NXP MC68302 Integrated Multi-Protocol Processor Reference Manual
NXP MC68302 is a powerful microprocessor with a rich set of features designed for demanding applications. It offers exceptional processing capabilities, including fast interrupt handling and efficient data movement. With its versatile instruction set and on-chip resources, the MC68302 excels in diverse applications such as industrial control, automotive systems, and telecommunications equipment. It provides reliable performance and enables efficient system design, making it an ideal choice for engineers seeking a robust and capable microprocessor solution.
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MICROPROCESSOR & MEMORY
TECHNOLOGIES GROUP
MC68302 Auto Baud Support Package Specifications
Revision 2.0
AUTO_BAUD
MC68302 AutoBaud Specificati o n - Rev.2.0
May 1, 1994
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Table of Contents
AutoBaud controller.................................................................................................1
1.1
Overview................................................................................................1
1.2
1.3
AutoBaud Channel Reception Process ..................................................2
AutoBaud Channel Transmit Process ....................................................3
1.4
1.5
1.5.1
1.5.2
1.5.3
1.5.4
1.5.5
1.5.5.1
1.5.6
1.5.7
1.5.8
1.5.8.1
1.5.8.2
1.5.9
1.6
Autobaud Parameter RAM ....................................................................4
AutoBaud Programming Model.............................................................5
Initialization of the AutoBaud Microcode Package.........................6
Preparing for the AutoBaud Process................................................6
Enter_Baud_Hunt Command...........................................................7
AutoBaud Command Descriptor......................................................7
AutoBaud LookUp Table................................................................9
LookUp Table Example ..........................................................10
Determining Character Length and Parity .....................................11
AutoBaud Reception Error Handling Procedure ...........................12
AutoBaud Transmission.................................................................12
Automatic Echo ......................................................................12
Smart Echo ..............................................................................13
Switching To Another Protocol .....................................................14
AutoBaud Programming Example.......................................................14
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1.0 AutoBaud controller
This document describes the operation, registers, and programming of the Autobaud
Microcode Package Revision 2.0. Below are the major features of Revision 2.0:
• Downloads into the MC68302 dual-port RAM.
• Requires 16 x the maximum baudrate for the sampling clock (can be internally or externally generated).
• Supports parity determination and character length detection on reception for 7-bit characters with parity, 8-bit characters with and without parity.
• Supports parity generation on transmission
• Supports 2 echo modes (automatic and smart echoing).
• Supports determining baudrates up to 115.2 kbaud.
1.1 Overview
The support for the AutoBaud function is contained in a RAM microcode that is downloaded into the dual-port RAM of the MC68302. The AutoBaud function determines the baudrate and format of an asynchronous data stream starting with a known character. This package may be used to implement the standard AT command set or other characters.
In order to use the AutoBaud mode, the Serial Communication Controller (SCC) is initially programmed to BISYNC mode. The SCC receiver then synchronizes on the falling edge of the
START bit. Once a start bit is detected, each bit received is processed by the AutoBaud controller.
The AutoBaud controller measures the length of the START bit to determine the receive baudrate and compares the length to values in a user supplied lookup table. After the baudrate is determined, the AutoBaud controller assembles the character and compares it against two userdefined characters. If a match is detected, the AutoBaud controller interrupts the host and returns the determined nominal start value from the lookup table. The AutoBaud controller continues to assemble the characters and interrupt the host until the host stops the reception process. The incoming message should contain a mixture of even and odd characters so that the user has enough information to decide on the proper character format ( length and parity). The host then uses the returned nominal start value from the lookup table, modifies the SCC Configuration
Register (SCON) to generate the correct baudrate, and reprograms the SCC to UART mode.
