8-bit address field in the instruction.
Only 128 Lowest bytes of internal Data RAM and SFRs can be directly addressed.
1.2.2
Indirect Addressing
In indirect addressing the instruction specifies a register which contains the address of
the operand. Both internal and external RAM can be indirectly addressed.
The address register for 8-bit addresses can be R0 or R1 of the selected register bank,
or the Stack Pointer. The address register for 16-bit addresses can only be the 16-bit
“data pointer” register, DPTR.
1.2.3
Register
Instructions
The register banks, containing registers R0 through R7, can be accessed by certain
instructions which carry a 3-bit register specification within the opcode of the instruction.
Instructions that access the registers this way are code efficient, since this mode eliminates an address byte. When the instruction is executed, one of the eight registers in the
selected bank is accessed. One of four banks is selected at execution time by the two
bank select bits in the PSW.
1.2.4
Register-specific
Instructions
Some instructions are specific to a certain register. For example, some instructions
always operate on the Accumulator, or Data Pointer, etc., so no address byte is needed
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to point to it. The opcode does this itself. Instructions that refer to the Accumulator as ‘A’
assemble as accumulator-specific opcodes.
1.2.5
Immediate
Constants
The value of a constant can follow the opcode in Program Memory. For example;
MOV A, # 100
loads the Accumulator with the decimal number 100. The same number could be specified in hex digits as 64H.
1.2.6
Indexed Addressing Only Program Memory can be accessed with indexed addressing, and it can only be
read. This addressing mode is intended for reading look-up tables in Program Memory.
A 16-bit base register (either DPTR or the Program Counter) points to the base of the
table, and the Accumulator is set up with the table entry number. The address of the
table entry in Program Memory is formed by adding the Accumulator data to the base
pointer.
Another type of indexed addressing is used in the “case jump” instruction. In this case
the destination address of a jump instruction is computed as the sum of the base pointer
and the Accumulator data.
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The 8051 Instruction Set
1.3
Arithmetic
Instructions
The menu of arithmetic instructions is listed in Table 1-2. The table indicates the
addressing modes that can be used with each instruction to access the <byte> operand.
For example, the ADD A, <byte> instruction can be written as:
ADD
ADD
ADD
ADD
A,7FH (direct addressing)
A,@ R0(indirect addressing)
A,R7
(register addressing)
A,# 127(immediate constant)
Table 1-2. A list of the Atmel 8051 Arithmetic Instructions.
Mnemonic
Operation
Execution Time in X1
Mode
@12 MHz (µs)
Addressing Modes
Dir
Ind
Reg
Im
m
ADD A, <byt>e
A = A + <byte>
X
X
X
X
ADDC A,
<byte>
A = A + <byte> + C
X
X
X
X
1
SUBB A,
<byte>
A = A – <byte> – C
X
X
X
X
1
INC A
A=A+1
Accumulator only
1
INC <byte>
<byte> = <byte> + 1
X
X
1
INC DPTR
DPTR = DPTR + 1
Data Pointer only
2
DEC A
A=A–1
Accumulator only
1
DEC <byte>
<byte> = <byte> – 1
X
1
MUL AB
B:A = B × A
ACC and B only
4
DIV AB
A = Int [A/B]
B = Mod [A/B]
ACC and B only
4
DA A
Decimal Adjust
Accumulator only
1
X
X
X
The execution times listed in Table 1-2 assume a 12 MHz clock frequency and X1
mode. All of the arithmetic instructions execute in 1 µs except the INC DPTR instruction,
which takes 2 µs, and the Multiply and Divide instructions, which take 4 µs.
Note that any byte in the internal Data Memory space can be incremented or decremented without going through the Accumulator.
One of the INC instructions operates on the 16-bit Data Pointer. The Data Pointer is
used to generate 16-bit addresses for external memory, so being able to increment it in
one 16-bit operation is a useful feature.
The MUL AB instruction multiplies the Accumulator by the data in the B register and puts
the 16-bit product into the concatenated B and Accumulator registers.
The DIV AB instruction divides the Accumulator by the data in the B register and leaves
the 8-bit quotient in the Accumulator, and the 8-bit remainder in the B register.
Oddly enough, DIV AB finds less use in arithmetic “divide” routines than in radix conversions and programmable shift operations. An example of the use of DIV AB in a radix
conversion will be given later. In shift operations, dividing a number by 2n shifts its n bits
to the right. Using DIV AB to perform the division completes the shift in 4 µs leaves the B
register holding the bits that were shifted out.
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The DA A instruction is for BCD arithmetic operations. In BCD arithmetic ADD and
ADDC instructions should always be followed by a DA A operation, to ensure that the
result is also in BCD. Note that DAA will not convert a binary number to BCD. The DA A
operation produces a meaningful result only as the second step in the addition of two
BCD bytes.
1.4
Logical
Instructions
Table 1-3 shows the list of logical instructions. The instructions that perform Boolean
operations (AND, OR, Exclusive OR, NOT) on bytes perform the operation on a bit-bybit basis. That is, if the Accumulator contains 00110101B and <byte> contains
01010011B, then
ANL A,<byte>
will leave the Accumulator holding 00010001B.
The addressing modes that can be used to access the <byte> operand are listed in
Table 1-3. Thus, the ANL A, <byte> instruction may take any of the following forms.
ANL
ANL
ANL
ANL
A,
A,
A,
A,
7FH(direct addressing)
@ R1(indirect addressing)
R6(register addressing)
# 53H(immediate constant)
All of the logical instructions that are Accumulator specific execute in 1 µs (using a
12 MHz clock and X1 mode). The others take 2 µs.
Table 1-3. A list of the Atmel 8051 Logical Instructions
Mnemonic
Operation
Execution Time
@ 12MHz (µs)
Addressing Modes
Dir
Ind
Reg
Imm
X
X
X
ANL A, <byte>
A = A AND <byte>
X
ANL <byte>, A
<byte> = <byte> AND A
X
1
ANL <byte>, #
data
<byte> = <byte> AND # data
X
2
ORL A, <byte>
A = A OR <byte>
X
ORL <byte>, A
<byte> = <byte> OR A
X
1
ORL <byte>, #
data
<byte> = <byte> OR # data
X
2
XRL A, <byte>
A = A XOR <byte>
X
XRL <byte>, A
<byte> = <byte> XOR A
X
1
XRL <byte>, #
data
<byte> = <byte> XOR # data
X
2
CLR A
A = 00H
Accumulator only
1
CLP A
A = NOT A
Accumulator only
1
RL A
Rotate ACC Left 1 bit
Accumulator only
1
RLC A
Rotate Left through Carry
Accumulator only
1
RR A
Rotate ACC Right 1 bit
Accumulator only
1
RRC A
Rotate Right through Carry
Accumulator only
1
SWAP A
Swap Nibbles in A
Accumulator only
1
Atmel 8051 Microcontrollers Hardware Manual
X
X
X
X
X
X
1
1
1
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The 8051 Instruction Set
Note that Boolean operations can be performed on any byte in the internal Data Memory
space without going through the Accumulator. The XRL <byte>, # data instruction, for
example, offers a quick and easy way to invert port bits, as in
XRL P1, #OFFH
If the operation is in response to an interrupt, not using the Accumulator saves the time
and effort to stack it in the service routine.
The Rotate instructions (RL A, RLC A, etc.) shift the Accumulator 1 bit to the left or right.
For a left rotation, the MSB rolls into the LSB position. For a right rotation, the LSB rolls
into the MSB position.
The SWAP A instruction interchanges the high and low nibbles within the Accumulator.
this is a useful operation in BCD manipulations. For example, if the Accumulator contains a binary number which is known to be less than 100, it can be quickly converted to
BCD by the following code:
MOV B, #10
DIV AB
SWAP A
ADD A,B
Dividing the number by 10 leaves the tens digit in the low nibble of the Accumulator, and
the ones digit in the B register. The SWAP and ADD instructions move the tens digit to
the high nibble of the Accumulator, and the ones digit to the low nibble.
1.5
Data Transfers
1.5.1
Internal RAM
Table 1-4 shows the menu of instructions that are available for moving data around
within the internal memory spaces, and the addressing modes that can be used with
each one. With a 12 MHz clock and X1 mode, all of these instructions execute in either
1 or 2 µs.
The MOV <dest>, <src> instruction allows data to be transferred between any two internal RAM or SFR locations without going through the Accumulator. Remember the Upper
128 bytes of data RAM can be accessed only by indirect, and SFR space only by direct
addressing.
Note that in all 8051 devices, the stack resides in on-chip RAM, and grows upwards.
The PUSH instruction first increments the Stack Pointer (SP), then copies the byte into
the stack. PUSH and POP use only direct addressing to identify the byte being saved or
restored, but the stack itself is accessed by indirect addressing using the SP register.
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This means the stack can go into the Upper 128, if they are implemented, but not into
SFR space.
Table 1-4. Atmel 8051 Data Transfer Instructions that Access Internal Data Memory
Space
Mnemonic
Operation
Execution Time
@ 12MHz (µs)
Addressing Modes
Dir
Ind
Reg
Imm
X
MOV A, <src>
A = <src>
X
X
X
MOV <dest>, A
<dest> = A
X
X
X
MOV <dest>,
<src>
<dest> = <src>
X
X
X
MOV DPTR, #
data 16
DPTR = 16-bit immediate
constant
PUSH <src>
INC SP: MOV “@SP”, <scr>
X
2
POP <dest>
MOV <dest>, “@SP”: DEC SP
X
2
XCH A, <byte>
ACC and <byte> Exchange Data
X
XCHD A, @Ri
ACC and @ Ri exchange low
nibbles
X
X
X
1
1
X
2
X
2
1
1
The Upper 128 are not implemented in the 8 standard 8051, nor in their ROMless. With
these devices, if the SP points to the Upper 128 PUSHed bytes are lost, and POPped
bytes are indeterminate.
The Data Transfer instructions include a 16-bit MOV that can be used to initialize the
Data Pointer (DPTR) for look-up tables in Program Memory, or for 16-bit external Data
Memory accesses.
The XCH A, <byte> instruction causes the Accumulator and addressed byte to
exchange data.
The XCHD A, @ Ri instruction is similar, but only the low nibbles are involved in the
exchange.
The see how XCH and XCHD can be used to facilitate data manipulations, consider first
the problem of shifting an 8-digit BCD number two digits to the right. Table 1-5 shows
how this can be done using direct MOVs, and for comparison how it can be done using
XCH instructions. To aid in understanding how the code works, the contents of the registers that are holding the BCD number and the content of the Accumulator are shown
alongside each instruction to indicate their status after the instruction has been
executed.
After the routine has been executed, the Accumulator contains the two digits that were
shifted out on the right. Performing the routine with direct MOVs uses 14 code bytes and
9 µs of execution time (assuming a 12 MHz clock and X1 mode). The same operation
with XCHs uses less code and executes almost twice as fast.
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The 8051 Instruction Set
Table 1-5. Shifting a BCD Number Two Digits to the Right
MOV A,2EH
MOV 2EH, 2DH
MOV 2DH, 2CH
MOV 2CH, 2BH
MOV 2BH, # 0
Note:
2B
2C
2D
2E
ACC
00
00
00
00
00
12
12
12
12
00
34
34
34
12
12
56
56
34
34
34
78
56
56
56
56
78
78
78
78
78
2A
2B
2C
2E
2E
ACC
00
00
00
00
00
12
00
00
00
00
34
34
12
12
12
56
56
56
34
34
78
78
78
78
56
00
12
34
56
78
Using direct MOVs: 14 bytes, 9 µs
CLR A
XCH A,2BH
XCH A,2CH
XCH A,2DH
XCH A,2EH
Note:
2A
Using XCHs: 9 bytes, 5 µs
Table 1-6. Shifting a BCD Number One Digit to the Right
2A
2B
2C
2D
2E
ACC
MOV R1,# 2EH
00
12
34
56
78
XX
MOV R0, # 2DH
00
12
34
56
78
XX
LOOP: MOV A, @R1
00
12
34
56
78
78
XCHD A, @R0
00
12
34
58
78
76
SWAP A
00
12
34
58
78
67
MOV @R1, A
00
12
34
58
67
67
DEC R1
00
12
34
58
67
67
DEC R0
00
12
34
58
67
67
loop for R1 = 2DH:
00
12
38
45
67
45
loop for R1 = 2CH:
00
18
23
45
67
23
loop for R1 = 2BH:
08
01
23
45
67
01
CLR A
08
01
23
45
67
00
XCH A,2AH
00
01
23
45
67
08
loop for R1 = 2EH:
CJNE R1, #2AH, LOOP
To right-shift by an odd number of digits, a one-digit shift must be executed. Table 1-6
shows a sample of code that will right-shift a BCD number one digit, using the XCHD
instruction. Again, the contents of the registers holding the number and of the Accumulator are shown alongside each instruction.
First, pointers R1 and R0 are set up to point to the two bytes containing the last four
BCD digits. Then a loop is executed which leaves the last byte, location 2EH, holding
the last two digits of the shifted number. The pointers are decremented, and the loop is
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repeated for location 2DH. The CJNE instruction (Compare and Jump if Not Equal) is a
loop control that will be described later.
The loop is executed from LOOP to CJNE for R1 = 2EH, 2DH, 2CH and 2BH. At that
point the digit that was originally shifted out on the right has propagated to location 2AH.
Since that location should be left with 0s, the lost digit is moved to the Accumulator.
1.6
External RAM
Table 1-7 shows a list of the Data Transfer instructions that access external Data Memory. Only indirect addressing can be used. The choice is whether to use a one-byte
address, @Ri, where Ri can be either R0 or R1 of the selected register bank, or a twobyte address, @DPTR. The disadvantage to using 16-bit addresses if only a few
Kbytes of external RAM are involved is that 16-bit addresses use all 8 bits of Port 2 as
address bus. On the other hand, 8-bit addresses allow one to address a few Kbytes of
RAM, as shown in Table 1-7, without having to sacrifice all of Port 2.
All of these instructions execute in 2 µs, with a 12 MHz clock (and X1 mode).
Note that in all external Data RAM accesses, the Accumulator is always either the destination or source of the data.
The read and write strobes to external RAM are activated only during the execution of a
MOVX instruction. Separately these signals are inactive, and in fact if they’re not going
to be used at all, their pins are available as extra I/O lines.
Table 1-7. Data Transfer Instructions that Access External Data Memory Space
1.7
Lookup Tables
Execution Time
@ 12MHz (µs)
Address Width
Mnemonic
Operation
8 bits
MOVX A, @Ri
Read external
RAM @ Ri
2
8 bits
MOVX @ Ri, A
Write external
RAM @ Ri
2
16 bits
MOVX A, @ DPTR
Read external
RAM @ DPTR
2
16 bits
MOVX @ DPTR, A
Write external
RAM @ DPTR
2
Table 1-8 shows the two instructions that are available for reading lookup tables in Program Memory. Since these instructions access only Program Memory, the lookup tables
can be read, not updated. The mnemonic is MOVC for “move constant”.
If the table access is to external Program Memory, then the read strobe is PSEN.
The first MOVC instruction in Table 1-8 can accommodate a table of up to 256 entries,
numbered 0 through 255. The number of the desired entry is loaded into the Accumulator, and the Data Pointer is set up to point to beginning of the table. Then
MOVC A, @A + DPTR
copies the desired table entry into the Accumulator.
