AT89_Series_HW
Introduction
This chapter presents a comprehensive description of the on-chip hardware features
of Atmel’s Flash-based microcontrollers. Included in this description are the following
items.
• 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 and Low Power Idle
The devices under consideration are listed in Table 1.
Figure 1 shows a functional block diagram of the AT89 Series microcontrollers.
Table 1. Atmel’s Flash Microcontrollers
Device Name
Program
Memory
Data Memory
Bytes
16-bit Timers
Technology
AT89C1051
1K Flash
64 RAM
1
CMOS
AT89C2051
2K Flash
128 RAM
2
CMOS
AT89C51
4K Flash
128 RAM
2
CMOS
AT89C52
8K Flash
256 RAM
3
CMOS
AT89C55
20K Flash
256 RAM
3
CMOS
AT89S8252
8K Flash
256 RAM
2K EEPROM
3
CMOS
AT89S53
12K Flash
256 RAM
3
CMOS
AT89 Series
Hardware
Description
Hardware
Description
Special Function Registers
A map of the on-chip memory area called Special Function Register (SFR) space is
shown in Figure 2. SFRs marked by parentheses are resident in the AT89C52 but not
in the AT89C51.
0499B-B–12/97
2-37
Figure 1. AT89 Series Flash-Based Microcontroller Architectural Block Diagram
2-38
Hardware Description
Hardware Description
Figure 2. SFR Map. (...) Indicates Resident in AT89C52, not in AT89C51.
8 Bytes
F8
F0
FF
B
F7
E8
E0
EF
ACC
E7
D8
DF
D0
PSW
C8
(T2CON)
D7
(T2MOD)
(RCAP2L)
(RCAP2H)
(TL2)
(TH2)
CF
C0
C7
B8
IP
BF
B0
P3
B7
A8
IE
AF
A0
P2
A7
98
SCON
90
P1
88
TCON
TMOD
TL0
TL1
80
P0
SP
DPL
DPH
SBUF
9F
97
TH0
TH1
8F
PCON
87
Not all of the addresses are occupied. Unoccupied
addresses are not implemented on the chip. Read
accesses to these addresses in general return random
data, and write accesses have no effect.
User software should not write 1s to these unimplemented
locations, since they may be used in future microcontrollers
to invoke new features. In that case, the reset or inactive
values of the new bits will always be 0, and their active values will be 1.
The functions of the SFRs are outlined in the following sections.
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.
Accumulator
ACC is the Accumulator register. The mnemonics for Accumulator-specific instructions, however, refer to the Accumulator simply as A.
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 initiates the transmission.) When data is moved from SBUF, it
comes from the receive buffer.
B Register
The B register is used during multiply and divide operations. For other instructions it can be treated as another
scratch pad register.
Data Pointer
The Data Pointer (DPTR) consists of a high byte (DPH) and
a low byte (DPL). Its function is to hold a 16-bit address. It
may be manipulated as a 16-bit register or as two independent 8-bit registers.
Ports 0 To 3
P0, P1, P2, and P3 are the SFR latches of Ports 0, 1, 2,
and 3, respectively.
Program Status Word
The PSW register contains program status information, as
detailed in Figure 3.
Timer Registers
Register pairs (TH0, TL0), (TH1, TL1), and (TH2, TL2) are
the 16-bit Counter registers for Timer/Counters 0, 1, and 2,
respectively.
Stack Pointer
The Stack Pointer Register is 8 bits wide. It is incremented
before data is stored during PUSH and CALL executions.
Capture Registers
The register pair (RCAP2H, RCAP2L) are the Capture registers for the Timer 2 Capture Mode. In this mode, in
2-39
response to a transition at the AT89C52’s T2EX pin, TH2
and TL2 are copied into RCAP2H and RCAP2L. Timer 2
also has a l6-bit auto-reload mode, and RCAP2H and
RCAP2L hold the reload value for this mode.
Control Registers
Special Function Registers IP, IE, TMOD, TCON, T2CON,
T2MOD, 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 of this chapter.
Figure 3. PSW: Program Status Word Register
(MSB)
CY
Symbol
(LSB)
AC
F0
Position
Name and Significance
CY
PSW.7
Carry flag.
AC
PSW.6
F0
RS1
RS0
Symbol
OV
—
Position
Name and Significance
OV
PSW.2
Overflow flag.
Auxillary Carry flag. (For BCD operations.)
—
PSW.1
User definable flag.
PSW.5
Flag 0. (Available to the user for general
purposes.)
P
PSW.0
RS1
PSW.4
Register bank select control bits 1 and 0.
Parity flag. Set/cleared by hardware each
instruction cycle to indicate an odd/even
number of 1 bits in the Accumulator, that
is, even parity.
RS0
PSW.3
Set/cleared by software to determine
working register bank.(1)
Note:
1. 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)
Figure 4. AT89C51 and AT89C52 Port Bit Latches and I/O Buffers
*See Figure 5 for details of the internal pullup.
2-40
P
Hardware Description
Hardware Description
Port Structures and Operation
All four ports in the AT89C51 and AT89C52 are bidirectional. Each consists of a latch (Special Function Registers
P0 through P3), an output driver, and an input buffer.
The output drivers of Ports 0 and 2, and the 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 two Port 1 pins (in the AT89C52)
are multifunctional. They are not only port pins, but also
provide the special features listed in the following table.
Port Pin
Alternate Function
P1.0(1)
T2 (Timer/Counter 2 external input)
P1.1(1)
T2EX (Timer/Counter 2 Capture/Reload
trigger)
Note:
P3.0
RXD (serial input port)
P3.1
TXD (serial output port)
P3.2
INT0 (external interrupt)
P3.3
INT1 (external interrupt)
P3.4
T0 (Timer/Counter 0 external input)
P3.5
T1 (Timer/Counter 1 external input)
P3.6
WR (external data memory write strobe)
P3.7
RD (external data memory read strobe)
1. P1.0 and P1.1 serve these alternate functions only
on the AT89C52.
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.
I/O Configurations
Figure 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 clocks 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 pin”
signal.
As shown in Figure 4, the output drivers of Ports 0 and 2
can be switched 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 1s are written to the P0 SFR.
If a P3 bit latch contains a 1, then the output level is controlled by the alternate output function signal, as shown in
Figure 4. The actual P3.X pin level is always available to
the pin’s alternate input function.
Ports 1, 2, and 3 have internal pullups. Port 0 has open
drain outputs. Each I/O line can be used independently 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 pullup but can be pulled low by an external source.
Port 0 has no internal pullups. The FET pullup in the P0
output driver (see Figure 4) is used only when the Port
emits 1s during external memory accesses. Otherwise, the
FET pullup is off. Consequently, P0 lines that are used as
output port lines are open drain. Writing a 1 to the bit latch
leaves both FET outputs off, so the pin floats. In this condition, it can be used as a high-impedance input.
Because Ports 1, 2, and 3 have fixed internal pullups, they
are sometimes called quasi-bidirectional ports. When configured as inputs, they pull high and source current (I IL )
when externally pulled low. Port 0, on the other hand, is
considered truly bidirectional, because it floats when configured as an input.
The reset function writes 1s to all the port latches in the
AT89C51and AT89C52. If a 0 is subsequently written to a
port latch, the latch can be reconfigured as an input if a 1 is
written to it.
Writing to a Port
When an instruction changes a port latch value, the new
value arrives at the latch during S6P2 of the final cycle of
the instruction. However, port latches are sampled by their
output buffers only during Phase 1 of any clock period.
(During Phase 2, the output buffer holds the value sampled
during the previous Phase 1). Consequently, the new value
in the port latch does not actually appear at the output pin
until the next Phase 1, which is at S1P1 of the next
machine cycle. See Figure 39 in the Internal Timing section.
If the change requires a 0-to-l transition in Port 1, 2, or 3, an
additional pullup is turned on during S1P1 and S1P2 of the
cycle in which the transition occurs to increase the transition speed. The extra pullup can source about 100 times
the current that the normal pullup can. The internal pullups
are field-effect transistors, not linear resistors. The pullup
arrangements are shown in Figure 5.
2-41
Figure 5. Ports 1 and 3 Internal Pullup Configurations. Port 2 is similar except that it holds the strong pullup on while emitting 1s that are address bits. (See text, “Accessing External Memory”.)
Note:
pFET1 is turned on for 2 osc. periods after Q makes a 0-to-1 transition. During this time, pFET1 also turns on pFET3 through
the inverter to form a latch which holds the 1. pFET2 is also on.
The pullup consists of three pFETs. An n-channel FET
(nFET) turns on when a logical 1 is applied to its gate, and
turns 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.
The pFET1 transistor in Figure 5 is turned on for 2 oscillator
periods after a 0-to-l transition in the port latch. While
pFET1 is on, it turns on pFET3 (a weak pullup) through the
inverter. This inverter and pFET3 form a latch that holds the
1.