Many rates are supported including: 150, 300, 600, 1200, 2400, 4800, 9600, 14.4K, 19.2K,
38.4K, 57.6K, 64K, 96K, and 115.2K. To estimate the performance of the AutoBaud microcode package, the performance table in Appendix A of the MC68302 user’s manual can be used. The maximum full-duplex rate for a BISYNC channel is one-tenth of the system clock rate. So a 16.67
MHz 68302 can support 115.2K AutoBaudrate with another low-speed channel (<50 kbps) and a
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20 MHz MC68302 can support 115.2K AutoBaudrate with 2 low-speed channels. The performance can vary depending on system loading, configuration, and echoing mode.
It is important that the highest priority SCC be used for the AutoBaud function, since it is running at a very high rate. Any SCC that is guaranteed to be idle during the search operation of the AutoBaud process will not impact the performance of Autobaud in an application. Idle is defined as not having any transmit or receive requests to/from the SCC FIFOs.
1.2 AutoBaud Channel Reception Process
The interface between the AutoBaud controller and the host processor is implemented with shared data structures in the SCC Parameter RAM and in external memory and through the use of a special command to the SCC.
The AutoBaud Controller uses Receive Buffer Descriptor number 7 (Rx BD7) for the
AutoBaud Command Descriptor. This Rx BD is initialized by the Host to contain a pointer to a lookup table residing in the external RAM (contains the maximum and nominal START bit length for each Baudrate). The Host also prepares two characters against which the AutoBaud Controller will compare the received character (usually these characters are ‘a’ and ‘A’ ), and the host initializes a pointer to a buffer in external memory where the assembled characters will be stored until the host stops the AutoBaud process. Finally, the host initializes the SCC Data
Synchronization Register (DSR) to $7FFF in order to synchronize on the falling edge of the start bit.
Once the data structures are initialized, the host programs the SCON register to provide a sampling clock that is 16X the maximum supported baudrate. The Host then issues the
Enter_Baud_Hunt command and enables the SCC in the BISYNC mode.
The AutoBaud Controller reception process starts when the START bit arrives. The
AutoBaud Controller then starts to measure the START bit length. With each byte received from the SCC that “belongs” to the START bit, the AutoBaud Controller increments the Start Length
Counter and compares it to the current lookup table entry. If the Start Length Counter passed the maximum bit length defined by the current table entry, the AutoBaud Controller switches to the next lookup table entry (the next slower baudrate). This process goes on until the AutoBaud
Controller recognizes the end of the Start bit. Then, the AutoBaud controller starts the character assembly process.
The character assembly process uses the nominal bit length, taken from the current lookup table entry, to sample each incoming bit in it’s center. Each bit received is stored to form an 8 bit character. When the assembly process is completed (a Stop bit is received), the character is compared against two user-defined characters.
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If the received character does not match any of the two user defined characters, the AutoBaud
Controller re-enters the Enter_Baud_Hunt process. The host is not notified until a match is encountered.
If a match is found, the character is written to the Received Control Character Register
(RCCR) with the corresponding status bit set in Rx BD7. The channel will generate the Control
Character Received (CCR) interrupt (bit 3 in the SCCE), if enabled. If the character matched, but a framing error was detected on the stop bit, the AutoBaud Controller will also set the framing error status bit in Rx BD7.
The AutoBaud Controller then continues to assemble the incoming characters and to store them in the external data buffer. The host receives a CCR interrupt after each character is received.The host is responsible for determining the end of the incoming message (for example, a carriage return), stopping the Autobaud Process, and reprogramming the SCC to UART mode.
The AutoBaud Controller returns the nominal start bit length value for the detected baudrate from the lookup table and a pointer to the last character received that was written to the external data buffer. The host must be able to handle each character interrupt in order to determine parity and character length (this information may be overwritten when the next character interrupt is presented to the host). The host uses the two received characters to determine 1) whether a properly formed “at” or “AT” was received, and 2) the proper character format (character length, parity).
Once this is decided, three possible actions can result. First, the host may decide that the data received was not a proper “at” or “AT”, and issue the Enter_Baud_Hunt command to cause the
AutoBaud controller to resume the search process. Second, the host may decide the “at” or “AT” is proper and simply continue to received characters in BISYNC mode. Third, the M68000 core may decide that the “at” or “AT” is proper, but a change in character length or parity is required.