The other MOVC instruction works the same way, except the Program Counter (PC) is
used as the table base, and the table is accessed through a subroutine. First the number of the desired entry is loaded into the Accumulator, and the subroutine is called:
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MOV A, ENTRY_NUMBER
CALLTABLE
The subroutine “TABLE” would look like this:
TABLE:MOVC A, @A + PC
RET
The table itself immediately follows the RET (return) instruction in Program Memory.
This type of table can have up to 255 entries, numbered 1 through 255. Number 0 cannot be used, because at the time the MOVC instruction is executed, the PC contains the
address of the RET instruction. An entry numbered 0 would be the RET opcode itself.
Table 1-8. Lookup Table Read Instructions
1.8
Boolean
Instructions
Mnemonic
Operation
Execution Time
@ 12MHz (µs)
MOVC A, @A + DPTR
Read Pgm Memory at (A + DPTR)
2
MOVC A, @A + PC
Read Pgm Memory at (A + PC)
2
8051 devices contain a complete Boolean (single-bit) processor. The internal RAM contains 128 addressable bits, and the SFR space can support up to 128 other addressable
bits. All of the port lines are bit-addressable, and each one can be treated as a separate
single-bit port. The instructions that access these bits are not just conditional branches,
but a complete menu of move, set, clear, complement, OR and AND instructions. These
kinds of bit operations are not easily obtained in other architectures with any amount of
byte-oriented software.
The instruction set for the Boolean processor is shown in Table 1-9. All bit accesses are
by direct addressing. Bit addresses 00H through 7FH are in the Lower 128, and bit
addresses 80H through FFH are in SFR space.
Table 1-9. 8051 Boolean Instructions
Mnemonic
Operation
Execution Time
@ 12MHz (µs)
ANL C,bit
ANL C,/bit
ORL C,bit
ORL C,/bit
MOV C,bit
MOV bit,C
CLR C
CLR bit
SETB C
SETB bit
CPL C
CPL bit
JC rel
JNC rel
JB bit,rel
JNB bit,rel
JBC bit,rel
C = C AND bit
C = C AND (NOT bit)
C = C OR bit
C = C OR (NOT bit)
C = bit
bit = C
C=0
bit = 0
C=1
bit = 1
C = NOT C
bit = NOT bit
Jump if C = 1
Jump if C = 0
Jump if bit = 1
Jump if bit = 0
Jump if bit = 1 ; CLR bit
2
2
2
2
1
2
1
1
1
1
1
1
2
2
2
2
2
Note how easily an internal flag can be moved to a port pin:
MOV C, FLAG
MOV P1.0, C
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Atmel 8051 Microcontrollers Hardware Manual
The 8051 Instruction Set
In this example, FLAG is the name of any addressable bit in the lower 128 or SFR
space. An I/O line (the LSB of Port 1, in the case) is set or cleared depending on
whether the flag bit is 1 or 0.
The Carry bit in the PSW is used as the single-bit Accumulator of the Boolean processor. Bit instructions that refer to the Carry bit as C assemble as Carry-specific
instructions (CLR C, etc.). The Carry bit also has a direct address, since it resides in the
PSW register, which is bit-addressable.
Note that the Boolean instruction set includes ANL and ORL operations, but not the XRL
(Exclusive OR) operation. An XRL operation is simple to implement in software. Suppose, for example, it is required to form the Exclusive OR of two bits:
C= bit1 XRL bit2
The software to do that could be as follows:
MOV C, bit1
JNB bit2, OVER
CPL C
OVER: (continue)
First, bit 1 is moved to the Carry. If bit 2 = 0, then C now contains the correct result. That
is, bit 1 XRL bit2 = bit1 if bit2 = 0. On the other hand, if bit2 = 1 C now contains the complement of the correct result. It need only be inverted (CPL C) to complete the operation.
This code uses the JNB instruction, one of a series of bit-test instructions which execute
a jump if the addressed bit is set (JC, JB, JBC) or if the addressed bit is not set (JNC,
JNB). In the above case, bit2 is being tested, and if bit2 = 0 the CPL C instruction is
jumped over.
JBC executes the jump if the addressed bit is set, and also clears the bit. Thus a flag
can be tested and cleared in one operation.
All the PSW bits are directly addressable, so the Parity bit, or the general purpose flags,
for example, are also available to the bit-test instructions.
1.8.1
Relative Offset
The destination address for these jumps is specified to the assembler by a label or by an
actual address in Program Memory. However, the destination address assembles to a
relative offset byte. This is a signed (two’s complement) offset byte which is added to the
PC in two’s complement arithmetic if the jump is executed.
The range of the jump is therefore -128 to +127 Program Memory bytes relative to the
first byte following the instruction.
1.9
Jump
Instructions
Table 1-10 shows the list of unconditional jumps.
Table 1-10. Unconditional Jumps in Atmel 8051
Mnemonic
Operation
Execution Time @ 12MHz (µs)
JMP addr
JMP @A + DPTR
CALL addr
RET
RETI
NOP
Jump to addr
Jump to A + DPTR
Call subroutine at addr
Return from subroutine
Return from interrupt
No operation
2
2
2
2
2
1
The table lists a single “JMP addr” instruction, but in fact there are three -SJMP, LJMP,
AJMP -which differ in the format of the destination address. JMP is a generic mnemonic
which can be used if the programmer does not care how the jump is encoded.
Atmel 8051 Microcontrollers Hardware Manual
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The 8051 Instruction Set
The SJMP instruction encodes the destination address as relative offset, as described
above. The instruction is 2 bytes long, consisting of the opcode and the relative offset
byte. The jump distance is limited to range of -128 to + 127 bytes relative to the instruction following the SJMP.
The LJMP instruction encodes the destination address as a 16-bit constant. The instruction is 3 bytes long, consisting of the opcode and two address bytes. The destination
address can be anywhere in the 64K Program Memory space.
The AJMP instruction encodes the destination address as an 11-bit constant. The
instruction is 2 bytes long, consisting of the opcode, which itself contains 3 of the 11
address bits, followed by another byte containing the low 8 bits of the destination
address. When the instruction is executed, these 11 bits are simply substituted for the
low 11 bits in the PC. The high 5 bits stay the same. Hence the destination has to be
within the same 2K block as the instruction following the AJMP.
In all cases the programmer specifies the destination address to the assembler in the
same way: as a label or as a 16-bit constant. The assembler will put the destination
address into the correct format for the given instruction. If the format required by the
instruction will not support the distance to the specified destination address, a “Destination out of range” message is written, into the list file.
The JMP @ A + DPTR instruction supports case jumps. The destination address is
computed at execution time as the sum of the 16-bit DPTR register and the Accumulator. Typically, DPTR is set up with the address of a jump table, and the Accumulator is
given an index to the table. In a 5-way branch, for example, an integer 0 through 4 is
loaded into the Accumulator.
The code to be executed might be as follows:
MOV
MOV
RL
JMP
DPTR, # JUMP_TABLE
A, INDEX_NUMBER
A
@ A + DPTR
The RLA instruction converts the index number (0 through 4) to an even number on the
range 0 through 8, because each entry in the jump table is 2 bytes long:
JUMP_TABLE:
AJMP CASE_0
AJMP CASE_1
AJMP CASE_2
AJMP CASE_3
AJMP CASE_4
Table 1-10 shows a single “CALL addr” instruction, but there are two of them -LCALL
and ACALL -which differ in the format in which the subroutine address is given to the
CPU. CALL is a generic mnemonic which can be used if the programmer does not care
which way the address is encoded.
The LCALL instruction uses the 16-bit address format, and the subroutine can be anywhere in the 64K Program Memory space. The ACALL instruction uses the 11-bit format,
and the subroutine must be in the same 2K block as the instruction following the ACALL.
In any case the programmer specifies the subroutine address to the assembler in the
same way: as a label or as a 16-bit constant. The assembler will put the address into the
correct format for the given instructions.
Subroutines should end a RET instruction, which returns execution following the CALL.
RETI is used to return from an interrupt service routine. The only difference between RET
and RETI is that RETI tells the interrupt control system that the interrupt in progress is
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The 8051 Instruction Set
done. If there is no interrupt in progress at the time RETI is executed, then the RETI is
functionally identical to RET.
Table 1-11 shows the list of conditional jumps available to the Atmel 8051 user. All of
these jumps specify the destination address by the relative offset method, and so are
limited to a jump distance of -128 to + 127 bytes from the instruction following the conditional jump instruction. Important to note, however, the user specifies to the assembler
the actual destination address the same way as the other jumps: as a label or a 16-bit
constant.
Table 1-11. Conditional Jumps in Atmel 8051 Devices
Mnemonic
Operation
Execution Time
@ 12MHz (µs)
Addressing Modes
DIR
IND
REG
IMM
JZ rel
Jump if A = 0
Accumulator only
2
JNZ rel
Jump if A ≠ 0
Accumulator only
2
DJNZ <byte>,rel
Decrement and jump if
not zero
X
CJNZ A,<byte>,rel
Jump if A = <byte>
X
CJNE <byte>,#data,rel
Jump if <byte> = #data
X
2
X
X
X
2
2
There is no Zero bit in the PSW. The JZ and JNZ instructions test the Accumulator data
for that condition.
The DJNZ instruction (Decrement and Jump if Not Zero) is for loop control. To execute a
loop N times, load a counter byte with N and terminate the loop with DJNZ to the beginning
of the loop, as shown below for N = 10:
MOV
COUNTER, # 10
LOOP:(begin loop)
*
*
*
(end loop)
DJNZ COUNTER, LOOP
(continue)
The CJNE instruction (Compare and Jump if Not Equal) can also be used for loop control as in Table 1-11. Two bytes are specified in the operand field of the instruction. The
jump is executed only if the two bytes are not equal. In the example of Figure 12, the two
bytes were the data in R1 and the constant 2AH. The initial data in R1 was 2EH. Every
time the loop was executed, R1 was decremented, and the looping was to continue until
the R1 data reached 2AH.
Another application of this instruction is in “greater than, less than” comparisons. The
two bytes in the operand field are taken as unsigned integers. If the first is less than the
second, then the Carry bit is set (1). If the first is greater than or equal to the second,
then the Carry bit is cleared.
1.10
Instruction Table Table 1-12 shows the Hex value of each instruction detailing the:
„ byte size
„ number of cycles
„ flags modified by the instruction
Atmel 8051 Microcontrollers Hardware Manual
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MOV A,R0
1-1
MOVX R0,A
1-1
Fx
SUBB A,R0
1-1,C,OV,AC
9x
Ex
MOV dir,R0
2-2
8x
DJNZ R0,rel
2-2
MOV R0,#imm
2-1
7x
Dx
XRL A,R0
1-1
6x
XCH A,R0
1-1
ANL A,R0
1-1
5x
Cx
ORL A,R0
1-1
4x
CJNE R0,#imm,rel
3-2,C
ADDC A,R0
1-1,C,OV,AC
3x
Bx
ADD A,R0
1-1,C,OV,AC
2x
MOV R0,dir
2-2
DEC R0
1-1
1x
Ax
INC R0
1-1
Ox
x8
MOVX R1,A
1-1
MOV A,R1
1-1
DJNZ R1,rel
2-2
XCH A,R1
1-1
CJNE R1,#imm,rel
3-2,C
MOV R1,dir
2-2
SUBB A,R1
1-1,C,OV,AC
MOV dir,R1
2-2
MOV R1,#imm
2-1
XRL A,R1
1-1
ANL A,R1
1-1
ORL A,R1
1-1
ADDC A,R1
1-1,C,OV,AC
ADD A,R1
1-1,C,OV,AC
DEC R1
1-1
INVC R1
1-1
x9
MOVX R2,A
1-1
MOV A,R2
1-1
DJNZ R2,rel
2-2
XCH A,R2
1-1
CJNE R2,#imm,rel
3-2,C
MOV R2,dir
2-2
SUBB A,R2
1-1,C,OV,AC
MOV dir,R2
2-2
MOV R2,#imm
2-1
XRL A,R2
1-1
ANL A,R2
1-1
ORL A,R2
1-1
ADDC A,R2
1-1,C,OV,AC
ADD A,R2
1-1,C,OV,AC
DEC R2
1-1
INC R2
1-1
xA
MOVX R3,A
1-1
MOV A,R3
1-1
DJNZ R3,rel
2-2
XCH A,R3
1-1
CJNE R3,#imm,rel
3-2,C
MOV R3,dir
2-2
SUBB A,R3
1-1,C,OV,AC
MOV dir,R3
2-2
MOV R3,#imm
2-1
XRL A,R3
1-1
ANL A,R3
1-1
ORL A,R3
1-1
ADDC A,R3
1-1,C,OV,AC
ADD A,R3
1-1,C,OV,AC
DEC R3
1-1
INC R3
1-1
xB
MOVX R4,A
1-1
MOV A,R4
1-1
DJNZ R4,rel
2-2
XCH A,R4
1-1
CJNE R4,#imm,rel
3-2,C
MOV R4,dir
2-2
SUBB A,R4
1-1,C,OV,AC
MOV dir,R4
2-2
MOV R4,#imm
2-1
XRL A,R4
1-1
ANL A,R4
1-1
ORL A,R4
1-1
ADDC A,R4
1-1,C,OV,AC
ADD A,R4
1-1,C,OV,AC
DEC R4
1-1
INC R4
1-1
xC
MOVX R5,A
1-1
MOV A,R5
1-1
DJNZ R5,rel
2-2
XCH A,R5
1-1
CJNE R5,#imm,rel
3-2,C
MOV R5,dir
2-2
SUBB A,R5
1-1,C,OV,AC
MOV dir,R5
2-2
MOV R5,#imm
2-1
XRL A,R5
1-1
ANL A,R5
1-1
ORL A,R5
1-1
ADDC A,R5
1-1,C,OV,AC
ADD A,R5
1-1,C,OV,AC
DEC R5
1-1
INC R5
1-1
xD
MOVX R6,A
1-1
MOV A,R6
1-1
DJNZ R6,rel
2-2
XCH A,R6
1-1
CJNE R6,#imm,rel
3-2,C
MOV R6,dir
2-2
SUBB A,R6
1-1,C,OV,AC
MOV dir,R6
2-2
MOV R6,#imm
2-1
XRL A,R6
1-1
ANL A,R6
1-1
ORL A,R6
1-1
ADDC A,R6
1-1,C,OV,AC
ADD A,R6
1-1,C,OV,AC
DEC R6
1-1
INC R6
1-1
xE
MOVX R7,A
1-1
MOV A,R7
1-1
DJNZ R7,rel
2-2
XCH A,R7
1-1
CJNE R7,#imm,rel
3-2,C
MOV R7,dir
2-2
SUBB A,R7
1-1,C,OV,AC
MOV dir,R7
2-2
MOV R7,#imm
2-1
XRL A,R7
1-1
ANL A,R7
1-1
ORL A,R7
1-1
ADDC A,R7
1-1,C,OV,AC
ADD A,R7
1-1,C,OV,AC
DEC R7
1-1
INC R7
1-1
xF
The 8051 Instruction Set
Table 1-12. 8051 Instruction Table