If the pin emits a 1, a negative glitch on the pin from some
external source can turn off pFET3, causing the pin to go
into a float state. pFET2 is a very weak pullup which is on
whenever the nFET is off, in traditional CMOS style. pFET2
is only about 1/10 the strength of pFET3. Its function is to
restore a 1 to the pin in the event the pin lost a 1 in a glitch.
modify-write instructions given in the following table read
the latch rather than the pin.
Mnemonic
Instruction
Example
ANL
Logical AND
ANL P1, A
ORL
Logical OR
ORL P2, A
XRL
Logical EX-OR
XRL P3, A
JBC
Jump if bit = 1 and
clear bit
JBC P1.1, LABEL
CPL
Complement bit, CPL
P3.0
INC
Increment
INC P2
DEC
Decrement
DEC P2
DJNZ
Decrement and jump if
not zero
DJNZ P3, LABEL
MOV, PX.Y, C
Move carry bit to bit Y
of Port X
CLR PX.Y
Clear bit Y of Port X
SETB PX.Y
Set bit Y of Port X
Port Loading and Interfacing
The output buffers of Ports 1, 2, and 3 can each drive 4 LS
TTL inputs. CMOS pins can be driven by open-collector
and open-drain outputs, but 0-to-l transitions will not be
fast. An input 0 turns off pullup pFET3, leaving only the
very weak pullup pFET2 to drive the transition.
In external bus mode, Port 0 output buffers can drive 8 LS
TTL inputs. As port pins, they require external pullups to
drive any inputs.
Read-Modify-Write Feature
Some instructions that read a port read the latch and others
read the pin. Read-modify-write instructions read the latch
rather than the pin, and these instructions read a value,
possibly change it, and then rewrite it to the latch. When
the destination operand is a port, or a port bit, the read2-42
Hardware Description
The last three instructions in this list are read-modify-write
instructions, because they read all 8 bits of the port byte,
modify the addressed bit, then write the new byte back to
the latch.
Read-modify-write instructions are directed to the latch
rather than the pin in order to avoid misinterpreting 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
Hardware Description
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.
Accessing External Memory
Accesses to external memory are either to program memory or to data memory. Accesses to external program
memory use the PSEN (program store enable) signal as
the read strobe. Accesses to external data memory use RD
or WR (alternate functions of P3.7 and P3.6) to strobe the
memory. Refer to Figures 36 through 38 in the Internal Timing section for more information.
Fetches from external program memory always use a 16bit address. Accesses to external data memory can use
either a 16-bit address (MOVX @DPTR) or an 8-bit
address (MOVX @Ri).
Whenever a l6-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 pullups during the entire time that they emit
address bits that are 1s (during the execution of a MOVX
@DPTR instruction.) During this time, the Port 2 latch (the
Special Function Register) does not have to contain 1s,
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 reappear in the next cycle.
If an 8-bit address is used (MOVX @Ri), the contents of the
Port 2 SFR remain at the Port 2 pins throughout the external memory cycle, which facilitates 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. The Address Latche Enable (ALE)
signal should be used to capture the address byte into an
external latch. The address byte is valid at the negative
transition 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 any information in the Port 0 SFR. If the
user writes to Port 0 during an external memory fetch, the
incoming code byte is corrupted. Therefore, do not write to
Port 0 if external program memory is used.
External program memory is accessed under the following
two conditions.
1. When the EA signal is active; or
2. When the program counter (PC) contains a number
larger than 0FFFH (1FFFH for the AT89C52).
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 pullups to
emit PC bits that are 1s.
Timer/Counters
The AT89C51 has two 16-bit Timer/Counter registers:
Timer 0 and Timer 1. The AT89C52 has these two
Timer/Counters, and in addition Timer 2. All three can be
configured to operate either as Timers or event Counters.
As a Timer, the register is incremented every machine
cycle. Thus, the register counts machine cycles. Since a
machine cycle consists of 12 oscillator periods, the count
rate is 1/12 of the oscillator frequency.
As a Counter, the register is incremented in response to a lto-0 transition at its corresponding external input pin, T0,
T1, or (in the AT89C52) T2. The external input is sampled
during S5P2 of every machine cycle. When the samples
show a high in one cycle and a low in the next cycle, the
count is incremented. The new count value appears in the
register during S3P1 of the cycle following the one in which
the transition was detected. Since 2 machine cycles (24
oscillator periods) are required to recognize a l-to-0 transition, the maximum count rate is 1/24 of the oscillator frequency. There are no restrictions on the duty cycle of the
external input signal, but it should be held for at least one
full machine cycle to ensure that a given level is sampled at
least once before it changes.
In addition to the Timer or Counter functions, Timer 0 and
Timer 1 have four operating modes: (13 bit timer, 16 bit
timer, 8 bit auto-reload, split timer). Timer 2 in the AT89C52
has three modes of operation: Capture, Auto-Reload, and
baud rate generator.
Timer 0 and Timer 1
Timer/Counters 1 and 0 are present in both the AT89C51
and AT89C52. The Timer or Counter function is selected
by control bits C/T in the Special Function Register TMOD
(Figure 6). These two Timer/Counters have four operating
modes, which are selected by bit pairs (M1, M0) in TMOD.
Modes 0, 1, and 2 are the same for both Timer/Counters,
but Mode 3 is different. The four modes are described in
the following sections.
Mode 0
Both Timers in Mode 0 are 8-bit Counters with a divide-by32 prescaler. Figure 7 shows the Mode 0 operation as it
applies to Timer 1.
2-43
In this mode, the Timer register is configured as a 13-bit
register. As the count rolls over from all 1s to all 0s, it sets
the Timer interrupt flag TF1. The counted input is enabled
to the Timer when TR1 = 1 and either GATE = 0 or INT1 =
1. Setting GATE=1 allows the Timer to be controlled by
external input INT1, to facilitate pulse width measurements.
TR1 is a control bit in the Special Function Register TCON
(Figure 8). GATE is in TMOD.
The 13-bit register consists of all 8 bits of TH1and the lower
5 bits of TL1. The upper 3 bits of TL1 are indeterminate and
should be ignored. Setting the run flag (TR1) does not clear
the registers.
Mode 0 operation is the same for Timer 0 as for Timer 1,
except that TR0, TF0 and INT0 replace the corresponding
Timer 1 signals in Figure 7. There are two different GATE
bits, one for Timer 1 (TMOD.7) and one for Timer 0
(TMOD.3).
2-44
Hardware Description
Hardware Description
Figure 6. TMOD: Timer/Counter Mode Control Register
(MSB)
(LSB)
GATE
C/T
M1
M0
GATE
C/T
Timer1
GATE
C/T
M1
Timer0
Gating control when set. Timer/Counter x is enabled only
while INTx pin is high and TRx control pin is set. When
cleared, Timer x is enabled whenever TRx control bit is
set.
Timer 0 gate bit
Timer or Counter Selector cleared for Timer operation
(input from internal system clock). Set for Counter
operation (input from Tx input pin).
Timer 0 M1 bit
M1
Mode bit 1
M0
Mode bit 0
M0
Timer 0 counter/timer select bit
Timer 0 M0 bit
M1
M0
Mode
Operating Mode
0
0
0
13-bit Timer Mode.
8-bit Timer/Counter THz with TLx as 5-bit prescaler.
0
1
1
16-bit Timer Mode.
16-bit Timer/Counters THx and TLx are cascaded; there is no prescaler.
1
0
2
8-bit Auto Reload.
8-bit auto-reload Timer/Counter THx holds a value which is to be
reloaded into TLx each time it overflows.
1
1
3
Split Timer Mode.
(Timer 0) TL0 is an 8-bit Timer/Counter controlled by the standard Timer
0 control bits. TH0 is an 8-bit timer only controlled by Timer 1 control bits.
1
1
3
(Timer 1) Timer/Counter 1 stopped.
Timer SFR
Purpose
Address
Bit-Addressable
TCON
Control
88H
Yes
TMOD
Mode
89H
No
TL0
Timer 0 low-byte
8AH
No
TL1
Timer 1 low-byte
8BH
No
TH0
Timer 0 high-byte
8CH
No
TH1
T2CON
Timer 1 high-byte
8DH
No
(1)
Timer 2 control
C8H
Yes
(1)
Timer 2 Mode
C9H
No
Timer 2 low-byte capture
CAH
No
Timer 2 high-byte capture
CBH
No
Timer 2 low-byte
CCH
No
Timer 2 high byte
CDH
No
T2MOD
RCAP2L(1)
RCAP2H
TL2
(1)
(1)
TH2(1)
Note:
1. AT89C52 only.
2-45
Figure 7. Timer/Counter 1 Mode 0: 13-Bit Counter
OSC
÷12
C/T=0
TL1
TH1
(5 BITS) (8 BITS)
TF1
INTERRUPT
C/T=1
CONTROL
T1 PIN
TR1
GATE
INT1 PIN
Figure 8. Timer/Counter 1 Mode 1: 16-Bit Counter
Mode 1
Mode 1 is the same as Mode 0, except that the Timer register is run with all 16 bits. The clock is applied to the combined high and low timer registers (TL1/TH1). As clock
pulses are received, the timer counts up: 0000H, 0001H,
0002H, etc. An overflow occurs on the FFFFH-to-0000H
overflow flag. The timer continues to count. The overflow
flag is the TF1 bit in TCON that is read or written by software. See Figure 8.