1.3 AutoBaud Channel Transmit Process
The AutoBaud microcode package supports two methods for transmission. The first method is automatic echo which is supported directly in the SCC hardware, and the second method is a smart echo or software transmit which is supported with an additional clock and software.
The automatic echo requires Revision C or later of the MC68302. Automatic echo is enabled by setting the DIAG bits in the SCC Mode Register (SCM) to ‘10’ and asserting the CD pin
(externally on SCC1 and on SCC2 and SCC3, either externally or by leaving the pin as a general purpose input). The ENT bit of the SCC should remain cleared. The transmitter is not used, so this echoing method does not impact performance.
The smart echo or software transmit requires use of an additional clock and the transmitter, so the overall performance could be affected if other SCCs are running. This method requires an additional clock for sampling the incoming bit stream since the baudrate generator (BRG) must be
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used to provide the correct frequency for transmission. The user needs to provide the sampling clock that will be used for the AutoBaud function on the RCLK pin (for example, a 1.8432 MHz clock for 115.2K). The clock that will be used for the SCC transmission can be provided to the
BRG from the system clock or on TIN1.The TIN1 and RCLK1 pins can be tied together externally. After the first two characters have been received and character length and parity determined, the host programs the DSR to $FFFF, enables the transmitter (by setting ENT), and programs the transmit character descriptor (overlays CONTROL Character 8). The host is interrupted after each character is transmitted.
1.4 Autobaud Parameter RAM
When configured to operate in the AutoBaud mode, the MC68302 overlays some entries of the
UART-Specific Parameter RAM as illustrated in Table 1 on page 6.
Table 1. AutoBaud Specific Parameter
Address Name
SCC Base + 9C * MAX_IDL
SCC Base + 9E MAX_BIT
SCC Base + A0 NOM_START
SCC Base + A2 * PAREC
SCC Base + A4 * FRMEC
SCC Base + A6 * NOSEC
SCC Base + A8 * BRKEC
SCC Base + AA * ABCHR1
SCC Base + AC * ABCHR2
SCC Base + AE RCCR
SCC Base + B0 * CHARACTER1
SCC Base + B2 * CHARACTER2
SCC Base + B4 * CHARACTER3
SCC Base + B6 * CHARACTER4
SCC Base + B8 * CHARACTER5
SCC Base + BA * CHR6/RxPTR
SCC Base + BC * CHR7RxPTR
SCC Base + BE * CHR8/TxBD
Width
Word
Word
Word
Word
Word
Word
Word
Word
Word
Word
Word
Word
Word
Word
Word
Word
Word
Word
Description
Maximum IDLE Characters
Current Maximum Start Bit Length
Current Nom. Start Bit (used to determine baudrate)
Receive Parity Error Counter
Receive Framing Error Counter
Receive Noise Counter
Receive Break Error Counter
User Defined Character1
User Defined Character2
Receive Control Character Register
CONTROL Character1
CONTROL Character2
CONTROL Character3
CONTROL Character4
CONTROL Character5
CONTRChar6/MSW of pointer to external Rx Buffer
CONTRChar7/LSW of pointer to external Rx Buffer
CONTROL Character8/Transmit BD
*– These values should be initialized by the user (M68000 core).
Note the new parameters that have been added to the table. They are MAX_BIT,
NOM_START, ABCHR1, ABCHR2, RxPTR (2 words), and TxBD. These parameters are of special importance to the AutoBaud controller. They must be written prior to issuing the
Enter_Baud_Hunt command.
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When the channel is operating in the AutoBaud hunt mode, the MAX_BIT parameter is used to hold the current maximum start bit length. The NOM_START location contains the current nominal start from the LookUp Table. After the AutoBaud is successful and the first character is matched, the user should use the NOM_START value from the Autobaud Specific Parameter
RAM to determine which baudrate from the LookUp table was detected. Also the Tx Internal
Data Pointer (at offset SCC Base + 94) will point to the last character received into external data buffer.