Atmel 8051 Microcontrollers Hardware Manual
NOP
1-1
JBC bit,rel
3-2
JB bit,rel
3-2
JNB bit,rel
3-2
JC rel
2-2
JNC rel
2-2
JZ rel
2-2
JNZ rel
2-2
SJMP rel
2-2
MOV DPTR,#imm16
3-2
ORL C,/bit
2-2,C
ANL C,/bit
2-2,C
PUSH dir
2-2
POP dir
2-2
MOVX A,@DPTR
1-2
MOVX @DPTR,A
1-2
Ox
1x
2x
3x
4x
5x
6x
7x
8x
9x
Ax
Bx
Cx
Dx
Ex
Fx
x0
Atmel 8051 Microcontrollers Hardware Manual
ACALL addr11
2-2
AJMP addr11
2-2
ACALL addr11
2-2
AJMP addr11
2-2
ACALL addr11
2-2
AJMP addr11
2-2
ACALL addr11
2-2
AJMP addr11
2-2
ACALL addr11
2-2
AJMP addr11
2-2
ACALL addr11
2-2
AJMP addr11
2-2
ACALL addr11
2-2
AJMP addr11
2-2
ACALL addr
2-2
AJMP addr
2-2
x1
MOVX @R0,A
1-2
MOVX A,@R0
1-2
SETB bit
2-1
CLR bit
2-1
CPL bit
2-1
MOV C,bit
2-1,C
MOV bit,C
2-2
ANL C,bit
2-2,C
ORL C,bit
2-2,C
XRL dir,A
2-1
ANL dir,A
2-1
ORL dir,A
2-1
RETI
1-2
RET
1-2
LCALL code
3-2
LJMP code
3-2
x2
MOVX @R1,A
1-2
MOVX A,@R1
1-2
SETB C
1-1,c=1
CLR C
1-1,C=0
CPL C
1-1,C
INC DPTR
1-2
MOVC A,@A+DPTR
1-2
MOVC A,@A+PC
1-2
JMP @A+DPTR
1-2
XRL dir,#imm
3-2
ANL dir,#imm
3-2
ORL dir,#imm
3-2
RLC A
1-1,C
RL A
1-1
RRC A
1-1.C
RR A
1-1
x3
CPL A
1-1
CLR A
1-1
DA A
1-1,C
SWAP A
1-1
CJNE A,#imm,rel
3-2,C
MUL AB
1-4,C=0,OV
SUBB A,#imm
2-1,C,OV,AC
DIV AB
1-4,C=0,OV
MOV A,#imm
2-1
XRL A,#imm
2-1
ANL A,#imm
2-1
ORL A,#imm
2-1
ADDC A,#imm
2-1,C,OV,AC
ADD A,#imm
2-1,C,OV,AC
DEC A
1-1
INC A
1-1
x4
MOV dir,A
2-1
MOV A,dir
2-1
DJNZ dir,rel
3-2
XCH A,dir
2-1
CJNE A,dir,rel
3-2,C
SUBB A,dir
2-1,C,OV,AC
MOV dir,dir
3-2
MOV dir,#imm
3-2
XRL A,dir
2-1
ANL A,dir
2-1
ORL A,dir
2-1
ADDC A,dir
2-1,C,OV,AC
ADD A,dir
2-1,C,OV,AC
DEC dir
2-1
INC dir
2-1
x5
MOVX @R0,A
1-1
MOV A,@R0
1-1
XCHD A,@R0
1-1
XCH A,@R0
1-1
CJNE @R0,#imm,rel
3-2,C
MOV @R0,dir
2-2
SUBB A,@R0
1-1,C,OV,AC
MOV dir,@R0
2-2
MOV @R0,#imm
2-1
XRL A,@R0
1-1
ANL A,@R0
1-1
ORL, A,@R0
1-1
ADDC A,@R0
1-1,C,OV,AC
ADD A,@R0
1-1,C,OV,AC
DEC @R0
1-1
INC @R0
1-1
x6
MOVX @R1,A
1-1
MOV A,@R1
1-1
XCHD A,@R1
1-1
XCH A,@R1
1-1
CJNE @R1,#imm,rel
3-2,C
MOV @R1,dir
2-2
SUBB A,@R1
1-1,C,OV,AC
MOV dir,@R1
2-2
MOV @R1,#imm
2-1
XRL A,@R1
1-1
ANL A,@R1
1-1
ORL A,@R1
1-1
ADDC A,@R1
1-1,C,OV,AC
ADD A,@R1
1-1,C,OV,AC
DEC @R1
1-1
INC @R1
1-1
x7
The 8051 Instruction Set
Table 1-12. 8051 Instruction Table (Continued)
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Atmel 8051 Microcontrollers Hardware Manual
1-16
Rev. 4316A–8051–01/04
Section 2
Common Features Description
2.1
Introduction
This chapter presents a comprehensive description of the on-chip hardware features of
the Atmel 8051 microcontrollers. Included in this description are:
„ The port drivers and how they function both as ports and, for Ports 0 and 2, in bus
operations
„ The Timer/Counters
„ The serial Interface
„ The Interrupt System
„ Reset
„ The reduced Power Modes
Atmel 8051 Microcontrollers Hardware Manual
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Common Features Description
Figure 2-1. 8051 Architecture Block Diagram
Note:
(*)For Timer 2 only.
Figure 2-1 shows a functional block diagram of the 80C51s.
2.2
Special Function
Registers
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A map of the on-chip memory area called SFR (Special Function Register) space is
shown in Figure 2-1. SFRs marked by parentheses are resident in the microcontroller
which have the Timer2 feature.Note that not all of the addresses are occupied. Read
accesses to these addresses will in general return random data.
Atmel 8051 Microcontrollers Hardware Manual
Common Features Description
Table 2-1. SFR Map
Bit
Addressable
8 Bytes
Non-bit Addressable
F8h
FFh
F0h
B
F7h
E8h
EFh
E0h
ACC
E7h
D8h
DFh
D0h
PSW
C8h
(T2CON)
D7h
(RCAP2L)
(RCAP2H)
(TL2)
(TH2)
CFh
C0h
C7h
B8h
IP
BFh
B0h
P3
B7h
A8h
IE
AFh
A0h
P2
A7h
98h
SCON
90h
P1
88h
TCON
TMOD
TL0
TL1
80h
P0
SP
DPL
DPH
0/8
1/9
2/A
3/B
Note:
SBUF
9Fh
97h
TH0
4/C
TH1
5/D
AUXR
6/E
CKCON
8Fh
PCON
87h
7/F
Reserved
User software should not write to the reserved locations, since they may be used in
derivative Atmel 8051 products to invoke new features. The functions of the SFRs are
described as below.
2.2.1
Accumulator
ACC is the Accumulator register. The mnemonics for accumulator-specific instructions,
however, refer to the accumulator simply as A.
2.2.2
B Register
The B register is used during multiply and divide operations. For other instructions it can
be treated as another scratch pad register.
2.2.3
Program Status
Word
The PSW register contains program status information as detailed in Table 2-2.
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Common Features Description
Table 2-2. PSW: Program Status Word Register
(MSB)
(LSB)
CY
AC
F0
RS1
RS0
OV
-
Symbol
Position
CY
PSW.7
Carry flag
AC
PSW.6
Auxiliary Carry flag.
(For BCD operations.)
F0
PSW.5
Flag 0
(Available to the user for general purposes.)
P
Name and Significance
Register bank Select control bits 1 & 0. Set/cleared
by software to determine working register bank (see
Note).
RS1
PSW.4
RS0
PSW.3
OV
PSW.2
Overflow flag.
-
PSW.1
(reserved)
P
PSW.0
Parity flag.
Set/cleared by hardware each instruction cycle to
indicate and odd/even number of “one” bits in the
accumulator, i.e., even parity.
Note: The contents of (RS1, RS0) enable the working register banks as follows
(0.0)-Bank 0(00H-07H)
(0.1)-Bank 1(08H-0FH)
(1.0)-Bank 2(10H-17H)
(1.1)-Bank 3(18H-1FH)
2.2.4
Stack Pointer
The Stack Pointer register is 8 bits wide. It is incremented before data is stored during
PUSH and CALL executions. While the stack may reside anywhere in on-chip RAM, the
Stack Pointer is initialized to 07H after a reset. This causes the stack to begin at location
08H.
2.2.5
Data Pointer
The Data Pointer (DPTR) consists of a high byte (DPH) and a low byte (DPL). Its
intended function is to hold a 16-bit address. It may be manipulated as a 16-bit register
or as two independent 8-bit registers.
2.2.6
Ports 0 to 3
P0, P1, P2 and P3 are the SFR latches of Ports 0, 1, 2 and 3, respectively.
2.2.7
Serial Data Buffer
The Serial Data Buffer is actually two separate registers, a transmit buffer and a receive
buffer register. When data is moved to SBUF, it goes to the transmit buffer where it is
held for serial transmission. (Moving a byte to SBUF is what initiates the transmission.)
When data is moved from SBUF, it comes from the receive buffer.
2.2.8
Timer Registers
Register pairs (TH0, TL0), (TH1, TL1), and (TH2, TL2) are the 16-bit counting registers
for Timer/Counters 0, 1, and 2, respectively.
2.2.9
Capture Registers
The register pair (RCAP2H, RCAP2L) are the capture register for the Timer 2 ‘capture
mode’. In this mode, in response to a transition at the 80C52’s T2EX pin, TH2 and TL2
are copied into RCAP2H and RCAP2L. Timer 2 also has a 16-bit auto-reload mode, and
RCAP2H and RCAP2L hold the reload value for this mode. More about Timer 2’s features in Section 1.6.
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Common Features Description
2.2.10
Control Registers
Special Function Registers IP, IE, TMOD, TCON, T2CON, SCON, and PCON contain
control and status bits for the interrupt system, the timer/counters, and the serial port.
They are described in later sections.
2.3
Oscillator and
Clock Circuit
XTAL1 and XTAL2 are the input and output of a single-stage on-chip inverter, which can
be configured with off-chip components as a Pierce oscillator, as shown in Figure 2-2.
The on-chip circuitry, and selection of off-chip components to configure the oscillator are
discussed in Section 1.12.
Figure 2-2. Crystal/Ceramic Resonator Oscillator
The oscillator, in any case, drives the internal clock generator. The clock generator provides the internal clocking signals to the chip. The internal clocking signals are at half
the oscillator frequency, and define the internal phases, states, and machine cycles,
which are described in the next section.
2.3.1
More about the Onchip Oscillator
This section not yet available.
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2.4
CPU Timing
2.4.1
X1 Mode (Standard
Mode)
A machine cycle consists of 6 states (12 oscillator periods). Each state is divided into a
Phase 1 half, during which the Phase 1 clock is active, and a Phase 2 half, during which
the Phase 2 clock is active. Thus, a machine cycle consists of 12 oscillator periods,
numbered S1P1 (State 1, Phase 1), through S6P2 (State 6, Phase 2). Each phase lasts
for one oscillator period. Each state lasts for two oscillator periods. Typically, arithmetic
and logical operations take place during Phase 1 and internal register-to-register transfers take place during Phase 2.
The diagrams in Figure 2-3 show the fetch/execute timing referenced to the internal
states and phases. Since these internal clock signals are not user accessible, the
XTAL2 oscillator signal and the ALE (Address Latch Enable) signal are shown for external reference. ALE is normally activated twice during each machine cycle: once during
S1P2 and S2P1, and again during S4P2 and S5P1.
Execution of one-cycle instruction begins at S1P2, when the opcode is latched into the
Instruction Register. If it is a two-byte instruction, the second byte is read during S4 of
the same machine cycle. If it is one-byte instruction, there is still a fetch at S4, but the
byte read (which would be the next opcode), is ignored, and the Program Counter is not
incremented. In any case, execution is complete at the end of S6P2. Figure 2-3A and
Figure 2-3B show the timing for a 1-byte, 1-cycle instruction and for a 2-byte, 1-cycle
instruction.
Most 80C51 instructions execute in one cycle. MUL (multiply) and DIV (divide) are the
only instructions that take more than two cycles to complete. They take four cycles.
Separately, two codes bytes are fetched from Program Memory during every machine
cycle. The only exception to this is when a MOVX instruction is executed. MOVX is a 1byte 2-cycle instruction that accesses external Data Memory. During a MOVX, two
fetches are skipped while the external Data Memory is being addressed and strobed.
Figure 2-3C and Figure 2-3D show the timing for a normal 1-byte, 2-cycle instruction
and for a MOVX instruction.
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Figure 2-3. 80C51 fetch/Execute Sequences.
2.4.2
X2 Mode
This section not yet available.
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2.5
Port Structures
and Operation
All four ports in the 80C51 are bidirectional. Each consists of a latch (Special Function
Register P0 through P3), an output driver, and an input buffer.
The output drivers of Ports 0 and 2, and input buffers of Port 0, are used in accesses to
external memory. In this application, Port 0 outputs the low byte of the external memory
address, time-multiplexed with the byte being written or read. Port 2 outputs the high
byte of the external memory address when the address is 16 bits wide. Otherwise the
Port 2 pins continue to emit the P2 SFR content.
All the Port 3 pins, and (in the case of Timer2) two Port 1 pins are multifunctional. They
are not only port pins, but also serve the functions of various special features as listed
below:
Port Pin
(1)
(1)
P1.0
P1.1
P3.0
P3.1
P3.2
P3.3
P3.4
P3.5
P3.6
P3.7
Note:
Alternate Function
T2 (Timer/Counter 2 external input)
T2EX (Timer/Counter 2 capture/reload trigger)
RXD (serial input port)
TXD (serial output port)
INT0 (external interrupt)
INT1 (external interrupt)
T0 (Timer/Counter 0 external input)
T1 (Timer/Counter 1 external input)
WR (external Data memory write strobe)
RD (external Data memory read strobe)
1. P1.0 and P1.1 serve these alternate functions only on the microcontrollers including
Timer2.
The alternate functions can only be activated if the corresponding bit latch in the port
SFR contains a 1. Otherwise the port pin is stuck at 0.
2.5.1
I/O Configurations
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Figure 2-4 shows a functional diagram of a typical bit latch and I/O buffer in each of the
four ports. The bit latch (one bit in the port’s SFR) is represented as a Type D flip-flop,
which will clock in a value from the internal bus in response to a “write to latch” signal
from the CPU. The Q output of the flip-flop is placed on the internal bus in response to a
“read latch” signal from the CPU. The level of the port pin itself is placed on the internal
bus in response to a “read pin” signal from the CPU. Some instructions that read a port
activate the “read latch” signal, and others activate the “read latch” signal, and others
activate the “read pin” signal.
Atmel 8051 Microcontrollers Hardware Manual
Common Features Description
Figure 2-4. 80C51 Port Bit Latches and I/O Buffers.
As shown in Figure 2-4, the output drivers of Ports 0 and 2 are switchable to an internal
ADDR and ADDR/DATA bus by an internal CONTROL signal for use in external memory accesses. During external memory accesses, the P2 SFR remains unchanged, but
the P0 SFR gets 1s written to it.
Also shown in Figure 2-4, is that if a P3 bit latch contains a 1, then the output level is
controlled by the signal labeled “alternate output function.” The actual P3.X pin level is
always available to the pin’s alternate input function, if any.