Mode 2
Mode 2 configures the Timer register as an 8-bit Counter
(TL1) with automatic reload, as shown in Figure 9. Overflow from TL1 not only sets TF1, but also reloads TL1 with
the contents of TH1, which is preset by software. The
reload leaves TH1 unchanged. Mode 2 operation is the
same for Timer/Counter 0.
Mode 3
Timer 1 in Mode 3 simply holds its count. The effect is the
same as setting TR1 = 0.
Timer 0 in Mode 3 establishes TL0 and TH0 as two separate counters. The logic for Mode 3 on Timer 0 is shown in
Figure 10. TL0 uses the Timer 0 control bits: C/T, GATE,
TR0, INT0, and TF0. TH0 is locked into a timer function
(counting machine cycles) and over the use of TR1 and
TF1 from Timer 1. Thus, TH0 now controls the Timer 1
interrupt.
Mode 3 is for applications requiring an extra 8-bit timer or
counter. With Timer 0 in Mode 3, the AT89C51 can appear
to have three Timer/Counters, and an AT89C52, can
appear to have four. 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. In this case, Timer 1 can still be used by the
serial port as a baud rate generator or in any application
not requiring an interrupt.
Figure 9. Timer/Counter 1 Mode 2: 8-Bit Auto-Reload
OSC
÷ 12
C/T=0
TL1
(8 BITS)
TF1
C/T=1
CONTROL
T1 PIN
RELOAD
TR1
GATE
INT0 PIN
2-46
Hardware Description
TH1
(8 BITS)
INTERRUPT
Hardware Description
Figure 10. Timer/Counter 0 Mode 3: Two 8-Bit Counters
Figure 11. TCON: Timer/Counter Control Register
(MSB)
TF1
(LSB)
TR1
TF0
TR0
IE1
IT1
IE0
IT0
Symbol
Position
Name and Significance
TF1
TCON.7
Timer 1 overflow flag. Set by hardware on Timer/Counter overflow. Cleared by hardware when processor
vectors to interrupt routine.
TR1
TCON.6
Timer 1 run control bit. Set/cleared by software to turn Timer/Counter on/off.
TF0
TCON.5
Timer 0 overflow flag. Set by hardware on Timer/Counter overflow. Cleared by hardware when processor
vectors to interrupt routine.
TR0
TCON.4
Timer 0 run control bit. Set/cleared by software to turn Timer/Counter on/off.
IE1
TCON.3
Interrupt 1 edge flag. Set by hardware when external interrupt edge detected. Cleared when interrupt
processed.
IT1
TCON.2
Interrupt 1 type control bit. Set/cleared by software to specify falling edge/low level triggered external
interrupts.
IE0
TCON.1
Interrupt 0 edge flag. Set by hardware when external interrupt edge detected. Cleared when interrupt
processed.
IT0
TCON.0
Interrupt 0 type control bit. Set/cleared by software to specify falling edge/low level triggered external
interrupts.
Timer 2
Timer 2 is a 16-bit Timer/Counter present only in the
AT89C52. This is a powerful addition to the other two just
discussed. Five extra special function registers are added
to accommodate Timer 2 which are: the timer registers,
TL2 and TH2, the timer control register, T2CON, and the
capture registers, RCAP2L and RCAP2H. Like Timers 0
and 1, it can operate either as a timer or as an event
counter, depending on the value of bit C/T2 in the Special
Function Register T2CON (Figure 12). Timer 2 has three
operating modes: capture, auto-reload, and baud rate generator, which are selected by bits in T2CON, as shown in
Table 2.
Table 2. Timer 2 Operation 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)
2-47
Figure 12. T2CON Timer/Counter 2 Control Register
(MSB)
TF2
(LSB)
EXF2
RCLK
TCLK
EXEN2
TR2
C/T2
CP/RL2
Symbol
Position
Name and Significance
TF2
T2CON.7
Timer 2 overflow flag set by a Timer overflow and must be cleared by software. TF2 will not be set when
either RCLK = 1 or TCLK = 1.
EXF2
T2CON.6
Timer 2 external flag set when either a capture or reload is caused by a negative transition on T2EX and
EXEN2 = 1. When Timer 2 interrupt is enabled, EXF2 = 1 will cause the CPU to vector to the Timer 2
interrupt routine. EXF2 must be cleared by software.
RCLK
T2CON.5
Receive clock flag. When set, causes the serial port to use Timer 2 overflow pulses for its receive clock
in Modes 1, 3 and Timer 1 provides transmit baud rate. RCLK = 0 causes Timer 1 overflow to be used for
the receive clock.
TCLK
T2CON.4
Transmit clock flag. When set, causes the serial port to use Timer 2 overflow pulses for its transmit clock
in Modes 1, 3 and Timer 1 provides transmit baud rate. TCLK = 0 causes Timer 1 overflows to be used
for the transmit clock.
EXEN2
T2CON.3
Timer 2 external enable flag. When set, allows a capture or reload to occur as a result of a negative
transition on T2EX if Timer 2 is not being used to clock the serial port. EXEN2 = 0 causes Timer 2 to
ignore events at T2EX.
TR2
T2CON.2
Start/stop control for Timer 2. A logic 1 starts the timer.
C/T2
T2CON.1
Timer or counter select. (Timer 2)
0 = Internal timer (OSC/12)
1 = External event counter (falling edge triggered).
CP/RL2
T2CON.0
Capture/Reload flag. When set, captures will occur on negative transitions at T2EX if EXEN2 = 1. When
cleared, auto-reloads will occur either with Timer 2 overflows or negative transitions at T2EX when
EXEN2 = 1. When either RCLK = 1 or TCLK = 1, this bit is ignored and the timer is forced to auto-reload
on Timer 2 overflow.
In the Capture Mode, the EXEN2 bit in T2CON selects two
options. If EXEN2 = 0, then Timer 2 is a 16-bit timer or
counter whose overflow sets bit TF2, the Timer 2 overflow
bit, which can be used to generate an interrupt. If EXEN2 =
1, then Timer2 performs the same way, but a l-to-0 transition at external input T2EX also causes the current value in
the Timer 2 registers, TL2 and TH2, to be captured into the
RCAP2L and RCAP2H registers, respectively. (RCAP2L
and RCAP2H are new Special Function Registers in the
AT89C52.) In addition, the transition at T2EX sets the
EXF2 bit in T2CON, and EXF2, like TF2, can generate an
interrupt.
The Capture Mode is illustrated in Figure 13.
In the auto-reload mode, the EXEN2 bit in T2CON also
selects two options. If EXEN2 = 0, then when Timer 2 rolls
over it sets TF2 and also reloads the Timer 2 registers with
the 16-bit value in the RCAP2L and RCAP2H registers,
which are preset by software. If EXEN2 = 1, then Timer 2
performs the same way, but a 1-to-0 transition at external
input T2EX also triggers the 16-bit reload and sets EXF2.
The auto-reload mode is illustrated in Figure 14.
The baud rate generator mode is selected by RCLK = 1
and/or TCLK = 1. This mode is described in conjunction
with the serial port. (Figure 17)
2-48
Hardware Description
Serial Interface
The serial port is full duplex, which means it can transmit
and receive simultaneously. It is also receive-buffered,
which means it can begin receiving a second byte before a
previously received byte has been read from the receive
register. (However, if the first byte still has not been read
when 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 separate receive register.
The serial port can operate in the following four modes.
Mode 0: Serial data enters and exits through RXD. TXD
outputs the shift clock. Eight data bits are transmitted/received, with the LSB first. The baud rate is fixed at
1/12 the oscillator frequency.
Hardware Description
Figure 13. Timer 2 In Capture Mode
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 ninth 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) can
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 ninth data bit, and a stop bit (1). In fact, Mode 3
is the same as Mode 2 in all respects except the baud rate,
which is variable in Mode 3.
In all four modes, transmission is initiated by any instruction
that uses SBUF as a destination register. Reception is initiated in Mode 0 by the condition RI = 0 and REN = 1.
Reception is initiated in the other modes by the incoming
start bit if REN =1.
The following example shows how to use the serial interrupt for multiprocessor communications. When the master
processor must transmit a block of data to one of several
slaves, it first sends out an address byte that 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 is interrupted by a data byte. An
address byte, however, interrupts all slaves, so that each
slave can examine the received byte and see if it is being
addressed. The addressed slave clears its SM2 bit and
prepares to receive the data bytes that follows. The slaves
that are not addressed set their SM2 bits and ignore the
data bytes.