NOTE
When the channel is operating in the UART mode, the
NOM_START_/BRKCR is used as the Break Count Register and must be initialized before a STOP_TRANSMIT command is issued.
The characters ABCHR1 and ABCHR2 are the AutoBaud characters that should be searched for by the AutoBaud controller. Typically these are ‘a’ and ‘A’ (i.e. $0061 and $0041) if using the
Hayes command set. These characters must be odd in order for the AutoBaud controller to correctly determine the length of the start bit. Characters are transmitted and received least significant bit first, so the AutoBaud controller detects the end of the Start Bit by the least significant bit of the character being a ‘1’.
The RxPTR is a 2 word location that contains a 32-bit pointer to a buffer in external memory used for assembling the received characters and must be initialized before the Enter_Baud_Hunt command is issued. WARNING: Since a length for this external buffer is not given, the user must provide enough space in memory for characters to be assembled and written until the Autobaud process is to avoid overwriting other data in memory. This location is not used as the
CHARACTER7 value in the control character table until the channel operates in normal UART mode. After reception begins in normal UART mode (i.e. the “a” or “A” is found), this entry is available again as a Control Character Table entry.
The TxBD entry is used as the transmit character descriptor for smart echo or software transmit. This location is not used as the CHARACTER8 value in the control character table until the channel operates in normal UART mode. After reception begins in normal UART mode (i.e. the “a” or “A” is found), this entry is available again as a Control Character Table entry.
1.5 AutoBaud Programming Model
The following sections describe the details of initializing the Autobaud microcode, preparing for the AutoBaud process, and the memory structures used.
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1.5.1 Initialization of the AutoBaud Microcode Package
After a power up or hardware reset, the following steps should be followed. Step 2 is not required after a hardware reset unless the dual-port RAM has been modified by the user after the reset.
1. Write $0700 to the BAR register. The base address of the internal ram after this action is $700000. If a different base address is used, then the S record file should be modified to the selected address.
2. Load the S-records file into the internal ram.
3. Write $0001 to address $0F8 in the supervisory space to enable the Autobaud package.
The user will no longer be able to read or write the dual-port RAM.
4. Write a software reset command to the CR register.
5. Continue with the normal initialization sequence.
NOTE: This initialization sequence is for REV B or later parts (mask 2B14M or later).
1.5.2 Preparing for the AutoBaud Process
The Host begins preparation for the Autobaud process with the following steps. Steps 1 and 2 are required if the SCC has been used after reset or after UART mode in order to re-enable the process.
1. Disable the SCC by clearing the ENR and ENT bits. (The host may wish to precede this action with the STOP_TRANSMIT commands to abort transmission in an orderly way).
2. Issue the ENTER_HUNT_MODE comand to the SCC (This ensures that an open buffer descriptor is closed).
3. Set up all the AutoBaud parameters in the AutoBaud specific parameter RAM shown in
Table 1 on page 6, the AutoBaud command descriptor shown in Table 2 on page 10, and the
LookUp Table shown in Table 3 on page 12. Of these three areas, the AutoBaud controller only modifies the AutoBaud specific parameter RAM and the first word of the AutoBaud command descriptor during its operation.
4. Write the SCON to configure the SCC to use the baudrate generator clock of 16x the maximum supported baudrate. A typical value is $4000 assuming a 1.8432 MHz clock rate on
TIN1 and a maximum baudrate of 115.2K, but this can change depending on the maximum baudrate and the EXTAL frequency.
6. Write the DSR of the SCC with the value $7FFF in order to detect the start bit.
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7. The Host initiates the AutoBaud search process by issuing the Enter_Baud_Hunt command
8. Write the SCM of the SCC with $1133 to configure it for BISYNC mode, with the REVD and RBCS bits set, software operation mode, and the transmitter disabled. After a few characters have been received, the transmitter can be enabled, and the software echo function may be performed after issuing the RESTART TRANSMIT command.