Ports 1, 2, and 3 have internal pull-ups. Ports 0 has open-drain outputs. Each I/O line
can be independently used as an input or an output. (Ports 0 and 2 may not be used as
general purpose I/O when being used as the ADDR/DATA BUS). To be used as an
input, the port bit latch must contain a 1, which turns off the output driver FET. Then, for
Ports 1, 2, and 3, the pin is pulled high by the internal pull-up, but can be pulled low by
an external source.
Port 0 differs in not having internal pull-ups. The pull-up FET in the P0 output driver (see
Figure 2-4A) is used only when the Port is emitting 1’s during external memory
accesses. Otherwise the pull-up FET is off. Consequently P0 lines that are being used
as output port lines are open drain. Writing a 1 to the bit latch leaves both output FETs
off, so the pin floats. In that conditions it can be used as a high-impedance input.
Because Ports 1, 2, and 3 have fixed internal pull-ups they are sometimes called “quasibidirectional” ports. When configured as inputs they pull high and will source current (IIL,
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in the data sheets) when externally pulled low. Port 0, on the other hand, is considered
“true” bidirectional, because when configured as an input it floats.
All the port latches in the 80C51 have 1’s written to them by the reset function. If a 0 is
subsequently written to a port latch, it can be re configured as an input by writing a 1 to
it.
2.5.2
Writing to a Port
In the execution of an instruction that changes the value in a port latch, the new value
arrives at the latch during S6P2 of the final cycle of the instruction. However, port
latches are in fact sampled by their output buffers only during Phase 1 of any clock
period. (During Phase 2 the output buffer holds the value it saw during the previous
Phase 1). Consequently, the new value in the port latch won’t actually appear at the output pin until the next Phase 1, which be at S1P1 of the next machine cycle.
If the change requires a 0-to-1 transition in Port 1, 2, or 3, an additional pull-up is turned
on during S1P1 and S1P2 of the cycle in which the transition occurs. This is done to
increase the transition speed. The extra pull-up can source about 100 times the current
that the normal pull-up can. It should be noted that the internal pull-ups are field-effect
transistors, not linear resistors. The pull-up arrangements are shown in Figure 2-5.
In the CMOS versions, the pull-up consists of three pFETs. It should be noted that an nchannel FET (nFET) is turned on when a logical 1 is applied to its gate, and is turned off
when a logical 0 is applied to its gate. A p-channel FET (pFET) is the opposite: it is on
when its gate sees a 0, and off when its gate sees a 1.
pFET 1 in Figure 2-5 is the transistor that is turned on 2 oscillator periods after a 0-to-1
transition in the port latch. While it’s on, it turns on pFET 3 (a weak pull-up), through the
inverter. This inverter and pFET form a latch which hold the 1.
Note that if the pin is emitting a 1, a negative glitch on the pin from some external source
can turn off pFET 3, causing the pin to go into a float state, pFET 2 is a very weak pullup which is on whenever the nFET is off, in traditional CMOS style. It’s only about 1/10
the strength of pFET3. Its function is to restore a 1 to the pin in the event the pin had a 1
and lost it to a glitch.
Figure 2-5. Ports 1 and 3 CMOS Internal Pull-up Configurations.
CMOS Configuration. pFET 1 is turned on for
2 osc. periods after Q makes a 1-to-0 transition. During this time, pFET 1 also turns on
pFET 3 through the inverter to form a latch
which holds the 1. pFET 2 is also on.
Port 2 is similar except that it holds the strong pull-up on while emitting 1s that are address bits.
(See “Accessing External Memory”.)
2.5.3
Port Loading and
Interfacing
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The output buffer of Ports 1, 2 and 3 can each drive 3LS TTL inputs. The pins can be
driven by open-collector and open-drain outputs, but note that 0-to-1 transition will not
be fast. In the CMOS device, an input 0 turns off pull-up P3, leaving only the weak pull-
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Common Features Description
up P2 to drive the transistor. Figure 2-6 shows an example where the port is driven by
an open drain transistor tN. The parasitic capacitance is equal to 1000pF.
Figure 2-6. Port Interfacing
The above diagram show the behavior of the port during 0 to 1 transition.
In the area A only pull-up P2 sinks the capacitor and takes 5 µs to switch from 0 volt to 2
volts. In the area B, pull-up P2 and P3 feed the capacitor and the time to charge the capacitor is
divide roughly by ten. So this figure shows it takes some machine cycles before having a true
high level during a 0-to-1 transition.
Figure 2-7. Port Behavior During 0-to-1 Transition
Voltage (V)
2.5.4
Read-Modify-Write
Feature
Some instructions that read a port read the latch and others read the pin. Which instructions perform what functions? The instructions that read the latch rather than the pin are
the ones that read a value, possibly change it, and then rewrite it to the latch. These are
called “read-modify-write” instructions. The instructions listed below are read-modifywrite instructions. When the destination operand is a port, or a port bit, these instructions read the latch rather than the pin:
ANL
(logical AND, e.G., ANL P1,A)
ORL
(logical OR, e.g., ORL P2,A)
XRL
(logical EX-OR, e.g., XRL P3,A)
JBC
(jump if bit = 1 and clear bit, e.g., JBC P1.1, LABEL)
CPL
(complement bit, e.g., CPL P3.0)
INC
(increment, e.g., INC P2)
DEC
(decrement, e.g., DEC P2)
DJNZ
(decrement and jump if not zero, e.g., DJNZ P3, LABEL)
MOV PX.Y,C(move carry bit to bit Y of Port X)
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CLR PX.Y(clear bit Y of Port X)
SETB PX.Y(set bit Y of Port X)
It is not obvious that the last three instructions in this list are read-modify-write instructions, but they are. They read the port byte, all 8 bits, modify the addressed bit, then
write the new byte back to the latch.
The reason that read-modify-write instructions are directed to the latch rather than the
pin is to avoid a possible misinterpretation of the voltage level at the pin. For example, a
port bit might be used to drive the base of a transistor. When a 1 is written to the bit, the
transistor is turned on. If the CPU then reads the same port bit at the pin rather than the
latch, it will read the base voltage of the transistor and interpret it as a 0. Reading the
latch rather than the pin will return the correct value of 1.
2.6
Accessing
Accesses to external memory are of two types: accesses to external Program Memory
External Memory and accesses to external Data Memory. Accesses to external Program Memory use signal PSEN (program store enable) as the read strobe. Accesses to external Data Memory use
RD or WR (alternate function of P3.7 and P3.6) to strobe the memory.
Fetches from external Program memory always use a 16-bit address. Accesses to
external Data Memory can use either a 16-bit address (MOVX @DPTR) or an 8-bit
address (MOVX @Ri).
Whenever a 16-bit address is used, the high byte of the address comes out on Port 2,
where it is held for the duration of the read or write cycle. Note that the Port 2 drivers use
the strong pull-ups during the entire time that they are emitting address bits that are 1’s.
This is during the execution of a MOVX @DPTR instruction. During this time the Port 2
latch (the Special Function register) does not have to contain 1’s, and the contents of
the Port 2 SFR are not modified. If the external memory cycle is not immediately followed by another external memory cycle, the undisturbed contents of the Port 2 SFR will
reappear in the next cycle.
If an 8-bit address is being used (MOVX @Ri), the contents of the Port 2 SFR remain at
the Port 2 pins throughout the external memory cycle. This will facilitate paging.
In any case, the low byte of the address is time-multiplexed with the data byte on Port 0.
The ADDR/DATA signal drives both FETs in the Port 0 output buffers. Thus, in this
application the Port 0 pins are not open-drain outputs, and do not require external pullups. Signal ALE (address latch enable) should be used to capture the address byte into
an external latch. The address byte is valid at the negative transitions of ALE. Then, in a
write cycle, the data byte to be written appears on Port 0 just before WR is activated, and
remains there until after WR is deactivated. In a read cycle, the incoming byte is accepted at
Port 0 just before the read strobe is deactivated.
During any access to external memory, the CPU writes 0FFH to the Port 0 latch (the
Special Function Register), thus obliterating whatever information the Port 0 SFR may
have been holding.
External program Memory is accessed under two conditions:
1. Whenever signal EA is active or
2. Whenever the program counter (PC) contains a number that is larger than the
memory size.
This requires that the ROMless versions have EA wired low to enable the lower program
bytes to be fetched from external memory.
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When the CPU is executing out of external Program Memory, all 8 bits of Port 2 are dedicated to an output function and may not be used for general purpose I/O. During
external program fetches they output the high byte of the PC. During this time the Port 2
drivers use the strong pull-ups to emit PC bits that are 1’s.
Figure 2-8. External Program Memory Execution
2.7
PSEN
The read strobe for external fetches is PSEN. PSEN is not activated for internal fetches.
When the CPU is accessing external Program Memory, PSEN is activated twice every cycle
(except during a MOVX instruction) whether or not the byte fetched is actually needed for the
current instruction. When PSEN is activated its timing is not the same as RD. A complete RD
cycle, including activation and deactivation of ALE and RD, takes 12 oscillator periods. A complete PSEN cycle, including activation and deactivation of ALE and PSEN, takes 6 oscillator
periods. The execution sequence for these two types of read cycles are shown in Figure 2-8 for
comparison.
2.8
ALE
The main function of ALE is to provide a properly timed signal to latch the low byte of an
address from P0 to an external latch during fetches from external Program Memory. For
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that purpose ALE is activated twice every machine cycle. This activation takes place
even when the cycle involves no external fetch. The only time an ALE pulse doesn’t
come out is during an access to external Data Memory. The first ALE of the second
cycle of a MOVX instructions is missing. The ALE disable mode, described in Section
2.8.2, disables the ALE output. Consequently, in any system that does not use external
Data Memory, ALE is activated at a constant rate of 1/6 the oscillator frequency, and
can be used for external clocking or timing purposes.
2.8.1
Overlapping
External Program
and Data Memory
Spaces
In some applications it is desirable to execute a program from the same physical memory that is being used to store data. In the 80C51, the external Program and Data
Memory spaces can be combined by ANDing PSEN and RD. A positive-logic AND of these
two signals produces an active-low read strobe that can be used for the combined physical
memory. Since the PSEN cycle is faster than the RD cycle, the external memory needs to be
fast enough to accommodate the PSEN cycle.
2.8.2
ALE Disable Mode
The ALE signal is used to demultiplex address and data buses on port 0 when used with
external program or data memory. Nevertheless, during internal code execution, ALE
signal is still generated.
In order to reduce EMI, ALE signal can be disabled by setting AO bit.
The AO bit is located in AUXR register at bit location 0 (See Table 2-3). As soon as AO
is set, ALE is no longer output but remains active during MOVX and MOVC instructions
and external fetches. During ALE disabling, ALE pin is weakly pulled high.
Table 2-3. AUXR Register
AUXR (S:8Eh)
Auxiliary Register
7
6
5
4
3
2
1
0
-
-
-
-
-
-
EXTRAM
AO
Bit
Bit
Number
Mnemonic
7
-
Reserved
The value read from this bit is indeterminate. Do not set this bit.
6
-
Reserved
The value read from this bit is indeterminate. Do not set this bit.
5
-
Reserved
The value read from this bit is indeterminate. Do not set this bit.
4
-
Reserved
The value read from this bit is indeterminate. Do not set this bit.
3
-
Reserved
The value read from this bit is indeterminate. Do not set this bit.
2
-
Reserved
The value read from this bit is indeterminate. Do not set this bit.
1
EXTRAM
0
AO
Description
EXTRAM select
Clear to map XRAM data in internal XRAM memory.
Set to map XRAM data in external XRAM memory.
ALE Output bit
Clear to restore ALE operation during internal fetches.
Set to disable ALE operation during internal fetches.
Reset Value = XXXX XX00b
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2.9
Timer/Counters
The Atmel 80C51 Microcontrollers implement two general purpose, 16-bit timers/counters. They are identified as Timer 0 and Timer 1, and can be independently
configured to operate in a variety of modes as a timer or as an event counter. When
operating as a timer, the timer/counter runs for a programmed length of time, then
issues an interrupt request. When operating as a counter, the timer/counter counts negative transitions on an external pin. After a preset number of counts, the counter issues
an interrupt request.
The various operating modes of each timer/counter are described in the following
sections.
2.9.1
Timer/Counter
Operations
A basic operation consists of timer registers THx and TLx (x= 0, 1) connected in cascade to form a 16-bit timer. Setting the run control bit (TRx) in TCON register (see
Figure 2-4) turns the timer on by allowing the selected input to increment TLx. When
TLx overflows it increments THx; when THx overflows it sets the timer overflow flag
(TFx) in TCON register. Setting the TRx does not clear the THx and TLx timer registers.
Timer registers can be accessed to obtain the current count or to enter preset values.
They can be read at any time but TRx bit must be cleared to preset their values, otherwise the behavior of the timer/counter is unpredictable.
The C/Tx# control bit (in TCON register) selects timer operation, or counter operation,
by selecting the divided-down peripheral clock or external pin Tx as the source for the
counted signal. TRx bit must be cleared when changing the mode of operation, otherwise the behavior of the timer/counter is unpredictable.
For timer operation (C/Tx# = 0), the timer register counts the divided-down peripheral
clock. The timer register is incremented once every peripheral cycle (6 peripheral clock
periods). The timer clock rate is FPER / 6, i.e. FOSC / 12 in standard mode or FOSC / 6 in
X2 mode.
For counter operation (C/Tx# = 1), the timer register counts the negative transitions on
the Tx external input pin. The external input is sampled every peripheral cycle. When
the sample is high in one cycle and low in the next one, the counter is incremented.
Since it takes 2 cycles (12 peripheral clock periods) to recognize a negative transition,
the maximum count rate is FPER / 12, i.e. FOSC / 24 in standard mode or FOSC / 12 in X2
mode. There are no restrictions on the duty cycle of the external input signal, but to
ensure that a given level is sampled at least once before it changes, it should be held for
at least one full peripheral cycle.
In addition to the “timer” or “counter” selection, Timer 0 and Timer 1 have four operating
modes from which to select which are selected by bit-pairs (M1, M0) in TMOD. Modes 0, 1,
and 2 are the same for both timer/counters. Mode 3 is different. The four operating modes are
described below.
Timer 2, has three modes of operation: ‘capture’, ‘auto-reload’ and ‘baud rate
generator’.
2.10
Timer 0
Timer 0 functions as either a timer or event counter in four modes of operation. Figure 29 to Figure 2-12 show the logical configuration of each mode.
Timer 0 is controlled by the four lower bits of the TMOD register (see Table 2-5) and bits
0, 1, 4 and 5 of the TCON register (see Table 2-4). TMOD register selects the method of
timer gating (GATE0), timer or counter operation (T/C0#) and mode of operation (M10
and M00). The TCON register provides timer 0 control functions: overflow flag (TF0), run
control bit (TR0), interrupt flag (IE0) and interrupt type control bit (IT0).
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For normal timer operation (GATE0= 0), setting TR0 allows TL0 to be incremented by
the selected input. Setting GATE0 and TR0 allows external pin INT0# to control timer
operation.
Timer 0 overflow (count rolls over from all 1s to all 0s) sets TF0 flag, generating an interrupt request.