SM2 has no effect in Mode 0 but can be used to check the
validity of the stop bit in Mode 1. In a Mode 1 reception, if
SM2=1, the receive interrupt is not activated unless a valid
stop bit is received.
Serial Port Control Register
The serial port control and status register is the Special
Function Register SCON, shown in Figure 15. This register
contains the mode selection bits, the 9th data bit for transmit and receive (TB8 and RB8), and the serial port interrupt
bits (TI and RI).
Multiprocessor Communications
Modes 2 and 3 have a special provision for multiprocessor
communications. In these modes, 9 data bits are received,
followed by a stop bit. The ninth bit 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 is activated only if RB8 = 1. This feature is enabled by setting bit
SM2 in SCON.
2-49
Figure 14. Timer 2 in Auto-Reload Mode (DCEN = 0)
OSC
÷ 12
C/T2 = 0
TL2
TH2
(8 BITS) (8 BITS)
C/T2 = 1
CONTROL
TR2
RELOAD
T2PIN
RCAP2L RCAP2H
TRANSITION
DETECTOR
TF2
TIMER 2
INTERRUPT
T2EX PIN
EXF2
CONTROL
EXEN2
Figure 15. SCON: Serial Port Control Register
(MSB)
SM0
(LSB)
SM1
SM2
REN
TB8
RB8
TI
RI
Symbol
Position
Name and Significance
SM0
SCON.7
Serial port mode bit 0 (see table below).
SM1
SCON.6
Serial port mode bit 1 (see table below).
SM2
SCON.5
Enables the multiprocessor communication feature in Modes 2 and 3. In Mode 2 or 3, if SM2 is set to 1,
then RI will not be activated if the received 9th data bit (RB8) is 0. In Mode 1, if SM2 = 1, then RI will not
be activated if a valid stop bit was not received. In Mode 0, SM2 should be 0.
REN
SCON.4
Enables serial reception. Set by software to enable reception. Clear by software to disable reception.
TB8
SCON.3
The 9th data bit that will be transmitted in Modes 2 and 3. Set or clear by software.
RB8
SCON.2
In Modes 2 and 3, the 9th data bit that was received. In Mode 1, if SM2 = 0, RB8 is the stop bit that was
received. In Mode 0, RB8 is not used.
TI
SCON.1
Transmit interrupt flag. 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, in any serial transmission. Must be cleared by software.
RI
SCON.0
Receive interrupt flag. Set by hardware at the end of the 8th bit time in Mode 0, or halfway through the
stop bit time in the other modes, in any serial reception (except see SM2). Must be cleared by software.
Where SM0, SM1 specify the serial port mode as follows:
2-50
SM0
SM1
Mode
Description
Baud Rate
0
0
0
Shift Register
fixed (fOSC./12)
0
1
1
8-bit UART
variable (set by timer)
1
0
2
9-bit UART
fixed (fOSC./64 or fOSC./32)
1
1
3
9-bit UART
variable (set by timer)
Hardware Description
Hardware Description
Baud Rates
The baud rate in Mode 0 is fixed as shown in the following
equation.
Oscillator Frequency
Mode 0 Baud Rate = ------------------------------------------------------12
The baud rate in Mode 2 depends on the value of the
SMOD bit in Special Function Register PCON. If SMOD = 0
(the value on reset), the baud rate is 1/64 of the oscillator
frequency. If SMOD = 1, the baud rate is 1/32 of the oscillator frequency, as shown in the following equation.
SMOD
2
Mode 2 Baud Rate = ----------------- × (Oscillator Frequency)
64
In the AT89C51, the Timer 1 overflow rate determines the
baud rates in Modes 1 and 3. In the AT89C52, these baud
rates can be determined by Timer 1, by Timer 2, or by both
(one for transmit and the other for receive).
Using Timer 1 to Generate Baud Rates
When Timer 1 is the baud rate generator, the baud rates in
Modes l and 3 are determined by the Timer 1 overflow rate
and the value of SMOD according to the following equation.
SMOD
2
Modes 1, 3
= ----------------- × (Timer 1 Overflow Rate)
Baud Rate
32
The Timer 1 interrupt should be disabled in this application.
The Timer itself can be configured for either timer or
counter operation in any of its 3 running modes. In the most
typical applications, it is configured for timer operation in
auto-reload mode (high nibble of TMOD = 0010B). In this
case, the baud rate is given by the following formula.
SMOD
Oscillator Frequency
2
Modes 1, 3
= ----------------- × ------------------------------------------------------Baud Rate
32
12 × [ 256 – ( TH1 ) ]
0001B), and using the Timer 1 interrupt to do a 16-bit software reload.
Figure 16 lists commonly used baud rates and how they
can be obtained from Timer 1.
Using Timer 2 to Generate Baud Rates
In the AT89C52, setting TCLK and/or RCLK in T2CON
selects Timer 2 as the baud rate generator (Figure 11).
Under these conditions, 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 17.
The baud rate generator mode is similar to the auto-reload
mode, in that a rollover in TH2 reloads the Timer 2 registers
with the 16-bit value in the RCAP2H and RCAP2L registers, which are preset by software.
In this case, the baud rates in Modes 1 and 3 are determined by the Timer 2 overflow rate according to the following equation.
Timer 2 Overflow Rate
Modes 1, 3 Baud Rate = -----------------------------------------------------------16
Timer 2 can be configured for either timer or counter operation. In the most typical applications, it is configured for
timer operation (C/T2 = 0). Normally, a timer increments
every machine cycle (thus at 1/12 the oscillator frequency),
but timer operation is different for Timer 2 when it is used
as a baud rate generator. As a baud rate generator, Timer
2 increments every state time (thus at 1/2 the oscillator frequency). In this case, the baud rate is given by the following formula.
Oscillator Frequency
Modes 1, 3
= ---------------------------------------------------------------------------------------------Baud Rate
32 × [ 65536 – ( RCAP2H,RCAP2L ) ]
where (RCAP2H, RCAP2L) is the content of RCAP2H and
Programmers can achieve very low baud rates with Timer 1
RCAP2L taken as a 16-bit unsigned integer.
by leaving the Timer 1 interrupt enabled, configuring the
Timer to run as a 16-bit timer (high nibble of TMOD =
Figure 16. Commonly Used Baud Rates Generated by Timer 1
Baud Rate
fOSC
SMOE
Timer 1
C/T
Mode
Reload Value
Mode 0 Max: 1 MHz
12 MHz
X
X
X
X
Mode 2 Max: 375K
12 MHz
1
X
X
X
Modes 1, 3: 62.5K
12 MHz
1
0
2
FFH
19.2K
11.059 MHz
1
0
2
FDH
9.6K
11.059 MHz
0
0
2
FDH
4.8K
11.059 MHz
0
0
2
FAH
2.4K
11.059 MHz
0
0
2
F4H
1.2K
11.059 MHz
0
0
2
E8H
137.5
11.986 MHz
0
0
2
1DH
110
6 MHz
0
0
2
72H
110
12 MHz
0
0
1
FEEBH
2-51
Figure 17. Timer 2 in Baud Rate Generator Mode
Figure 17 shows Timer 2 as a baud rate generator. This figure is valid only if RCLK + TCLK = 1 in T2CON. A rollover
in TH2 does not set TF2 and does 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.
If EXEN2 is set, a l-to-0 transition in T2EX sets EXF2 but
does not cause a reload from (RCAP2H, RCAP2L) to (TH2,
TL2). Thus, when Timer 2 is used as a baud rate generator,
T2EX can be used as an extra external interrupt.
When Timer 2 is running (TR2 = 1) as a timer in the baud
rate generator mode, programmers should not read from or
write to TH2 or TL2. Under these conditions, Timer 2 is
incremented every state time, and the results of a read or
write may not be accurate. The RCAP registers may be
read, but should not be written to, because a write might
overlap a reload and cause write and/or reload errors. Turn
Timer 2 off (clear TR2) before accessing the Timer 2 or
RCAP registers, in this case.
More About Mode 0
Serial data enters and exits through RXD. TXD outputs the
shift clock. Eight data bits are transmitted/received, with the
LSB first. The baud rate is fixed at 1/12 the oscillator frequency.
Figure 18 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 ninth position of the transmit
shift register and tells the TX Control block to begin a transmission. The internal timing is such that one full machine
cycle will elapse between “write to SBUF” and activation of
SEND.
2-52
Hardware Description
SEND transfers the output of the shift register to the alternate output function line of P3.0, and also transfers 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 one position to the right.
As data bits shift out to the right, 0s come in from the left.
When the MSB of the data byte is at the output position of
the shift register, the 1 that was initially loaded into the
ninth position is just to the left of the MSB, and all positions
to the left of that contain 0s. This condition flags the TX
Control block to do one last shift, then deactivate SEND
and set TI. Both of these actions occur at S1P1 of the tenth
machine cycle after “write to SBUF.”
Reception is initiated by the condition REN = 1 and R1 = 0.