In general, the AutoBaud controller uses the same data structure as that of the UART controller. The first character (if matched) is stored in the Receiver Control Character Register and the external data buffer, and the status of that character is reported in the AutoBaud Command
Descriptor. After the first character, each incoming character is then stored in the buffer pointed to by RxPTR, and the status is updated in the Autobaud Command Descriptor. The Tx Internal Data
Pointer (at offset SCC Base + 94) is updated to point to the last character stored in the external data buffer.
1.5.3 Enter_Baud_Hunt Command
This command instructs the AutoBaud Controller to begin searching for the baudrate of a user predefined character. Prior to issuing the command the M68000 prepares the AutoBaud
Command Descriptor to contain the lookup table size and pointer.
The Enter_Baud_Hunt uses the GCI command with opcode = 11, and the channel number set for the corresponding SCC. For example, with SCC1, the value written to the Command Register would be $71.
1.5.4 AutoBaud Command Descriptor
The AutoBaud Controller uses the Receive Buffer Descriptor number 7 (Rx BD7) as an
AutoBaud Command Descriptor. The AutoBaud Command Descriptor is used by the M68000 core to transfer command parameters to the AutoBaud Controller, and by the Autobaud Controller to report information concerning the received character.
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The structure of the AutoBaud Command descriptor for the AutoBaud Process is shown in
Table 2. The first word of the descriptor or the status word is updated after every character is received.
Table 2. Autobaud Command Descriptor
4 Offset 15 14 13 12 11 10
2
4
6
8
Function Code
9 8 7 6
FE M2 M1
LookUp Table Size
LookUp Table Pointer
5 3
EOT
2 1 0
OV CD
FE – Framing Error (Bit 10)
If this bit is set, a character with a framing error was received. A framing error is detected by the AutoBaud Controller when no stop bit is detected in the received data. FE will be set for a 9 bit character (8 bits + parity) if the parity bit is ‘0’. NOTE : the user must clear this bit when it is set.
M2 - Match Character2 (Bit 9)
When this bit is set, the character received matched the User Defined Character 2. The received character is written into the Receive Control Character Register (RCCR).
M1 - Match Character1 (Bit 8)
When this bit is set, the character received matched the User Defined Character 1. The received character is written into the Receive Control Character Register (RCCR).
EOT - End Of Table (bit 3)
When this bit is set, the AutoBaud Controller measured start length exceeded the maximum start length of the last entry in the LookUp table (lowest baudrate). NOTE : the user must clear this bit when it is set.
OV - Overrun (bit 1)
If this bit is set, a receiver overrun occurred during AutoBaud reception. NOTE : the user must clear this bit when it is set.
CD - Carrier Detect Lost (bit 0)
If this bit is set, the carrier detect signal was negated during AutoBaud reception. NOTE : the user must clear this bit when it is set.
Lookup Table Size -
LookUp Table Size is the number of baudrate entries in the external lookup table.
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LookUp Table Pointer -
The LookUp Table Pointer is the address in the external RAM where the lookup table begins. WARNING: The LookUp Table cannot cross a 64k memory block boundary.
1.5.5 AutoBaud LookUp Table
The AutoBaud Controller uses an external LookUp table to determine the baudrate while in the process of receiving a character. The LookUp table contains two entries for each supported baudrate. The first entry is the maximum START length for the particular baudrate, and the second entry is the nominal length for a 1/2 START bit.
To determine the two values for each table entry, first calculate the AutoBaud sampling rate
(EQ 2). To do this EQ 1 must be used until EQ 2 is satisfied. The sampling rate is the lowest speed baudrate that can be generated by the SCC baudrate generator that is over a threshold defined in EQ 2.
BRG Clk Rate = System Clock or TIN1 / ((Clock Divider bits in SCON) + 1) (EQ 1) assuming that the DIV bit in SCON is set to 0, (otherwise an additional “divide-by-4” must be included).