It is important to stop timer/counter before changing mode.
2.10.1
Mode 0 (13-bit
Timer)
Mode 0 configures timer 0 as a 13-bit timer which is set up as an 8-bit timer (TH0 register) with a modulo 32 prescaler implemented with the lower five bits of the TL0 register
(see Figure 2-9). The upper three bits of TL0 register are indeterminate and should be
ignored. Prescaler overflow increments the TH0 register.
As the count rolls over from all 1’s to all 0’s, it sets the timer interrupt flag TF0. The
counted input is enabled to the Timer when TR0 = 1 and either GATE = 0 or INT0 = 1.
(Setting GATE = 1 allows the Timer to be controlled by external input INT0, to facilitate pulse
width measurements). TR0 is a control bit in the Special Function register TCON (Table 2-4).
GATE is in TMOD.
The 13-bit register consists of all 8 bits of TH0 and the lower 5 bits of TL0. The upper 3
bits of TL0 are indeterminate and should be ignored. Setting the run flag (TR0) does not
clear the registers.
Mode 0 operation is the same for Timer 0 as for Timer 1. Substitute TR0, TF0 and INT0
for the corresponding Timer 1 signals in Table 2-10. There are two different GATE bits, one for
Timer 1 (TMOD.7) and one for Timer 0 (TMOD.3).
Figure 2-9. Timer/Counter x (x = 0 or 1) in Mode 0
PERIPH
CLOCK
÷6
0
THx
(8 bits)
1
Tx
TLx
(5 bits)
Overflow
TFx
TCON reg
Timer x
Interrupt
Request
C/Tx#
TMOD reg
INTx#
TRx
GATE
TCON reg
TMOD reg
2.10.2
Mode 1 (16-bit
Timer)
Mode 1 is the same as Mode 0, except that the Timer register is being run with all 16
bits.
Mode 1 configures timer 0 as a 16-bit timer with the TH0 and TL0 registers connected in
cascade (see Figure 2-10). The selected input increments the TL0 register.
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Figure 2-10. Timer/Counter x (x = 0 or 1) in Mode 1
PERIPH
CLOCK
÷6
0
THx
(8 bits)
1
TLx
(8 bits)
Overflow
TFx
TCON reg
Timer x
Interrupt
Request
Tx
C/Tx#
TMOD reg
INTx#
TRx
TCON reg
GATE
2.10.3
Mode 2 (8-bit Timer
with Auto-Reload)
Mode 2 configures timer 0 as an 8-bit timer (TL0 register) that automatically reloads
from the TH0 register (see Table 2-6 on page 37). TL0 overflow sets TF0 flag in the
TCON register and reloads TL0 with the contents of TH0, which is preset by software.
When the interrupt request is serviced, hardware clears TF0. The reload leaves TH0
unchanged. The next reload value may be changed at any time by writing it to the TH0
register.
Mode 2 operation is the same for Timer/Counter 1.
Figure 2-11. Timer/Counter x (x = 0 or 1) in Mode 2
PERIPH
CLOCK
÷6
0
TLx
(8 bits)
1
Overflow
TFx
TCON reg
Timer x
Interrupt
Request
Tx
C/Tx#
TMOD reg
INTx#
GATE
2.10.4
Mode 3 (Two 8-bit
Timers)
TRx
TCON reg
THx
(8 bits)
Mode 3 configures timer 0 so that registers TL0 and TH0 operate as separate 8-bit timers (see Figure 2-12). This mode is provided for applications requiring an additional 8-bit
timer or counter. TL0 uses the timer 0 control bits C/T0# and GATE0 in the TMOD register, and TR0 and TF0 in the TCON register in the normal manner. TH0 is locked into a
timer function (counting FPER /6) and takes over use of the timer 1 interrupt (TF1) and
run control (TR1) bits. Thus, operation of timer 1 is restricted when timer 0 is in mode 3.
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Common Features Description
Figure 2-12. Timer/Counter 0 in Mode 3: Two 8-bit Counters
PERIPH
CLOCK
÷6
0
1
TL0
(8 bits)
Overflow
TH0
(8 bits)
Overflow
TF0
TCON.5
Timer 0
Interrupt
Request
TF1
TCON.7
Timer 1
Interrupt
Request
T0
C/T0#
TMOD.2
INT0#
TR0
TCON.4
GATE
PERIPH
CLOCK
÷6
TR1
TCON.6
2.11
2.11.1
Timer 1
Mode 0 (13-bit
Timer)
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4316A–8051–01/04
Timer 1 is identical to timer 0, except for mode 3, which is a hold-count mode. The following comments help to understand the differences:
•
Timer 1 functions as either a timer or event counter in three modes of operation.
Figure 2-9 to Figure 2-11 show the logical configuration for modes 0, 1, and 2. Timer
1’s mode 3 is a hold-count mode.
•
Timer 1 is controlled by the four high-order bits of the TMOD register (see Table 2-5
on page 37) and bits 2, 3, 6 and 7 of the TCON register (see Table 2-4 on page 36).
The TMOD register selects the method of timer gating (GATE1), timer or counter
operation (C/T1#) and mode of operation (M11 and M01). The TCON register
provides timer 1 control functions: overflow flag (TF1), run control bit (TR1), interrupt
flag (IE1) and interrupt type control bit (IT1).
•
Timer 1 can serve as the baud rate generator for the serial port. Mode 2 is best
suited for this purpose.
•
For normal timer operation (GATE1 = 0), setting TR1 allows TL1 to be incremented
by the selected input. Setting GATE1 and TR1 allows external pin INT1# to control
timer operation.
•
Timer 1 overflow (count rolls over from all 1s to all 0s) sets the TF1 flag generating
an interrupt request.
•
When timer 0 is in mode 3, it uses timer 1’s overflow flag (TF1) and run control bit
(TR1). For this situation, use timer 1 only for applications that do not require an
interrupt (such as a baud rate generator for the serial port) and switch timer 1 in and
out of mode 3 to turn it off and on.
•
It is important to stop timer/counter before changing modes.
Mode 0 configures Timer 1 as a 13-bit timer, which is set up as an 8-bit timer (TH1 register) with a modulo-32 prescaler implemented with the lower 5 bits of the TL1 register
(see Figure 2-9). The upper 3 bits of the TL1 register are ignored. Prescaler overflow
increments the TH1 register.
Atmel 8051 Microcontrollers Hardware Manual
Common Features Description
Figure 2-13. Timer/Counter 1 Mode 0: 13-bit Counter
Periph
Clock
/6
2.11.2
Mode 1 (16-bit
Timer)
Mode 1 configures Timer 1 as a 16-bit timer with the TH1 and TL1 registers connected
in cascade (see Figure 2-10). The selected input increments the TL1 register.
2.11.3
Mode 2 (8-bit Timer
with Auto Reload)
Mode 2 configures Timer 1 as an 8-bit timer (TL1 register) with automatic reload from
the TH1 register on overflow (see Figure 2-11). TL1 overflow sets the TF1 flag in the
TCON register and reloads TL1 with the contents of TH1, which is preset by software.
The reload leaves TH1 unchanged.
Figure 2-14. Timer/Counter 1 Mode 2: 8-bit Auto-reload
Periph
Clock
6
INT1 Pin
2.11.4
Mode 3 (Halt)
Placing Timer 1 in mode 3 causes it to halt and hold its count. This can be used to halt
Timer 1 when TR1 run control bit is not available i.e., when Timer 0 is in mode 3.
2.11.5
Interrupt
Each timer handles one interrupt source; that is the timer overflow flag TF0 or TF1. This
flag is set every time an overflow occurs. Flags are cleared when vectoring to the timer
interrupt routine. Interrupts are enabled by setting ETx bit in IE0 register. This assumes
interrupts are globally enabled by setting EA bit in the IE0 register.
Atmel 8051 Microcontrollers Hardware Manual
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Common Features Description
Figure 2-15. Timer Interrupt System
Timer 0
Interrupt Request
TF0
TCON.5
ET0
IE0.1
Timer 1
Interrupt Request
TF1
TCON.7
ET1
IE0.3
2.11.6
Timer Registers
Table 2-4. TCON Register - TCON (S:88h)
Timer/Counter Control Register.
7
6
5
4
3
2
1
0
TF1
TR1
TF0
TR0
IE1
IT1
IE0
IT0
Bit
Number
Bit
Mnemonic Description
7
TF1
Timer 1 Overflow Flag
Cleared by hardware when processor vectors to interrupt routine.
Set by hardware on timer/counter overflow, when the timer 1 register overflows.
6
TR1
Timer 1 Run Control Bit
Clear to turn off timer/counter 1.
Set to turn on timer/counter 1.
5
TF0
Timer 0 Overflow Flag
Cleared by hardware when processor vectors to interrupt routine.
Set by hardware on timer/counter overflow, when the timer 0 register overflows.
4
TR0
Timer 0 Run Control Bit
Clear to turn off timer/counter 0.
Set to turn on timer/counter 0.
3
IE1
Interrupt 1 Edge Flag
Cleared by hardware when interrupt is processed if edge-triggered (see IT1).
Set by hardware when external interrupt is detected on INT1# pin.
2
IT1
Interrupt 1 Type Control Bit
Clear to select low level active (level triggered) for external interrupt 1 (INT1#).
Set to select falling edge active (edge triggered) for external interrupt 1.
1
IE0
Interrupt 0 Edge Flag
Cleared by hardware when interrupt is processed if edge-triggered (see IT0).
Set by hardware when external interrupt is detected on INT0# pin.
0
IT0
Interrupt 0 Type Control Bit
Clear to select low level active (level triggered) for external interrupt 0 (INT0#).
Set to select falling edge active (edge triggered) for external interrupt 0.
Reset Value = 0000 0000b
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Atmel 8051 Microcontrollers Hardware Manual
Common Features Description
Table 2-5. TMOD Register - TMOD (S:89h)
Timer/Counter Mode Control Register
7
6
5
4
3
2
1
0
GATE1
C/T1#
M11
M01
GATE0
C/T0#
M10
M00
Bit Number
Bit Mnemonic
Description
7
GATE1
Timer 1 Gating Control Bit
Clear to enable timer 1 whenever the TR1 bit is set.
Set to enable timer 1 only while the INT1# pin is high and TR1 bit is set.
6
C/T1#
Timer 1 Counter/Timer Select Bit
Clear for timer operation: timer 1 counts the divided-down system clock.
Set for Counter operation: timer 1 counts negative transitions on external pin T1.
5
M11
4
M01
3
GATE0
Timer 0 Gating Control Bit
Clear to enable timer 0 whenever the TR0 bit is set.
Set to enable timer/counter 0 only while the INT0# pin is high and the TR0 bit is set.
2
C/T0#
Timer 0 Counter/Timer Select Bit
Clear for timer operation: timer 0 counts the divided-down system clock.
Set for counter operation: timer 0 counts negative transitions on external pin T0.
1
M10
0
M00
Timer 1 Mode Select Bits
M11 M01
Operating mode
0
0
Mode 0: 8-bit timer/counter (TH1) with 5-bit prescaler (TL1).
0
1
Mode 1: 16-bit timer/counter.
1
0
Mode 2: 8-bit auto-reload timer/counter (TL1). Reloaded from TH1 at overflow.
1
1
Mode 3: timer 1 halted. Retains count.
Timer 0 Mode Select Bit
Operating mode
M10 M00
0
0
Mode 0: 8-bit timer/counter (TH0) with 5-bit prescaler (TL0).
0
1
Mode 1: 16-bit timer/counter.
1
0
Mode 2: 8-bit auto-reload timer/counter (TL0). Reloaded from TH0 at overflow.
1
1
Mode 3: TL0 is an 8-bit timer/counter.
TH0 is an 8-bit timer using timer 1’s TR0 and TF0 bits.
Reset Value = 0000 0000b
Table 2-6. TH0 Register - TH0 (S:8Ch)
Timer 0 High Byte Register.
7
Bit
Number
7:0
6
5
4
3
2
1
0
Bit
Mnemonic Description
High Byte of Timer 0.
Reset Value = 0000 0000b
Atmel 8051 Microcontrollers Hardware Manual
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Common Features Description
Table 2-7. TL0 Register - TL0 (S:8Ah)
Timer 0 Low Byte Register
7
Bit
Number
6
5
4
3
2
1
0
3
2
1
0
3
2
1
0
Bit
Mnemonic Description
7:0
Low Byte of Timer 0.
Reset Value = 0000 0000b
Table 2-8. TH1 Register - TH1 (S:8Dh)
Timer 1 High Byte Register
7
Bit
Number
6
5
4
Bit
Mnemonic Description
7:0
High Byte of Timer 1.
Reset Value = 0000 0000b
Table 2-9. TL1 Register - TL1 (S:8Bh)
Timer 1 Low Byte Register
7
Bit
Number
7:0
6
5
4
Bit
Mnemonic Description
Low Byte of Timer 1.
Reset Value = 0000 0000b
When Timer 0 is in Mode 3, Timer 1 can be turned on and off by switching it out of and
into its own Mode 3, or can still be used by the serial port as a baud rate generator, or in
fact, in any application not requiring an interrupt.
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Atmel 8051 Microcontrollers Hardware Manual
Common Features Description
2.12
Timer 2
Timer 2 is a 16-bit timer/counter which is present in most of the Atmel 8051 microcontrollers.The count is maintained by two 8-bit timer registers, TH2 and TL2, that are
cascade connected. Like Timers 0 and 1, it can operate either as a timer or as an event
counter.
It is controlled by the T2CON register (See Table 2-11) and the T2MOD register (See
Table 2-12). Timer 2 operation is similar to Timer 0 and Timer 1. C/T2 selects FOSC/6
(timer operation) or external pin T2 (counter operation) as timer register input. Setting
TR2 allows TL2 to be incremented by the selected input.
It has three operating modes: ‘capture’, ‘autoload’ and ‘baud rate generator’, which are
selected by bits in T2CON as shown in Table 2-10.
Table 2-10. Timer 2 Operating Modes
RCLK + TCLK
CP/RL2
TR2
Mode
0
0
1
16-bit auto-reload
0
1
1
16-bit capture
1
X
1
baud rate generator
X
X
0
(off)
In the capture mode there are two options which are selected by bit EXEN2 in T2CON. If
EXEN2 = 0, then Timer 2 is a 16-bit timer or counter which upon overflowing sets bit
TF2, the Timer 2 overflow bit, which can be used to generate an interrupt. If EXEN2 = 1,
then Timer 2 still does the above, but with the added feature that a 1-to-0 transition at
external input T2EX causes the current value in the Timer 2 registers, TL2 and TH2, to
be captured into registers RCAP2L and RCAP2H, respectively. RCAP2L and RCAP2H
are new Special Function Registers in the 80C52, 83C154 and 83C154D. In addition,
the transition at T2EX causes bit EXF2 in T2CON to be set, and EXF2, like TF2, can
generate an interrupt.
The capture mode is illustrated in Figure 2-16.