At S6P2 of the next machine cycle, the RX Control unit
writes the bits 11111110 to the receive shift register and
activates RECEIVE in the next clock phase.
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
machine cycle in which RECEIVE is active, the contents of
the receive shift register are shifted one position to the left.
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.
As data bits come in from the right, 1s shift out to the left.
When the 0 that was initially loaded into the right-most position arrives at the left-most position in the shift register, it
flags the RX Control block to do one last shift and load
SBUF. At S1P1 of the l0th machine cycle after the write to
SCON that cleared RI, RECEIVE is cleared and RI is set.
Hardware Description
Figure 18. Serial Port Mode 0
2-53
Figure 19. Serial Port Mode 1. TCLK, RCLK and Timer 2 are Present In the AT89C52 Only.
TIMER 2
OVERFLOW
TIMER 1
OVERFLOW
WRITE
TO
SBUF
÷2
SMOD
= 0
AT89C51 INTERNAL BUS
TB8
SMOD
= 1
S
D Q
CL
SBUF
TXD
ZERO DETECTOR
"0"
"1"
SHIFT DATA
START
TCLK
TX CONTROL
RX CLOCK
÷ 16
"0"
SEND
TI
SERIAL
PORT
INTERRUPT
"1"
RCLK
÷ 16
SAMPLE
LOAD
SBUF
SHIFT
1FFH
RX CLOCK RI
START
RX CONTROL
1-TO-0
TRANSITION
DETECTOR
BIT
DETECTOR
INPUT SHIFT REG.
(9 BITS)
RXD
SHIFT
LOAD
SBUF
SBUF
READ
SBUF
AT89C51 INTERNAL BUS
TX
CLOCK
WRITE TO SBUF
SEND
DATA
SHIFT
S1P1
D0
TXD
TI
D1
D2
RXD
D4
D5
D6
D7
STOP BIT
÷ 16 RESET
START BIT
BIT DETECTOR SAMPLE TIMES
SHIFT
RI
2-54
D3
START BIT
RX
CLOCK
RECEIVE
TRANSMIT
Hardware Description
D0
D1
D2
D3
D4
D5
D6
D7
STOP BIT
Hardware Description
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 AT89C51, the baud rate is determined by the
Timer 1 overflow rate. In the AT89C52 the baud rate is
determined either by the Timer 1 overflow rate, the Timer 2
overflow rate, or both. In this case, one Timer is for transmit, and the other is for receive.
Figure 19 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 ninth 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 divideby-16 counter. Thus, the bit times are synchronized to the
divide-by-16 counter, not to the “write to SBUF” signal.
The transmission begins when SEND is activated, 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, 0s are clocked in from the
left. When the MSB of the data byte is at the output position
of the shift register, the 1 that was initially loaded into the
ninth position is just to the left of the MSB, and all positions
to the left of that contain 0s. This condition flags the TX
Control unit to do one last shift, then deactivate SEND and
set TI. This occurs at the tenth divide-by-16 rollover after
“write to SBUF.”
Reception is initiated by a 1-to-0 transition detected at
RXD. For this purpose, RXD is sampled at a rate of 16
times the established baud rate. 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.
The 16 states of the counter divide each bit time into 16ths.
At the seventh, eighth, and ninth counter states of each bit
time, the bit detector samples the value of RXD. The value
accepted is the value that was seen in at least 2 of the 3
samples. This is done to reject noise. In order to reject false
bits, if the value accepted during the first bit time is not 0,
the receive circuits are reset and the unit continues looking
for another l-to-0 transition. If the start bit is valid, it is
shifted into the input shift register, and reception of the rest
of the frame proceeds.
As data bits come in from the right, 1s shift out to the left.
When the start bit arrives at the leftmost position in the shift
register, (which is a 9-bit register in mode 1), it flags the RX
Control block to do one last shift, load SBUF and RB8, and
set RI. The signal to load SBUF and RB8 and to set RI is
generated if, and only if, the following conditions are met at
the time the final shift pulse is generated.
1. RI = 0 and
2. Either SM2 = 0, or the received stop bit = 1
If either of these two conditions is not met, the received
frame is irretrievably lost. If both conditions are met, the
stop bit goes into RB8, the 8 data bits go into SBUF, and RI
is activated. At this time, whether or not the above conditions are met, the unit continues looking for a l-to-0 transition in RXD.
More About Modes 2 and 3
Eleven bits are transmitted (through TXD), or received
(through RXD): a start bit (0), 8 data bits (LSB first), a programmable ninth data bit, and a stop bit (1). On transmit,
the ninth data bit (TB8) can be assigned the value of 0 or 1.
On receive, the ninth data bit goes into RB8 in SCON. The
baud rate is programmable to either 1/32 or 1/64 of the
oscillator frequency in Mode 2. Mode 3 may have a variable
baud rate generated from either Timer 1 or 2, depending on
the state of TCLK and RCLK.
Figures 20 and 21 show a functional diagram of the serial
port in Modes 2 and 3. The receive portion is exactly the
same as in Mode 1. The transmit portion differs from Mode
1 only in the ninth bit of the transmit shift register.
Transmission is initiated by any instruction that uses SBUF
as a destination register. The “write to SBUF” signal also
loads TB8 into the ninth bit position of the transmit shift register and flags the TX Control unit that a transmission is
requested. Transmission 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 divideby-16 counter, not to the “write to SBUF” signal.
The transmission begins when SEND is activated, 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. The first shift clocks a 1 (the stop bit) into the ninth bit
position of the shift register. Thereafter, only 0s are clocked
in. Thus, as data bits shift out to the right, 0s are clocked in
from the left. When TB8 is at the output position of the shift
register, then the stop bit is just to the left of TB8, and all
positions to the left of that contain 0s. This condition flags
the TX Control unit to do one last shift, then deactivate
SEND and set TI. This occurs at the 11th divide-by-16 rollover after “write to SBUF.”
Reception is initiated by a 1-to-0 transition detected at
RXD. For this purpose, RXD is sampled at a rate of 16
times the established baud rate. When a transition is
detected, the divide-by-16 counter is immediately reset,
and 1FFH is written to the input shift register.
2-55
Figure 20. Serial Port Mode 2
2-56
Hardware Description
Hardware Description
Figure 21. Serial Port Mode 3. TCLK, RCLK, and Timer 2 are Present in AT89C52 only
TIMER 2
OVERFLOW
TIMER 1
OVERFLOW
WRITE
TO
SBUF
÷2
SMOD
= 0
AT89C51 INTERNAL BUS
TB8
SMOD
= 1
S
D Q
CL
SBUF
TXD
ZERO DETECTOR
"0"
"1"
SHIFT DATA
START
TCLK
TX CONTROL
÷16
"0"
RX CLOCK
SEND
TI
SERIAL
PORT
INTERRUPT
"1"
RCLK
÷16
SAMPLE
LOAD
SBUF
SHIFT
1FFH
RX CLOCK RI
START
RX CONTROL
1-TO-0
TRANSITION
DETECTOR
BIT
DETECTOR
INPUT SHIFT REG.
(9 BITS)
RXD
SHIFT
LOAD
SBUF
SBUF
READ
SBUF
AT89C51 INTERNAL BUS
TX
CLOCK
WRITE TO SBUF
SEND
DATA
SHIFT
S1P1
TRANSMIT
D0
TXD
TI
D1
D2
D3
D4
D5
D6
D7
TB8
START BIT
STOP BIT
STOP BIT GEN
RECEIVE
RX
CLOCK
÷16 RESET
RXD
START BIT
BIT DETECTOR SAMPLE TIMES
D0
D1
D2
D3
D4
D5
D6
D7
RB8
STOP
BIT
SHIFT
RI
2-57
At the seventh, eighth and ninth counter states of each bit
time, the bit detector samples the value of RXD. The value
accepted is the value that was seen in at least 2 of the 3
samples. If the value accepted during the first bit time is not
0, the receive circuits are reset and the unit continues looking for another l-to-0 transition. If the start bit proves valid, it
is shifted into the input shift register, and reception of the
rest of the frame proceeds.
As data bits come in from the right, 1s shift out to the left.
When the start bit arrives at the leftmost position in the shift
register (which in Modes 2 and 3 is a 9-bit register), it flags
the RX Control block to do one last shift, load SBUF and
RB8, and set RI. The signal to load SBUF and RB8 and to
set RI is generated if, and only if, the following conditions
are met at the time the final shift pulse is generated:
1. RI = 0, and
2. Either SM2 = 0 or the received 9th data bit = 1
If either of these conditions is not met, the received frame is
irretrievably lost, and RI is not set. If both conditions are
met, the received ninth data bit goes into RB8, and the first
8 data bits go into SBUF. One bit time later, whether the
above conditions were met or not, the unit continues looking for a 1-to-0 transition at the RXD input.
Note that the value of the received stop bit is irrelevant to
SBUF, RB8, or RI.