Figure 2-16. Timer 2 in Capture Mode
Periph
Clock
/6
In the auto-reload mode there are again two options, which are selected by bit EXEN2 in
T2CON. If EXEN2 = 0, then when Timer 2 rolls over it not only sets TF2 but also causes
the Timer 2 registers to be reloaded with the 16-bit value in registers RCAP2L and
RCAP2H, which are preset by software. If EXEN2 = 1, then Timer 2 still does the above,
Atmel 8051 Microcontrollers Hardware Manual
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Common Features Description
but with the added feature that a 1-to-0 transition at external input T2EX will also trigger
the 16-bit reload and set EXF2.
2.12.1
Auto-reload Mode
The auto-reload mode configures timer 2 as a 16-bit timer or event counter with automatic reload. This feature is controlled by the DCEN bit in the T2MOD register (See
Table 2-12). Setting the DCEN bit enables timer 2 to count up or down as shown in Figure 2-17. In this mode the T2EX pin controls the counting direction.
When T2EX is high, timer 2 up-counts. Timer overflow occurs at FFFFh which sets the
TF2 flag and generates an interrupt request. The overflow also causes the 16-bit value
in the RCAP2H and RCAP2L registers to be loaded into the timer registers TH2 and
TL2.
When T2EX is low, timer 2 down-counts. Timer underflow occurs when the count in the
timer registers, TH2 and TL2, equals the value stored in the RCAP2H and RCAP2L registers. The underflow sets TF2 flag and reloads FFFFh into the timer registers.
The EXF2 bit toggles when timer 2 overflow or underflow occurs, depending on the
direction of the count. EXF2 does not generate an interrupt. This bit can be used to provide 17-bit resolution.
Figure 2-17. Auto-reload Mode Up/Down Counter
Periph
CLOCK
:6
0
1
TR2
T2CON.2
CT/2
T2CON.1
T2
(DOWN COUNTING RELOAD VALUE)
FFh
(8-bit)
FFh
T2EX:
(8-bit)
1=UP
2=DOWN
TOGGLE T2CONreg
EXF2
TL2
(8-bit)
TH2
(8-bit)
TF2
TIMER 2
INTERRUPT
T2CONreg
RCAP2L RCAP2H
(8-bit)
(8-bit)
(UP COUNTING RELOAD VALUE)
2.12.2
Programmable
Clock-output
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In clock-out mode, timer 2 operates as a 50% duty-cycle, programmable clock generator
(See Figure 2-18). The input clock increments TL2 at frequency F OSC /2. The timer
repeatedly counts to overflow from a loaded value. At overflow, the contents of the
RCAP2H and RCAP2L registers are loaded into TH2 and TL2. In this mode, timer 2
overflows do not generate interrupts. The formula gives the clock-out frequency,
Atmel 8051 Microcontrollers Hardware Manual
Common Features Description
depending on the system oscillator frequency and the value in the RCAP2H and
RCAP2L registers:
F
× 2 x2
osc
Clock – OutFrequency = ----------------------------------------------------------------------------------------4 × ( 65536 – RCAP2H ⁄ RCAP2L )
Note:
X2 bit is located in the CKCON register.
In X2 mode, FOSC=FXTAL. In standard mode, FOSC=FXTAL/2.
For a 16 MHz system clock, timer 2 has a programmable frequency range of 61 Hz
(FOSC/216) to 4 MHz (FOSC/4). The generated clock signal is brought out to the T2 pin
(P1.0).
Timer 2 is programmed for the clock-out mode as follows:
•
Set T2OE bit in the T2MOD register.
•
Clear C/T2 bit in the T2CON register.
•
Determine the 16-bit reload value from the formula and enter it in the
RCAP2H/RCAP2L registers.
•
Enter a 16-bit initial value in timer registers TH2/TL2. It can be the same as the
reload value, or different, depending on the application.
•
To start the timer, set TR2 run control bit in the T2CON register.
It is possible to use timer 2 as a baud rate generator and a clock generator simultaneously. For this configuration, the baud rates and clock frequencies are not
independent since both functions use the values in the RCAP2H and RCAP2L registers.
Figure 2-18. Clock-out Mode
:6
FCLK PERIPH
TR2
T2CON
TL2
(8-bit)
TH2
(8-bit)
OVERFLOW
RCAP2L RCAP2H
(8-bit) (8-bit)
Toggle
T2
Q
D
T2OE
T2MOD
T2EX
EXF2
T2CON
EXEN2
T2CON
2.12.3
Timer Registers
TIMER 2
INTERRUPT
Table 2-11. T2CON Register - T2CON (S:C8h)
Timer 2 Control Register
7
6
5
4
3
2
1
0
TF2
EXF2
RCLK
TCLK
EXEN2
TR2
C/T2#
CP/RL2#
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Common Features Description
Bit
Number
7
Bit
Mnemonic Description
TF2
Timer 2 overflow Flag
TF2 is not set if RCLK=1 or TCLK = 1.
Must be cleared by software.
Set by hardware on timer 2 overflow.
6
EXF2
Timer 2 External Flag
Set when a capture or a reload is caused by a negative transition on T2EX pin if
EXEN2=1.
Set to cause the CPU to vector to timer 2 interrupt routine when timer 2 interrupt
is enabled.
Must be cleared by software.
5
RCLK
Receive Clock bit
Clear to use timer 1 overflow as receive clock for serial port in mode 1 or 3.
Set to use timer 2 overflow as receive clock for serial port in mode 1 or 3.
4
TCLK
Transmit Clock bit
Clear to use timer 1 overflow as transmit clock for serial port in mode 1 or 3.
Set to use timer 2 overflow as transmit clock for serial port in mode 1 or 3.
3
EXEN2
2
TR2
1
C/T2#
0
CP/RL2#
Timer 2 External Enable bit
Clear to ignore events on T2EX pin for timer 2 operation.
Set to cause a capture or reload when a negative transition on T2EX pin is
detected, if timer 2 is not used to clock the serial port.
Timer 2 Run control bit
Clear to turn off timer 2.
Set to turn on timer 2.
Timer/Counter 2 select bit
Clear for timer operation (input from internal clock system: FOSC).
Set for counter operation (input from T2 input pin).
Timer 2 Capture/Reload bit
If RCLK=1 or TCLK=1, CP/RL2# is ignored and timer is forced to auto-reload on
timer 2 overflow.
Clear to auto-reload on timer 2 overflows or negative transitions on T2EX pin if
EXEN2=1.
Set to capture on negative transitions on T2EX pin if EXEN2=1.
Reset Value = 0000 0000b
Bit addressable
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Atmel 8051 Microcontrollers Hardware Manual
Common Features Description
Table 2-12. T2MOD Register - T2MOD (S:C9h)
Timer 2 Mode Control Register
7
6
5
4
3
2
1
0
-
-
-
-
-
-
T2OE
DCEN
Bit
Number
Bit
Mnemonic Description
7
-
Reserved
The value read from this bit is indeterminate. Do not set this bit.
6
-
Reserved
The value read from this bit is indeterminate. Do not set this bit.
5
-
Reserved
The value read from this bit is indeterminate. Do not set this bit.
4
-
Reserved
The value read from this bit is indeterminate. Do not set this bit.
3
-
Reserved
The value read from this bit is indeterminate. Do not set this bit.
2
-
Reserved
The value read from this bit is indeterminate. Do not set this bit.
1
T2OE
Timer 2 Output Enable bit
Clear to program P1.0/T2 as clock input or I/O port.
Set to program P1.0/T2 as clock output.
0
DCEN
Down Counter Enable bit
Clear to disable timer 2 as up/down counter.
Set to enable timer 2 as up/down counter.
Reset Value = XXXX XX00b
Not bit addressable
Table 2-13. TH2 Register -TH2 (S:CDh)
Timer 2 High Byte Register
7
6
5
4
3
2
1
0
-
-
-
-
-
-
-
-
Bit
Number
7:0
Bit
Mnemonic Description
High Byte of Timer 2.
Reset Value = 0000 0000b
Not bit addressable
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Common Features Description
Table 2-14. TL2 Register - TL2 (S:CCh)
Timer 2 Low Byte Register
7
6
5
4
3
2
1
0
-
-
-
-
-
-
-
-
Bit
Number
Bit
Mnemonic Description
7:0
Low Byte of Timer 2.
Reset Value = 0000 0000b
Not bit addressable
Table 2-15. RCAP2H Register -RCAP2H (S:CBh)
Timer 2 Reload/Capture High Byte Register
7
6
5
4
3
2
1
0
-
-
-
-
-
-
-
-
Bit
Number
Bit
Mnemonic Description
7:0
High Byte of Timer 2 Reload/Capture.
Reset Value = 0000 0000b
Not bit addressable
Table 2-16. RCAP2L Register - RCAP2L (S:CAh)
Timer 2 Reload/Capture Low Byte Register
7
6
5
4
3
2
1
0
-
-
-
-
-
-
-
-
Bit
Number
7:0
Bit
Mnemonic Description
Low Byte of Timer 2 Reload/Capture.
Reset Value = 0000 0000b
Not bit addressable
2.13
Serial Interface
It provides both synchronous and asynchronous communication modes. It operates as a
Universal Asynchronous Receiver and Transmitter (UART) in three full-duplex modes
(Modes 1, 2 and 3). Asynchronous transmission and reception can occur simultaneously
and at different baud rates.
It is also receive-buffered, meaning it can commence reception of a second byte before
a previously received byte has been read from the receive register. (However, if the first
byte still hasn’t been read by the time reception of the second byte is complete, one of
the bytes will be lost). The serial port receive and transmit registers are both accessed
at Special Function Register SBUF. Writing to SBUF loads the transmit register, and
reading SBUF accesses a physically second receive register.
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Atmel 8051 Microcontrollers Hardware Manual
Common Features Description
The serial port can operate in 4 modes:
Mode 0: Serial data enters and exits through RXD. TXD outputs the shift clock. 8 bits
are transmitted/received: 8 data bits (LSB first). The baud rate is fixed at 1/12 the oscillator frequency.
Mode 1: 10 bits are transmitted (through TXD) or received (through RXD): a start bit (0),
8 data bits (LSB first), and a stop bit (1). On receive, the stop bit goes into RB8 in Special Function Register SCON. The baud rate is variable.
Mode 2: 11 bits are transmitted (through TXD) or received (through RXD): a start bit (0),
8 data bits (LSB first), a programmable 9th data bit, and a stop bit (1). On transmit, the
9th data bit (TB8 in SCON) can be assigned the value of 0 or 1. Or, for example, the parity bit (P, in the PSW) could be moved into TB8. On receive, the 9th data bit goes into
RB8 in Special Function register SCON, while the stop bit is ignored. The baud rate is
programmable to either 1/32 or 1/64 the oscillator frequency.
Mode 3: 11 bits are transmitted (through TXD) or received (through RXD): a start bit (0),
8 data bits (LSB first), a programmable 9th data bit and a stop bit (1). In fact, Mode 3 is
the same as Mode 2 in all respects except the baud rate. The baud rate in Mode 3 is
variable.
In all four modes, transmission is initiated in Mode 0 by the condition RI = 0 and REN =
1. Reception is initiated in Mde 0 by the condition RI = 0 and REN = 1. Reception is initiated in the other modes by the incoming start bit if REN = 1.
Serial I/O port includes the following enhancements:
•
Framing error detection
•
Automatic address recognition
The serial port control and status register is the Special Function Register SCON,
shown in Table 2-17. This register contains not only the mode selection bits, but also the
9th data bit for transmit and receive (TB8 and RB8), and the serial port interrupts bits (TI
and RI).
Atmel 8051 Microcontrollers Hardware Manual
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Common Features Description
Table 2-17. SCON Register
SCON - Serial Control Register (98h)
7
6
5
4
3
2
1
0
FE/SM0
SM1
SM2
REN
TB8
RB8
TI
RI
Bit
Bit
Number
Mnemonic
Description
Framing Error bit (SMOD0=1)
FE
Clear to reset the error state, not cleared by a valid stop bit.
Set by hardware when an invalid stop bit is detected.
SMOD0 must be set to enable access to the FE bit
7
SM0
Serial port Mode bit 0
Refer to SM1 for serial port mode selection.
SMOD0 must be cleared to enable access to the SM0 bit
Serial port Mode bit 1
6
SM1
SM0
SM1
Mode
Description
Baud Rate
0
0
0
Shift Register
FCPU PERIPH/6
0
1
1
8-bit UART
Variable
1
0
2
9-bit UART
FCPU PERIPH
/32 or /16
1
1
3
39-bit UART
Variable
Serial port Mode 2 bit / Multiprocessor Communication Enable bit
5
SM2
4
REN
3
TB8
Clear to disable multiprocessor communication feature.
Set to enable multiprocessor communication feature in mode 2 and 3, and
eventually mode 1. This bit should be cleared in mode 0.
Reception Enable bit
Clear to disable serial reception.
Set to enable serial reception.
Transmitter Bit 8 / Ninth bit to transmit in modes 2 and 3
2
RB8
o transmit a logic 0 in the 9th bit.
Set to transmit a logic 1 in the 9th bit.
Receiver Bit 8 / Ninth bit received in modes 2 and 3
Cleared by hardware if 9th bit received is a logic 0.
Set by hardware if 9th bit received is a logic 1.
In mode 1, if SM2 = 0, RB8 is the received stop bit. In mode 0 RB8 is not used.
1
0
TI
Transmit Interrupt flag
Clear to acknowledge interrupt.
Set by hardware at the end of the 8th bit time in mode 0 or at the beginning of
the stop bit in the other modes.
RI
Receive Interrupt flag
Clear to acknowledge interrupt.
Set by hardware at the end of the 8th bit time in mode 0, see Figure 2-26. and
Figure 2-27. in the other modes.
Reset Value = 0000 0000b
Bit addressable
2.13.1
Baud Rates
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The baud rate in Mode 0 is fixed:
Atmel 8051 Microcontrollers Hardware Manual
Common Features Description
The baud rate in Mode 2 depends on the value of bit SMOD in Special Function Register PCON.
If SMOD = 0 (which is its value on reset), the baud rate is 1/64 the oscillator frequency.
If SMOD = 1, the baud rate is 1/32 the oscillator frequency.
In the 80C51, the baud rates in Modes 1 and 3 are determined by the Timer 1 overflow
rate. In case of Timer2, these baud rates can be determined by Timer 1, or by Timer 2,
or by both (one for transmit and the other for receive).
Baud Rate Selection for
UART for Mode 1 and 3
The Baud Rate Generator for transmit and receive clocks can be selected separately via
the T2CON and BDRCON registers.
Figure 2-19. Baud Rate Selection
TIMER1
0
TIMER2
TIMER_BRG_RX
0
1
/ 16
Rx Clock
1
RCLK
RBCK
INT_BRG
TIMER1
0
TIMER2
TIMER_BRG_TX
0
1
1
/ 16
Tx Clock
TCLK
TBCK
INT_BRG
2.13.2
Baud Rate Selection
Table for UART
TCLK
RCLK
TBCK
RBCK
Clock Source
Clock Source
(T2CON)
(T2CON)
(BDRCON)
(BDRCON)
UART Tx
UART Rx
0
0
0
0
Timer 1
Timer 1
1
0
0
0
Timer 2
Timer 1
0
1
0
0
Timer 1
Timer 2
1
1
0
0
Timer 2
Timer 2
X
0
1
0
INT_BRG
Timer 1
X
1
1
0
INT_BRG
Timer 2
0
X
0
1
Timer 1
INT_BRG
1
X
0
1
Timer 2
INT_BRG
X
X
1
1
INT_BRG
INT_BRG
2.13.3
Internal Baud Rate
Generator (BRG)
When the internal Baud Rate Generator is used, the Baud Rates are determined by the
BRG overflow depending on the BRL reload value, the value of SPD bit (Speed Mode)
in BDRCON register and the value of the SMOD1 bit in PCON register.
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Common Features Description
Figure 2-20. Internal Baud Rate
Periph
Clock
FPER
/6
0
auto reload counter
overflow
BRG
/2
1
0
INT_BRG
1
BRL
SPD
SMOD1
BRR
•
The baud rate for UART is token by formula:
Baud_Rate =
2SMOD1 x FPER
6(1-SPD) x 32 x [256 - (BRL)]
(BRL) = 256 -
2SMOD1 x FPER
6(1-SPD) x 32 x Baud_Rate
Table 2-18. Example of computed value when X2=1, SMOD1=1, SPD=1
Example of computed value when X2=1, SMOD1=1, SPD=1
Baud Rates
FOSCA = 16.384 MHz
FOSCA = 24 MHz
BRL
Error (%)
BRL
Error (%)
115200
247
1.23
243
0.16
57600
238
1.23
230
0.16
38400
229
1.23
217
0.16
28800
220
1.23
204
0.16
19200
203
0.63
178
0.16
9600
149
0.31
100
0.16
4800
43
1.23
-
-
Table 2-19. Example of computed value when X2=0, SMOD1=0, SPD=0
Example of computed value when X2=0, SMOD1=0, SPD=0
Baud Rates
FOSCA = 16.384 MHz
FOSCA = 24 MHz
BRL
Error (%)
BRL
Error (%)
4800
247
1.23
243
0.16
2400
238
1.23
230
0.16
1200
220
1.23
204
0.16
600
185
0.16
152
0.16
The baud rate generator can be used for mode 1 or 3 (refer to Figure 2-22 on page 50),
but also for mode 0 for UART, thanks to the bit SRC located in BDRCON register (Table
2-29.)
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Common Features Description
2.13.4
Using Timer 1 to
Generate Baud
Rates
When Timer 1 is used as the baud rate generator, the baud rates in Modes 1 and 3 are
determined by the Timer 1 overflow rate and the value of SMOD as follows:
The Timer 1 interrupt should be disabled in this application. The Timer itself can be configured for either “timer” or “counter” operation, and in any of its 3 running modes. In the
most typical applications, it is configured for “timer” operation, in the auto-reload mode
(high nibble of TMOD = 0010B). In that case, the baud rate is given by the formula
One can achieve very low baud rates with Timer 1 by leaving the Timer 1 interrupt
enabled, and configuring the Timer to run as a 16-bit timer (high nibble of TMOD =
0001B), and using the Timer 1 interrupt to do a 16-bit software reload.
Figure 2-21 lists various commonly used baud rates and how they can be obtained from
Timer 1.
Figure 2-21. Timer 1 Generated Commonly Used Baud Rates
Fosc (MHz)
11.0592
12
14.7456
16
20
SMOD
150
40h
30h
00h
300
A0h
98h
80h
75h
52h
0
600
D0h
CCh
C0h
BBh
A9h
0
1200
E8h
E6h
E0h
DEh
D5h
0
2400
F4h
F3h
F0h
EFh
EAh
0
F3h
EFh
EFh
Baudrate
4800
4800
FAh
F8h
9600
FDh
FCh
9600
19200
2.13.5
Using Timer 2 to
Generate Baud
Rates
0
1
F5h
0
F5h
FDh
FCh
38400
FEh
76800
FFh
0
1
1
Timer 2 is selected as the baud rate generator by setting TCLK and/or RCLK in T2CON
(Table 2-11). Note then the baud rates for transmit and receive can be simultaneously
different. Setting RCLK and/or TCLK puts Timer 2 into its baud rate generator mode, as
shown in Figure 2-22.
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Common Features Description
Figure 2-22. Timer 2 in Baud Rate Generator Mode.
Periph
Clock
1
The baud rate generator mode is similar to the auto-reload mode, in that a rollover in
TH2 causes the Timer 2 registers to be reloaded with the 16-bit value in registers
RCAP2H and RCAP2L, which are preset by software.
Now, the baud rates in Modes 1 and 3 are determined by Timer 2’s overflow rate as
follows:
The Timer can be configured for either “timer” or “counter” operation. In the most typical
applications, it is configured for “timer” operation (C/T2 = 0). “Timer” operation is a little different for Timer 2 when it’s being used as a baud rate generator. Separately as a timer it would
increment every machine cycle (thus at 1/12 the oscillator frequency). As a baud rate generator,
however, it increment every state time (thus at 1/2 the oscillator frequency).
Timer 2 as a baud rate generator is shown in Figure 2-22. This Figure is valid only if
RCLK + TCLK = 1 in T2CON. Note that a rollover in TH2 does not set TF2, and will not
generate an interrupt. Therefore, the Timer 2 interrupt does not have to be disabled
when Timer 2 is in the baud rate generator mode. Note too, that if EXEN2 is set, a 1-to0 transition in T2EX will set EXF2 but will not cause a reload from (RCAP2H, RCAP2L)
to (TH2, TL2). Thus when Timer 2 is in use as a baud rate generator, T2EX can be used
as an extra external interrupt, if desired.
It should be noted that when Timer 2 is running (TR2 = 1) in “Timer” function in the baud
rate generator mode, one should not try to read or write TH2 or TL2. Under these conditions the Timer is being incremented every state time, and the results of a read or write
may not be accurate. The RCAP registers may be read, but shouldn’t be written to,
because a write might overlap a reload and cause write and/or reload errors. In this
case, turn the Timer off (clear TR2) before accessing the Timer 2 or RCAP registers.
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Common Features Description
Figure 2-23. Timer 2 Generated Commonly Used Baud Rates
Fosc (MHz)
6
11.0592
12
16
Baudrate (RCAP2H RACP2L)
110
F9-57
EE-3F
300
FD-8F
FB-80
FB-1E
F9-7D
600
FE-C8
FD-C0
FD-8F
FC-BF
1200
FF-64
FE-E0
FE-C8
FE-5F
2400
FF-B2
FF-70
FF-64
FF-30
4800
FF-D9
FF-B8
FF-B2
FF-98
9600
FF-DC
FF-D9
FF-CC
19200
FF-EE
FF-E6
38400
FF-F7
FF-F3
56800
FF-FA
XX-XX are values of RCAP2H-RCAP2L
2.13.6
More About Mode 0
Serial data enters and exits through RXD. TXD outputs the shift clock. 8 bits are transmitted/received: 8 data bits (LSB first). The baud rate is fixed at 1/12 the oscillator
frequency.
Figure 2-24 shows a simplified functional diagram of the serial port in mode 0, and associated timing.
Transmission is initiated by any instruction that uses SBUF as a destination register.
The “write to SBUF” signal at S6P2 also loads a 1 into the 9th bit position of the transmit
shift register and tells the TX Control block to commence a transmission. The internal
timing is such that one full machine cycle will elapse between “write to SBUF”, and activation of SEND.
SEND enables the output of the shift register to the alternate output function line of P3.0,
and also enables SHIFT CLOCK to the alternate output function line of P3.1. SHIFT
CLOCK is low during S3, S4, and S5 of every machine cycle, and high during S6, S1
and S2. At S6P2 of every machine cycle in which SEND is active, the contents of the
transmit shift register are shifted to the right one position.
As data bits shift out to the right, zeros come in from the left. When the MSB of the data
byte is at the output position of the shift register, then the 1 that was initially loaded into
the 9th position, is just to the left of the MSB, and all positions to the left of that contain
zeros. This condition flags the TX Control block to do one last shift and then deactivate
SEND and set T1. Both of these actions occur at S1P1 of the 10th machine cycle after
“write to SBUF.”
Reception is initiated by the condition REN = 1 and RI = 0. At S6P2 of the next machine
cycle, the RX Control unit writes the bits 11111110 to the receive shift register, and in
the next clock phase activates RECEIVE.
RECEIVE enables SHIFT CLOCK to the alternate output function line of P3.1. Shift
CLOCK makes transitions at S3P1 and S6P1 of every machine cycle. At S6P2 of every
cycle in which RECEIVE is active, the contents of the receive shift register are shifted to
the left one position. The value that comes in from the right is the value that was sampled at the P3.0 pin at S5P2 of the same machine cycle.
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As data bits come in from the right, 1’s shift out to the left. When the 0 that was initially
loaded into the rightmost position arrives at the leftmost position in the shift and load
SBUF. At S1P1 of the 10th machine cycle after the write to SCON that cleared RI,
RECEIVE is cleared and RI is set.
2.13.7
More About Mode 1
Ten bits are transmitted (through TXD), or received (through RXD): a start bit (0), 8 data
bits (LSB first), and a stop bit (1). On receive, the stop bit goes into RB8 in SCON. In the
80C51 the baud rate is determined by the Timer 1 overflow rate. In the microcontroller
having Timer 2 feature, it is determined either by the Timer 1 overflow rate, or the Timer
2 overflow rate, or both (one for transmit and the other for receive).
Figure 2-25 shows a simplified functional diagram of the serial port in Mode 1, and associated timings for transmit and receive.
Transmission is initiated by any instruction that uses SBUF as a destination register.
The “write to SBUF” signal also loads a 1 into the 9th bit position of the transmit shift
register and flags the TX Control unit that a transmission is requested. Transmission
actually commences at S1P1 of the machine cycle following the next rollover in the
divide-by-16 counter. (Thus, the bit times are synchronized to the divide-by-16 counter,
not to the “write to SBUF” signal).
The transmission begins with activation of SEND, which puts the start bit at TXD. One bit
time later, DATA is activated, which enables the output bit of the transmit shift register to TXD.
The first shift pulse occurs one bit time after that.
As data bits shift out to the right, zeros are clocked in from the left. When the MSB of the
data byte is at the output position of the shift register, then the 1 that was initially loaded
into the 9th position is just to the left of the MSB, and all positions to the left of that contain zeroes. This condition flags the TX Control unit to do one last shift and then
deactivate SEND and set TI. This occurs at the 10th divide-by-16 rollover after “write to SBUF”.
Reception is initiated by a detected 1-to-0 transition at RXD. For this purpose RXD is
sampled at a rate of 16 times whatever baud rate has been established. When a transition is detected, the divide-by-16 counter is immediately reset, and 1FFH is written into
the input shift register. Resetting the divide-by-16 counter aligns its rollovers with the
boundaries of the incoming bit times.
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Common Features Description
Figure 2-24. Serial Port Mode 0
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Common Features Description
2.14
Framing Error
Detection
Framing bit error detection is provided for the three asynchronous modes (modes 1, 2
and 3). To enable the framing bit error detection feature, set SMOD0 bit in PCON register (see Figure 2-25).
Figure 2-25. Framing Error Block Diagram
SM0/FE
SM1
SM2
REN
TB8
RB8
TI
RI
SCON (98h)
Set FE bit if stop bit is 0 (framing error) (SMOD0 = 1)
SM0 to UART mode control (SMOD0 = 0)
SMOD SMOD0
-
POF
GF1
GF0
PD
PCON (87h)
IDL
To UART framing error control
When this feature is enabled, the receiver checks each incoming data frame for a valid
stop bit. An invalid stop bit may result from noise on the serial lines or from simultaneous
transmission by two CPUs. If a valid stop bit is not found, the Framing Error bit (FE) in
SCON register (see Table 2-17) bit is set.
Software may examine FE bit after each reception to check for data errors. Once set,
only software or a reset can clear FE bit. Subsequently received frames with valid stop
bits cannot clear FE bit. When FE feature is enabled, RI rises on stop bit instead of the
last data bit (see Figure 2-26 and Figure 2-27).
Figure 2-26. UART Timings in Mode 1
RXD
D0
D1
D2
D3
D4
D5
D6
D7
Data byte
Start
bit
Stop
bit
RI
SMOD0=X
FE
SMOD0=1
Figure 2-27. UART Timings in Modes 2 and 3
RXD
D0
Start
bit
D1
D2
D3
D4
Data byte
D5
D6
D7
D8
Ninth Stop
bit
bit
RI
SMOD0=0
RI
SMOD0=1
FE
SMOD0=1
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Common Features Description
2.15
Automatic Address Recognition
2.15.1
Multiprocessor
Communications
Implemented in hardware, automatic address recognition enhances the multiprocessor
communication feature by allowing the serial port to examine the address of each
incoming command frame. Only when the serial port recognizes its own address, the
receiver sets RI bit in SCON register to generate an interrupt. This ensures that the CPU
is not interrupted by command frames addressed to other devices.
To support automatic address recognition, a device is identified by a given address and
a broadcast address.
Note:
The multiprocessor communication and automatic address recognition features cannot
be enabled in mode 0 (i.e. setting SM2 bit in SCON register in mode 0 has no effect).
If desired, you may enable the automatic address recognition feature in mode 1. In this
configuration, the stop bit takes the place of the ninth data bit. Bit RI is set only when the
received command frame address matches the device’s address and is terminated by a
valid stop bit.
Modes 2 and 3 have a special provision for multiprocessor, communications. In these
modes, 9 data bits are received. The 9th one goes into RB8. Then comes a stop bit. The
port can be programmed such that when the stop bit is received, the serial port interrupt
will be activated only if RB8 = 1. This feature is enabled by setting bit SM2 in SCON. A
way to use this feature in multiprocessor systems is as follows.
When the master processor wants to transmit a block of data to one of several slaves, it
first sends out an address byte which identifies the target slave. An address byte differs
from a data byte in that the 9th bit is 1 in an address byte and 0 in a data byte. With SM2
= 1, no slave will be interrupt by a data byte. An address byte, however, will interrupt all
slaves, so that each slave can examine the received byte and see if it is being
addressed. The addressed slave will clear its SM2 bit and prepare to receive the data
bytes that will be coming. The slaves that weren’t being addressed leaved their SM2s
set and go on about their business, ignoring the coming data bytes.
SM2 has no effect in Mode 0, and in Mode 1 can be used to check the validity of the
stop bit. In a Mode 1 reception, if SM2 = 1, the receive interrupt will not be activated
unless a valid stop bit is received.
2.15.2
Given Address
Each device has an individual address that is specified in SADDR register; the SADEN
register is a mask byte that contains don’t-care bits (defined by zeros) to form the
device’s given address. The don’t-care bits provide the flexibility to address one or more
slaves at a time. The following example illustrates how a given address is formed.
To address a device by its individual address, the SADEN mask byte must be 1111
1111b.
For example:
SADDR0101 0110b
SADEN1111 1100b
Given0101 01XXb
The following is an example of how to use given addresses to address different slaves:
Slave A:SADDR1111 0001b
SADEN1111 1010b
Given1111 0X0Xb
Slave B:SADDR1111 0011b
SADEN1111 1001b
Given1111 0XX1b
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Common Features Description
Slave C:SADDR1111 0011b
SADEN1111 1101b
Given1111 00X1b
The SADEN byte is selected so that each slave may be addressed separately.
For slave A, bit 0 (the LSB) is a don’t-care bit; for slaves B and C, bit 0 is a 1. To communicate with slave A only, the master must send an address where bit 0 is clear (e.g.