In the AT89C52, the Timer 2 Interrupt is generated by the
logical OR of TF2 and EXF2. Neither of these flags is
cleared by hardware when the service routine is vectored
to. In fact, the service routine may have to determine
whether TF2 or EXF2 generated the interrupt, and the bit
must be cleared in software.
Figure 22. Interrupt Sources
Interrupts
The AT89C51 provides 5 interrupt sources: two external
interrupts, two timer interrupts, and a serial port interrupt.
The AT89C52 provides 6 with the extra timer. These are
shown in Figure 22.
The External Interrupts INT0 and INT1 can each be either
level-activated or transition-activated, depending on bits
IT0 and IT1 in Register TCON. The flags that actually generate these interrupts are the IE0 and IE1 bits in TCON.
When the service routine is vectored to, hardware clears
the flag that generated an external interrupt only if the interrupt was transition-activated. If the interrupt was level-activated, then the external requesting source (rather than the
on-chip hardware) controls the request flag.
The Timer 0 and Timer 1 Interrupts are generated by TF0
and TF1, which are set by a rollover in their respective
Timer/Counter registers (except for Timer 0 in Mode 3).
When a timer interrupt is generated, the on-chip hardware
clears the flag that generated it when the service routine is
vectored to.
The Serial Port Interrupt is generated by the logical OR of
RI and TI. Neither of these flags is cleared by hardware
when the service routine is vectored to. In fact, the service
routine normally must determine whether RI or TI generated the interrupt, and the bit must be cleared in software.
2-58
Hardware Description
Figure 23. IE: Interrupt Enable Register
(MSB)
EA
(LSB)
—
ET2
ES
ET1
EX1
ET0
EX0
Enable bit = 1 enables the interrupt.
Enable bit = 0 disables it.
Symbol
Position
Function
EA
IE.7
Global enable/disable. Disables all
interrupts. If EA = 0, no interrupt will be
acknowledged. If EA = 1, each interrupt
source is individually enabled or disabled
by setting or clearing its enable bit.
—
IE.6
Undefined/reserved.
ET2
IE.5
Timer 2 Interrupt enable bit (AT89C52).
ES
IE.4
Serial Port Interrupt enable bit.
ET1
IE.3
Timer 1 Interrupt enable bit.
EX1
IE.2
External Interrupt 1 enable bit.
ET0
IE.1
Timer 0 Interrupt enable bit.
EX0
IE.0
External Interrupt 0 enable bit.
User software should never write 1s to unimplemented bits, since they
may be used in future AT89 Series products.
Hardware Description
Priority Level Structure
Figure 24. IP: Interrupt Priority Register
(MSB)
—
(LSB)
—
PT2
PS
PT1
PX1
PT0
PX0
Priority bit = 1 assigns high priority.
Priority bit = 0 assigns low priority.
Symbol
Position
Function
—
IP.7
reserved
—
IP.6
reserved
PT2
IP.5
Timer 2 Interrupt priority bit.
PS
IP.4
Serial Port Interrupt priority bit.
PT1
IP.3
Timer 1 Interrupt priority bit.
PX1
IP.2
External Interrupt 1 priority bit.
PT0
IP.1
Timer 0 Interrupt priority bit.
PX0
IP.0
External Interrupt 0 priority bit.
Each interrupt source can also be individually programmed
to one of two priority levels by setting or clearing a bit in
Special Function Register IP (interrupt priority) at address
0B8H (Figure 24). IP is cleared after a system reset to
place all interrupts at the lower priority level by default. A
low-priority interrupt can be interrupted by a high-priority
interrupt but not by another low-priority interrupt. A high-priority interrupt can not be interrupted by any other interrupt
source.
If two requests of different priority levels 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 determines which
request is serviced. Thus within each priority level there is a
second priority structure determined by the polling
sequence, as follows.
User software should never write 1s to unimplemented bits, since they
may be used in future AT89 Series products.
All of the bits that generate interrupts can be set or cleared
by software, with the same result as though they had been
set or cleared by hardware. That is, interrupts can be generated and pending interrupts can be canceled in software.
Each of these interrupt sources can be individually enabled
or disabled by setting or clearing a bit in Special Function
Register IE (interrupt enable) at address OA8H. As well as
individual enable bits for each interrupt source, there is a
global enable/disable bit that is cleared to disable all interrupts or set to turn on interrupts (see Figure 23).
Figure 23 shows that bit position IE.6 is unimplemented. In
the AT89C51, bit position IE.5 is also unimplemented. User
software should not write 1s to these bit positions, since
they may be used in future microcontrollers.
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.
The IP register contains a number of unimplemented bits.
IP.7 and IP.6 are vacant in the AT89C52, and in the
AT89C51 these bits and IP.5 are vacant. User software
should not write 1s to these bit positions, since they may be
used in future products.
Figure 25. Interrupt Response Timing Diagram
C1
S5P2
C2
6
INTERRUPT
GOES ACTIVE
C3
C4
C5
S6
INTERRUPT
LATCHED
INTERRUPTS
ARE POLLED
LONG CALL TO
INTERRUPT
VECTOR ADDRESS
INTERRUPT
ROUTINE
This is the fastest possible response when C2 is the final cycle of an instruction other than RETI or an access to IE or IP .
2-59
How Interrupts Are Handled
The interrupt flags are sampled at S5P2 of every machine
cycle. The samples are polled during the following machine
cycle. The AT89C52 Timer 2 interrupt cycle is different, as
described in the Response Time Section. 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 hardware generated LCALL is not blocked 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 write to the
IE or IP registers.
Any of these three conditions will block the generation of
the LCALL to the interrupt service routine. Condition 2
ensures that the instruction in progress will be completed
before vectoring to any service routine. Condition 3
ensures that if the instruction in progress is RETI or any
access to IE or IP, then at least one more instruction will be
executed before any interrupt is vectored to.
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. If an active interrupt flag is
not being serviced because of one of the above conditions
and is not still active when the blocking condition is
removed, the denied interrupt will not be serviced. In other
words, the fact 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
25.
Note that if an interrupt of higher priority level goes active
prior to S5P2 of the machine cycle labeled C3 in Figure 25,
then in accordance with the above rules it will be serviced
during C5 and C6, without any instruction of the lower priority routine having been executed.
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 does not. It
never clears the Serial Port or Timer 2 flags. This must be
done in the user’s software. The processor clears an external interrupt flag (IE0 or IE1) only if it was transition-activated. 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
2-60
Hardware Description
depends on the source of the interrupt being serviced, as
shown in the following table.
Interrupt
Source
Vector Address
External 0
IE0
0003H
Timer 0
TF0
000BH
External 1
IE1
0013H
Timer 1
TF1
001BH
Serial Port
RI or TI
0023H
Timer 2
TF2 or EXF2
002BH
System Reset
RST
0000H
Note:
When vectoring to an interrupt the flag that caused the
interrupt is automatically cleared by hardware. The
exceptions are RI and TI for serial port interrupts, and
TF2 and EXF2 for Timer 2 interrupts. Since there are two
possible sources for each of these interrupts, it is not
practical for the CPU to clear the interrupt flag. These
bits must be tested in the ISR to determine the source of
the interrupt, and then the interrupting flag is cleared by
software.
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.
Interrupt Flag Bits
Interrupt
Flag
SFR Register
and Bit Position
External 0
IE0
TCON.1
External 1
IE1
TCON.3
Timer 1
TF1
TCON.7
Timer 0
TF0
TCON.5
Serial port
TI
SCON.1
Serial port
RI
SCON.0
Timer 2
TF2
T2CON.7 (AT89C52)
Timer 2
EXF2
T2CON.6 (AT89C52)
Hardware Description
When an interrupt is accepted the following action occurs:
1. The current instruction completes operation.
2. The PC is saved on the stack.
3. The current interrupt status is saved internally.
4. Interrupts are blocked at the level of the interrupts.
5. The PC is loaded with the vector address of the ISR
(interrupt service routine).
6. The ISR executes.
The ISR executes and takes action in response to the interrupt. The ISR finishes with RETI (return from interrupt)
instruction. This retrieves the old value of the PC from the
stack and restores the old interrupt status. Execution of the
main program continues where it left off.
External Interrupts
The external sources can be programmed to be level-activated or transition-activated by setting or clearing bit IT1 or
IT0 in Register TCON. If ITx = 0, 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 machine cycle,
and then hold it low for at least one machine 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 the external source
must deactivate the request before the interrupt service
routine is completed, or else another interrupt will be generated.
Response Time
The INT0 and INT1 levels are inverted and latched into the
interrupt flags IE0 and IE1 at S5P2 of every machine cycle.
Similarly, the Timer 2 flag EXF2 and the Serial Port flags RI
and TI are set at S5P2. The values are not actually polled
by the circuitry until the next machine cycle.
The Timer 0 and Timer 1 flags, TF0 and TF1, are set at
S5P2 of the cycle in which the timers overflow. The values
are then polled by the circuitry in the next cycle. However,
the Timer 2 flag TF2 is set at S2P2 and is polled in the
same cycle in which the timer overflows.
If a request is active and conditions are right for it to be
acknowledged, a hardware subroutine call to the requested
service routine will be the next instruction executed. The
call itself takes two cycles. Thus, a minimum of three complete machine cycles elapsed between activation of an
external interrupt request and the beginning of execution of
the first instruction of the service routine. Figure 25 shows
interrupt response timings.
A longer response time results 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 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. 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 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 9 cycles.
Single-Step Operation
The AT89C51 interrupt structure allows single-step execution with very little software overhead. As previously noted,
an interrupt request will not be serviced while an interrupt of
equal priority level is still in progress, nor will it be serviced
after RETI until at least one other instruction has been executed. Thus, once an interrupt routine has been entered, it
cannot be re-entered until at least one instruction of the
interrupted program is executed. One way to use this feature for single-stop operation is to program one of the external interrupts (for example, INT0) to be level-activated. The
service routine for the interrupt will terminate with the following code.
JNB
P3.2,$ ;Wait Here Till INT0 Goes High
JB
P3.2,$ ;Now Wait Here Till it Goes Low
RETI
;Go Back and Execute One Instruction
If the INT0 pin, which is also the P3.2 pin, is held normally
low, the CPU will go right into the External Interrupt 0 routine and stay there until INT0 is pulsed (from low to high to
low). Then it will execute RETI, go back to the task program, execute one instruction, and immediately reenter the
External Interrupt 0 routine to await the next pulsing of
P3.2. One step of the task program is executed each time
P3.2 is pulsed.
Reset
The reset input is the RST pin, which is the input to a
Schmitt Trigger.
A reset is accomplished by holding the RST pin high for at
least two machine cycles (24 oscillator periods), while the
oscillator is running. The CPU responds by generating an
internal reset, with the timing shown in Figure 26.
2-61
The external reset signal is asynchronous to the internal
clock. The RST pin is sampled during State 5 Phase 2 of
every machine cycle. The port pins will maintain their current activities for 19 oscillator periods after a logic 1 has
been sampled at the RST pin; that is, for 19 to 31 oscillator
periods after the external reset signal has been applied to
the RST pin.
While the RST pin is high, ALE and PSEN are weakly
pulled high. After RST is pulled low, it will take 1 to 2
machine cycles for ALE and PSEN to start clocking. For
this reason, other devices can not be synchronized to the
internal timings of the AT89C51.
Driving the ALE and PSEN pins to 0 while reset is active
could cause the device to go into an indeterminate state.
The internal reset algorithm writes 0s to all the SFRs
except the port latches, the Stack Pointer, and SBUF. The
port latches are initialized to FFH, the Stack Pointer to 07H,
and SBUF is indeterminate. Table 3 lists the SFRs and
their reset values.
The internal RAM is not affected by reset. On power-up the
RAM content is indeterminate.
Note:
There is no internal pulldown reset pin on NMOS
devices, unlike that of Atmel’s CMOS microcontroller
devices.
Figure 26. Reset Timing
12 OSC. PERIODS
S5
S6
S1
S2
S3
S4
S5
S6
S1
S2
S3
S4
S5
S6
S1
S2
S3
S4
INTERNAL RESET SIGNAL
RST:
SAMPLE RST
SAMPLE RST
ALE:
PSEN:
P0:
INST
ADDR
INST
ADDR
INST
ADDR
11 OSC. PERIODS
Power-On Reset
For CMOS devices, the external resistor can be removed
because the RST pin has an internal pulldown. The capacitor value can then be reduced to 1 µF in Figure 27.
When power is turned on, the circuit holds the RST pin high
for an amount of time that depends on the capacitor value
and the rate at which it charges. To ensure a valid reset,
the RST pin must be held high long enough to allow the
oscillator to start up plus two machine cycles.
On power-up, VCC should rise within approximately 10 ms.
The oscillator start-up time depends on the oscillator frequency. For a 10 MHz crystal, the start-up time is typically
1 ms. For a 1 MHz crystal, the start-up time is typically 10
ms.
With the given circuit, reducing VCC quickly to 0 causes the
RST pin voltage to momentarily fall below 0V. However,
this voltage is internally limited and will not harm the
device.
2-62
Hardware Description
INST
ADDR
INST
ADDR
19 OSC. PERIODS
Note:
The port pins will be in a random state until the oscillator
has started and the internal reset algorithm has written
1s to them.
Powering up the device without a valid reset could cause
the CPU to start executing instructions from an indeterminate location. This is because the SFRs, specifically
the Program Counter, may not get properly initialized.
Hardware Description
Figure 27. Power-On Reset Circuit
Table 3. Reset Values of the SFRs
VCC
+
10µf
VCC
AT89C51
RST
8.2KΩ
GND
Power-Saving Modes of Operation
The Atmel Microcontrollers have two power-reducing
modes, Idle and Power Down. The input through which
backup power is supplied during these operations is VCC.
Figure 28 shows the internal circuitry which implements
these features. In the Idle mode (IDL = 1), the oscillator
continues to run and the Interrupt, Serial Port, and Timer
blocks continue to be clocked, but the clock signal is gated
off to the CPU. In Power Down (PD = 1), the oscillator is
frozen. The Idle and Power Down modes are activated by
setting bits in Special Function Register PCON. The
address of this register is 87H. Figure 29 details its contents.
Idle Mode
An instruction that sets PCON.0 is the last instruction executed before the Idle mode begins. In the Idle mode, the
internal clock signal is gated off to the CPU, but not to the
Interrupt, Timer, and Serial Port functions. The CPU status
is preserved in its entirety: the Stack Pointer, Program
Counter, Program Status Word, Accumulator, and all other
registers maintain their data during Idle. The port pins hold
the logical states they had at the time Idle was activated.
ALE and PSEN hold at logic high levels.
There are two ways to terminate the Idle. Activation of any
enabled interrupt will cause PCON.0 to be cleared by hardware, terminating the Idle mode. The interrupt will be serviced, and following RETI the next instruction to be executed will be the one following the instruction that put the
device into Idle.
SFR Name
Reset Value
PC
0000H
ACC
00H
B
00H
PSW
00H
SP
07H
DPTR
0000H
P0-P3
FFH
IP (AT89C51)
XXX00000B
IP (AT89C52)
XX000000B
IE (AT89C51)
0XX00000B
IE (AT89C52)
0X000000B
TMOD
00H
T2MOD (AT89C52)
XXXXXX00B
TCON
00H
T2CON (AT89C52)
00H
TH0
00H
TL0
00H
TH1
00H
TL1
00H
TH2 (AT89C52)
00H
TL2 (AT89C52)
00H
RCAP2H (AT89C52)
00H
RCAP2L (AT89C52)
00H
SCON
00H
SBUF
Indeterminate
PCON (CHMOS)
0XXX0000B
The flag bits GF0 and GF1 can be used to indicate whether
an interrupt occurred during normal operation or during an
Idle. For example, an instruction that activates Idle can also
set one or both flag bits. When Idle is terminated by an
interrupt, the interrupt service routine can examine the flag
bits.
The other way of terminating the Idle mode is with a hardware reset. Since the clock oscillator is still running, the
hardware reset must be held active for only two machine
cycles (24 oscillator periods) to complete the reset.
The signal at the RST pin clears the IDL bit directly and
asynchronously. At this time, the CPU resumes program
execution from where it left off; that is, at the instruction following the one that invoked the Idle Mode. As shown in Figure 26, two or three machine cycles of program execution
2-63
may take place before the internal reset algorithm takes
control. On-chip hardware inhibits access to the internal
RAM during this time, but access to the port pins is not
inhibited. To eliminate the possibility of unexpected outputs
at the port pins, the instruction following the one that
invokes Idle should not write to a port pin or to external
data RAM.
Figure 28. Idle and Power-Down Hardware
XTAL 2
XTAL 1
OSC
INTERRUPT,
SERIAL PORT,
TIMER BLOCKS
CLOCK
GEN.
CPU
PD
Power Down Mode
An instruction that sets PCON 1 is the last instruction executed before Power Down mode begins. In the Power
Down mode, the on-chip oscillator stops. With the clock frozen, all functions are stopped, but the on-chip RAM and
Special Function Registers are held. The port pins output
the values held by their respective SFRs. ALE and PSEN
output lows.
The only exit from Power Down for the AT89C series is a
hardware reset. Reset redefines all the SFRs but does not
change the on-chip RAM.
In the Power Down mode of operation, VCC can be reduced
to as low as 2V. However, VCC must not be reduced before
the Power Down mode is invoked, and V CC must be
restored to its normal operating level before the Power
Down mode is terminated. The reset that terminates Power
Down also frees the oscillator. The reset should not be activated before VCC is restored to its normal operating level
and must be held active long enough to allow the oscillator
to restart and stabilize (normally less than 10 msec).