1111 0000b).
For slave A, bit 1 is a 1; for slaves B and C, bit 1 is a don’t care bit. To communicate with
slaves B and C, but not slave A, the master must send an address with bits 0 and 1 both
set (e.g. 1111 0011b).
To communicate with slaves A, B and C, the master must send an address with bit 0 set,
bit 1 clear, and bit 2 clear (e.g. 1111 0001b).
2.15.3
Broadcast Address
A broadcast address is formed from the logical OR of the SADDR and SADEN registers
with zeros defined as don’t-care bits, e.g.:
SADDR 0101 0110b
SADEN 1111 1100b
Broadcast =SADDR OR SADEN1111 111Xb
The use of don’t-care bits provides flexibility in defining the broadcast address, however
in most applications, a broadcast address is FFh. The following is an example of using
broadcast addresses:
Slave A:SADDR1111 0001b
SADEN1111 1010b
Broadcast1111 1X11b,
Slave B:SADDR1111 0011b
SADEN1111 1001b
Broadcast1111 1X11B,
Slave C:SADDR=1111 0010b
SADEN1111 1101b
Broadcast1111 1111b
For slaves A and B, bit 2 is a don’t care bit; for slave C, bit 2 is set. To communicate with
all of the slaves, the master must send an address FFh. To communicate with slaves A
and B, but not slave C, the master can send and address FBh.
2.15.4
Reset Addresses
On reset, the SADDR and SADEN registers are initialized to 00h, i.e. the given and
broadcast addresses are XXXX XXXXb (all don’t-care bits). This ensures that the serial
port will reply to any address, and thus, that it is backwards compatible with the 80C51
microcontrollers that do not support automatic address recognition.
Table 2-20. SADEN Register
SADEN - Slave Address Mask Register (B9h)
7
6
5
4
3
2
1
0
Reset Value = 0000 0000b
Not bit addressable
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Common Features Description
Table 2-21. SADDR Register
SADDR - Slave Address Register (A9h)
7
6
5
4
3
2
1
0
Reset Value = 0000 0000b
Not bit addressable
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Common Features Description
UART Registers
Table 2-22. SCON Register
SCON - Serial Control Register (98h)
7
6
5
4
3
2
1
0
FE/SM0
SM1
SM2
REN
TB8
RB8
TI
RI
Bit
Bit
Number
Mnemonic
Description
Framing Error bit (SMOD0=1)
FE
Clear to reset the error state, not cleared by a valid stop bit.
Set by hardware when an invalid stop bit is detected.
SMOD0 must be set to enable access to the FE bit
7
SM0
Serial port Mode bit 0
Refer to SM1 for serial port mode selection.
SMOD0 must be cleared to enable access to the SM0 bit
Serial port Mode bit 1
6
SM1
SM0
SM1
Mode
Description
0
0
0
Shift Register FCPU PERIPH/6
Baud Rate
0
1
1
8-bit UART
1
0
2
9-bit UART
FCPU PERIPH /32 or /16
1
1
3
9-bit UART
Variable
Variable
Serial port Mode 2 bit / Multiprocessor Communication Enable bit
5
SM2
4
REN
3
TB8
Clear to disable multiprocessor communication feature.
Set to enable multiprocessor communication feature in mode 2 and 3, and eventually mode 1. This bit should be
cleared in mode 0.
Reception Enable bit
Clear to disable serial reception.
Set to enable serial reception.
Transmitter Bit 8 / Ninth bit to transmit in modes 2 and 3.
2
RB8
o transmit a logic 0 in the 9th bit.
Set to transmit a logic 1 in the 9th bit.
Receiver Bit 8 / Ninth bit received in modes 2 and 3
Cleared by hardware if 9th bit received is a logic 0.
Set by hardware if 9th bit received is a logic 1.
In mode 1, if SM2 = 0, RB8 is the received stop bit. In mode 0 RB8 is not used.
1
TI
Transmit Interrupt flag
Clear to acknowledge interrupt.
Set by hardware at the end of the 8th bit time in mode 0 or at the beginning of the stop bit in the other modes.
0
RI
Receive Interrupt flag
Clear to acknowledge interrupt.
Set by hardware at the end of the 8th bit time in mode 0, see Figure 2-26. and Figure 2-27. in the other modes.
Reset Value = 0000 0000b
Bit addressable
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Common Features Description
Table 2-23. SADEN Register
SADEN - Slave Address Mask Register for UART (B9h)
7
6
5
4
3
2
1
0
3
2
1
0
3
2
1
0
Reset Value = 0000 0000b
Table 2-24. SADDR Register
SADDR - Slave Address Register for UART (A9h)
7
6
5
4
Reset Value = 0000 0000b
Table 2-25. SBUF Register
SBUF - Serial Buffer Register for UART (99h)
7
6
5
4
Reset Value = XXXX XXXXb
Table 2-26. BRL Register
BRL - Baud Rate Reload Register for the internal baud rate generator, UART (9Ah)
7
6
5
4
3
2
1
0
Reset Value = 0000 0000b
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Common Features Description
Table 2-27. T2CON Register
T2CON - Timer 2 Control Register (C8h)
7
6
5
4
3
2
1
0
TF2
EXF2
RCLK
TCLK
EXEN2
TR2
C/T2#
CP/RL2#
Bit
Bit
Number
Mnemonic
7
TF2
Description
Timer 2 overflow Flag
Must be cleared by software.
Set by hardware on timer 2 overflow, if RCLK = 0 and TCLK = 0.
6
EXF2
Timer 2 External Flag
Set when a capture or a reload is caused by a negative transition on T2EX pin if EXEN2=1.
When set, causes the CPU to vector to timer 2 interrupt routine when timer 2 interrupt is enabled.
Must be cleared by software. EXF2 doesn’t cause an interrupt in Up/down counter mode (DCEN = 1)
5
RCLK
Receive Clock bit for UART
Cleared to use timer 1 overflow as receive clock for serial port in mode 1 or 3.
Set to use timer 2 overflow as receive clock for serial port in mode 1 or 3.
4
TCLK
Transmit Clock bit for UART
Cleared to use timer 1 overflow as transmit clock for serial port in mode 1 or 3.
Set to use timer 2 overflow as transmit clock for serial port in mode 1 or 3.
3
EXEN2
2
TR2
1
C/T2#
0
CP/RL2#
Timer 2 External Enable bit
Cleared to ignore events on T2EX pin for timer 2 operation.
Set to cause a capture or reload when a negative transition on T2EX pin is detected, if timer 2 is not used to clock the
serial port.
Timer 2 Run control bit
Cleared to turn off timer 2.
Set to turn on timer 2.
Timer/Counter 2 select bit
Cleared for timer operation (input from internal clock system: FCLK PERIPH).
Set for counter operation (input from T2 input pin, falling edge trigger). Must be 0 for clock out mode.
Timer 2 Capture/Reload bit
If RCLK=1 or TCLK=1, CP/RL2# is ignored and timer is forced to auto-reload on timer 2 overflow.
Cleared to auto-reload on timer 2 overflows or negative transitions on T2EX pin if EXEN2=1.
Set to capture on negative transitions on T2EX pin if EXEN2=1.
Reset Value = 0000 0000b
Bit addressable
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Common Features Description
Table 2-28. PCON Register
PCON - Power Control Register (87h)
7
6
5
4
3
2
1
0
SMOD1
SMOD0
-
POF
GF1
GF0
PD
IDL
Bit
Bit
Number
Mnemonic
7
SMOD1
6
SMOD0
5
-
4
POF
Power-Off Flag
Cleared to recognize next reset type.
Set by hardware when VCC rises from 0 to its nominal voltage. Can also be set by software.
3
GF1
General purpose Flag
Cleared by user for general purpose usage.
Set by user for general purpose usage.
2
GF0
General purpose Flag
Cleared by user for general purpose usage.
Set by user for general purpose usage.
1
PD
Power-Down mode bit
Cleared by hardware when reset occurs.
Set to enter power-down mode.
0
IDL
Idle mode bit
Cleared by hardware when interrupt or reset occurs.
Set to enter idle mode.
Description
Serial port Mode bit 1 for UART
Set to select double baud rate in mode 1, 2 or 3.
Serial port Mode bit 0 for UART
Cleared to select SM0 bit in SCON register.
Set to select FE bit in SCON register.
Reserved
The value read from this bit is indeterminate. Do not set this bit.
Reset Value = 00X1 0000b
Not bit addressable
Power-off flag reset value will be 1 only after a power on (cold reset). A warm reset
doesn’t affect the value of this bit.
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Common Features Description
Table 2-29. BDRCON Register
BDRCON - Baud Rate Control Register (9Bh)
7
6
5
4
3
2
1
0
-
-
-
BRR
TBCK
RBCK
SPD
SRC
Bit
Number
Bit
Mnemonic
7
-
Reserved
The value read from this bit is indeterminate. Do not set this bit
6
-
Reserved
The value read from this bit is indeterminate. Do not set this bit
5
-
Reserved
The value read from this bit is indeterminate. Do not set this bit.
4
BRR
Baud Rate Run Control bit
Cleared to stop the internal Baud Rate Generator.
Set to start the internal Baud Rate Generator.
3
TBCK
Transmission Baud rate Generator Selection bit for UART
Cleared to select Timer 1 or Timer 2 for the Baud Rate Generator.
Set to select internal Baud Rate Generator.
2
RBCK
Reception Baud Rate Generator Selection bit for UART
Cleared to select Timer 1 or Timer 2 for the Baud Rate Generator.
Set to select internal Baud Rate Generator.
1
SPD
0
SRC
Description
Baud Rate Speed Control bit for UART
Cleared to select the SLOW Baud Rate Generator.
Set to select the FAST Baud Rate Generator.
Baud Rate Source select bit in Mode 0 for UART
Cleared to select FOSC/12 as the Baud Rate Generator (FCLK PERIPH/6 in X2 mode).
Set to select the internal Baud Rate Generator for UARTs in mode 0.
Reset Value = XXX0 0000b
Not bit addressable
2.16
Interrupts
If two requests of different priority Ievels are received simultaneously, the request of
higher priority level is serviced. If requests of the same priority level are received
simultaneously, an internal polling sequence determine which request is serviced, Thus
within each priority level is a second priority structure determined by the polling
sequence, as follows:
Table 2-30. Interrupt Priority Level
Source
Priority Within Level
1
IE0
(highest)
2
TF0
3
IE1
4
TF1
5
RI + TI
6
TF2 + EXF2
(lowest)
Note that the "priority within level" structure is only used to resolve simultaneous
requests of the same priority level.
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Common Features Description
2.16.1
How Interrupts Are
Handled
The interrupt flags are sampled at SsP2 of every machine cycle. The samples are
polled during the following machine cycle. If one of the flags was in a set condition at
S5P2 of the preceding cycle, the polling cycle will find it and the interrupt system will
generate an LCALL to the appropriate service routine, provided this hardwaregenerated LCALL is not clocked by any of the following conditions:
1. An interrupt of equal or higher priority level is already in progress.
2. The current (polling) cycle is not the final cycle in the execution of the instruction
in progress.
3. The instruction in progress is RETI or any access to the IE or IP registers.
The polling cycle is repeated with each machine cycle, and the values polled are the
values that were present at S5P2 of the previous machine cycle. Note then that if an
interrupt flag is active but not being responded to for one of the above conditions, if the
flag is not still active when the blocking condition is removed, the denied interrupt will
not be serviced. In other words, the facts that the interrupt flag was once active but not
serviced is not remembered. Every polling cycle is new.
The polling cycle/LCALL sequence is illustrated in Figure 2-28.
Note that if an interrupt of higher priority level goes active prior to S5P2 of the machine
cycle labeled C3 in Figure 2-28, then in accordance with the above rules it will be
vectored to during CS and C6, without any instruction of the lower priority routine
having been executed.
Figure 2-28. Interrupt Response Timing Diagram
Thus the processor acknowledges an interrupt request by executing a hardware
generated LCALL to the appropriate servicing routine.
In some cases it also clears the flag that generated the interrupt, and in other cases it
doesn’t. It never clears the Serial Port or Timers 2 flags. This has to be done in the
user’s software. It clears an external interrupt flag (IEO or IE1) only if it was transitionactivated.
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The hardware-generated LCALL pushes the contents of the Program Counter onto the
stack (but it does not save the PSW) and reloads the PC with an address that depends
on the source of the interrupt being vectored to, as shown below.
Source
Vector Address
IE0
0003H
TF0
000BH
IE1
0013H
TF1
001BH
RI + TI
0023H
TF2 + EXF2
002BH
Execution proceeds from that location until the RETI instruction is encountered. The
RETI instruction informs the processor that this interrupt routine is no longer in
progress, then pops the top two bytes from the stack and reloads the Program Counter.
Execution of the interrupted program continues from where it left off.
Note that a simple RET instruction would also have returned execution to the
interrupted program, but it would have left the interrupt control system thinking an
interrupt was still in progress.
2.16.2
External Interrupts
The external sources can be programmed to be level-activated or transition-activated
by setting or clearing bit IT1 or ITO in Register TCON.
If ITx = O, external interrupt x is triggered by a detected low at the INTx pin. If ITx = 1,
external interrupt x is edge-triggered. In this mode if successive samples of the INTx
pin show a high in one cycle and a low in the next cycle, interrupt request flag IEx in
TCON is set. Flag bit IEx then requests the interrupt.
Since the external interrupt pins are sampled once each machine cycle, an input high
or low should hold for at least 12 oscillator periods to ensure sampling. If the external
interrupt is transition-activated, the external source has to hold the request pin high for
at least one cycle, and then hold it low for at least one cycle to ensure that the
transition is seen so that interrupt request flag IEx will be set. IEx will be automatically
cleared by the CPU when the service routine is called.
If the external interrupt is level-activated, the external source has to hold the request
active until the requested interrupt is actually generated. Then it has to deactivate the
request before the interrupt service routine is completed, or el se another interrupt will
be generated.
2.16.3
Response Time
The INTO and INT1 levels are inverted and latched into IEO and IE1 and S5P2 of
every machine cycle. The values are not actualIy polIed by the circuitry until the next
machine cycle. If a request is active and conditions are right for it to be acknowledged,
a hardware subroutine calI to the requested service routine will be the next instruction
to be executed. The calI itself takes two cycles. Thus, a minimum of three complete
machine cycles elapse between activation of an external interrupt request and the
beginning of execution of the service routine. Figure 29. shows interrupt response
timings.
A longer response time would result if the request is blocked by one of the 3 previously
listed conditions. If an interrupt of equal or higher priority level is already in progress,
the additional wait time obviously depends on the nature of the other interrupt’s service
routine. If the instruction in progress is not in its final cycle, the additional wait time
cannot be more than 3 cycles, since the longest instructions (MUL and DIV) are only 4
cycles long, and if the instruction in progress is RETI or an access to IE or IP, the
additional wait time cannot be more than 5 cycles (a maximum of one more cycle to
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Common Features Description
complete the instruction in progress, plus 4 cycles to complete the next instruction if the
instruction is MUL or DIV).
Thus, in a single-interrupt system, the response time is always more than 3 cycles and
less than 8 cycles.
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