Programming
IDL
Figure 29. PCON Power Control Register
(MSB)
SMOD
(LSB)
—
—
—
GF1
GF0
PD
IDL
Symbol
Position
Function
SMOD
PCON.7
Double Baud rate bit. When set to a 1 and
Timer 1 is used to generate baud rate, and
the Serial Port is used in modes 1, 2, or 3.
—
PCON.6
(Reserved)
—
PCON.5
(Reserved)
—
PCON.4
(Reserved)
GF1
PCON.3
General-purpose flag.bit.
GF0
PCON.2
General-purpose flag.bit.
PD
PCON.1
Power Down bit. Setting this bit activates
power down operation.
IDL
PCON.0
Idle mode bit. Setting this bit activates idle
mode operation.
If 1s are written to PD and IDL at the same time, PD takes precedence.
The reset value of PCON is (0XXX000).User software should never write
1s to unimplemented bits, since they may be used in future products.
The on-chip Flash allows the program memory to be reprogrammed in-system or by a conventional nonvolatile memory programmer. A list of programming companies that
support Atmel’s products can be found on the Atmel Bulletin Board and in the Microcontroller Programmer Support
section of this Data Book. To access the bulletin board, dial
408-436-4309.
The AT89C51/52 programs at VPP = 12V using one 100msec PROG pulse per byte programmed. This results in a
programming time of approximately 1.5 msec per byte, for
a total programming time of 6 sec for the 4K byte device
and 12 sec for the 8K byte device.
Detailed procedures for programming and verifying each
device are given in the data sheets.
Program Memory Locks
In some microcontroller applications, the program memory
must be secure from software piracy. Atmel has responded
to this need by implementing a program memory locking
scheme in all of its devices. While it is impossible for anyone to guarantee absolute security against all levels of
technological sophistication, the program memory locks
present a substantial barrier against illegal readout of protected software.
Table 4. Flash AT89C51 and AT89C52
Device Name
Flash Bytes
Ckt Type
VPP
Time Required to Program Entire Array
AT89C51
4K
CMOS
12V
6 seconds
AT89C52
8K
CMOS
12V
12 seconds
The procedure for programming the lock bits is detailed in the data sheets.
2-64
Hardware Description
Hardware Description
Table 5 lists the Lock Bits and their corresponding effects
on the microcontroller.
Erasing the Flash also erases the Lock Bits, returning the
microcontroller to full functionality.
Table 5. Program Lock Bits and Their Features
Program Lock Bits
Protection Type
Mode
LB1
LB2
LB3
1
U
U
U
No program lock features
enabled.
2
P
U
U
MOVC instructions
executed from external
program memory are
disabled from fetching
code bytes from internal
memory, EA is sampled
and latched on reset, and
further programming of the
Flash is disabled.
3
P
P
U
Same as 2, also verify is
disabled.
4
P
P
P
Same as 3, also external
execution is disabled.
Notes:
1. P = Programmed; U = Unprogrammed
2. Any other combination of the Lock Bits is not defined.
While the device is in ONCE mode, the Port 0 pins go into a
float state, and the other port pins and ALE and PSEN are
weakly pulled high. The oscillator circuit remains active.
While the device is in this mode, an emulator or test CPU
can be used to drive the circuit. A reset restores normal
operation.
On-Chip Oscillators
The crystal specifications and capacitance values (C1 and
C2 in Figure 30) are not critical. 30 pF can be used in these
positions at any frequency with good quality crystals. A
ceramic resonator can be used in place of the crystal in
cost-sensitive applications. When a ceramic resonator is
used, C1 and C2 are normally selected to be of somewhat
higher values, typically, 47 pF. The manufacturer of the
ceramic resonator should be consulted for recommendations on the values of these capacitors.
In general, crystals used with these devices typically have
the following specifications.
ESR (Equivalent Series Resistance) see Figure 31
CO (Shunt Capacitance)
7.0 pF max.
CL (Load Capacitance)
30 pF + 3 pF
Drive Level
1 mW
Frequency, tolerance and temperature range are determined by the system requirements.
Figure 30. Using the On-Chip Oscillator
Table 6. Program Protection
Device
Lock Bits
AT89C51
LB1, LB2, LB3
AT89C52
LB1, LB2, LB3
AT89C2051
LB1, LB2
AT89C1051
LB1, LB2
When Lock Bit 1 is programmed, the logic level at the EA
pin is sampled and latched during reset. If the device is
powered up without a reset, the latch initializes to a random
value, and holds that value until reset is activated. The
latched value of EA must agree with the current logic level
at that pin in order for the device to function properly.
ONCE™ Mode
The ONCE (“on-circuit emulation”) mode facilitates testing
and debugging of systems using the device without requiring the device to be removed from the circuit. The ONCE
mode is invoked by taking the following steps.
1. Pull ALE low while the device is in reset and PSEN is
high;
2. Hold ALE low as RST is deactivated.
2-65
Figure 31. ESR versus Frequency
The on-chip oscillator circuitry shown in Figure 32, consists
of a single stage linear inverter intended for use as a crystal-controlled, positive reactance oscillator.
To drive the parts with an external clock source, apply the
external clock signal to XTAL1, and leave XTAL2 floating,
as shown in Figure 33.
ESR in OHMS
500
400
300
200
100
4
8
12
16
CRYSTAL FREQUENCY in MHz
Figure 32. On-Chip Oscillator Circuitry for the AT89C51
VCC
TO INTERNAL
TIMING CKTS
Q2
D1
400 Ω
Rf
XTAL1
XTAL2
D2
Q1
Q3
Q4
PD
GND
Note:
2-66
In Atmel’s CMOS microcontrollers the Oscillator Specification differs from that in NMOS versions.
Hardware Description
Hardware Description
Figure 33. Using an External Clock Source
AT89
NC
EXTERNAL
OSCILLATOR
SIGNAL
XTAL2
XTAL1
GND
CMOS GATE
Internal Timing
Figures 34 through 37 show the various strobe and port
signals being clocked internally. The figures do not show
rise and fall times of the signals, nor do they show propagation delays between the XTAL signal and events at other
pins.
Rise and fall times are dependent on the external loading
that each pin must drive. They are often taken to be about
10 ns, measured between 0.8V and 2.0V.
Propagation delays are different for different pins. For a
given pin the delays vary with pin loading, temperature,
VCC, and manufacturing lot. If the XTAL waveform is taken
as the timing reference, propagation delays may vary from
25 to 125 ns.
The AC Timings section of the data sheets do not reference any timing to the XTAL waveform. Rather, they relate
the critical edges of control and input signals to each other.
The timings published in the data sheet include the effects
of propagation delays under the specified test conditions.
2-67
Figure 34. External Program Memory Fetches
STATE 1 STATE 2 STATE 3 STATE 4 STATE 5 STATE 6 STATE 1 STATE 2
P1 P2 P1 P2 P1 P2 P1 P2 P1 P2 P1 P2 P1 P2 P1 P2
XTAL:
ALE:
PSEN:
DATA
SAMPLED
DATA
SAMPLED
PCL
OUT
P0:
PCL
OUT
PCH OUT
P2:
DATA
SAMPLED
PCL
OUT
PCH OUT
PCH OUT
Figure 35. External Data Memory Read Cycle
STATE 4 STATE 5 STATE 6 STATE 1 STATE 2 STATE 3 STATE 4 STATE 5
P1 P2 P1 P2 P1 P2 P1 P2 P1 P2 P1 P2 P1 P2 P1 P2
XTAL:
ALE:
RD:
PCL OUT IF
PROGRAM MEMORY
IS EXTERNAL
DATA SAMPLED
DPL OR RI
OUT
P0:
P2:
2-68
PCH OR
P2 SFR
Hardware Description
FLOAT
DPH OR P2 SFR OUT
FLOAT
PCL
OUT
PCH OR
P2 SFR
Hardware Description
Figure 36. External Data Memory Write Cycle
STATE 4 STATE 5 STATE 6 STATE 1 STATE 2 STATE 3 STATE 4 STATE 5
P1 P2 P1 P2 P1 P2 P1 P2 P1 P2 P1 P2 P1 P2 P1 P2
XTAL:
ALE:
WR:
PCL OUT IF
PROGRAM MEMORY
IS EXTERNAL
DPL OR RI
OUT
P0:
P2:
PCH OR
P2 SFR
PCL
OUT
DATA OUT
DPH OR P2 SFR OUT
PCH OR
P2 SFR
Figure 37. Port Operation
STATE 4 STATE 5 STATE 6 STATE 1 STATE 2 STATE 3 STATE 4 STATE 5
P1 P2 P1 P2 P1 P2 P1 P2 P1 P2 P1 P2 P1 P2 P1 P2
XTAL:
P0, P1, P2, P3
P0, P1, P2, P3
RST
RST
INPUTS SAMPLED:
MOV PORT, SRC:
OLD DATA
NEW DATA
SERIAL PORT
SHIFT CLOCK
(MODE 0)
RXD PIN SAMPLED
RXD SAMPLED
2-69
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