Sampling Rate = BRG Clk Rate, where BRG Clk rate >= (Max Desired UART Baud Rate) x 16 (EQ 2)
For instance, if a 115.2K baudrate is desired, with a 16.67 MHz system clock, the minimum sampling rate possible is 1.843 MHz = 115.2K x 16. This exact frequency can be input to RCLK1 or TIN1 as the sample clock. If the system clock is to be used, a 16.67 MHz system clock cannot produce an exact baudrate clock of 1.843 MHz. The lowest one that can be used is Baud Rate =
16.67 MHz / (7+1) = 2.083 MHz. Thus, 2.083 MHz is the sampling rate, and the SCON should be set to $000e to produce this.
Once the sampling rate is known, the other two equations follow easily. The maximum
START bit length is calculated by the following equation:
Maximum START length = (Sampling Rate/Recognized baudrate) x 1.05
(EQ 3)
Thus, for the first entry in the table, the Maximum Start Length is 1.8432 Mhz/115200 x 1.05
= 17 for an external sample clock. The value 1.05 is a suggested margin that allows characters
5% larger than the nominal character rate to be accepted. In effect, the margin determines the
“split point” between what is considered to be a 56.7K character rate and what is a 38.4K character rate. The margin should not normally be less than 1.03 due to clocking differences between UARTs.
The nominal START bit length is calculated by:
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Nominal START length = (Sampling Rate/Recognized baudrate) / 2 (EQ 4)
8.
For the 115.2K example, in the first entry of the table, this would be 1.8432 MHz/115.2K/2 =
The structure of the LookUp table is shown in Table 3 on page 12. The table starts with the maximum UART baudrate supported and ends with the minimum UART baudrate supported.
Table 3. AutoBaud LookUp Table Format
OFFSET from LookUp Table
Pointer
0
2
4
6
•
•
(LookUp Table Size - 1) * 4
[(LookUp Table Size - 1) * 4] + 2
DESCRIPTION
Maximum START Length
Nominal START Length
Maximum START Length
Nominal START Length
Maximum START Length
Nominal START Length
Maximum START Length
Nominal START Length
NOTE
If less margin is used in the calculation of the Maximum START length above, it is possible to distinguish between close UART rates such as 64K and 57.6K. However variations in RS232 drivers of up to 4%, plus nominal clocking rate variations of 3%, plus the fact that the sampling rate may not perfectly divide into the desired UART rate, can make this distinction difficult to achieve in some scenarios.
1.5.5.1
LookUp Table Example
Table 4 is an example AutoBaud Lookup Table. The Maximum Start and Nominal Start values are derived assuming a 1.8432 MHz sampling clock on TIN1 or RCLK and a shift factor of 5%.
Table 4. LookUp Table Example
Desired
Baudrate
115200
57600
38400
28800
19200
14400
12000
9600
Maximum
Start
17
34
50
67
101
134
161
202
Nominal
Start
8
16
24
32
48
64
77
96
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Table 4. LookUp Table Example
Desired
Baudrate
7200
4800
2400
1200
600
300
110
Maximum
Start
269
403
806
1613
3226
6451
17594
Nominal
Start
128
192
384
768
1536
3072
8378
1.5.6 Determining Character Length and Parity
Table 5 on page 13 shows the different possible character lengths and parity that will be discussed. The following paragraphs will discuss for each case how to determine the parity.
Table 5. Character Lengths and Parity Cases
Case #
1
Character
Length
7-bit
Parity Notes
2
3
4
5
7-bit
8-bit
8-bit
8-bit no parity, 1 stop bit Not Supported even parity odd parity parity=1 parity=0 no parity
Parity is indicated by the most significant bit of the byte same as 7-bit, parity=0 even parity odd parity parity=0 parity=1
Parity is indicated by which characters generate a FE interrupt
Not Supported
• Case 1– This case can not be supported because the Autobaud can not separate the first character from the second character.
• Case 2– As each character is assembled, it is stored into a complete byte. Assuming that the characters are ASCII characters with 7-bit codes, the 8th bit of the byte will contain the parity bit. If the parity is either even or odd, then after receiving an odd character and an even character, the 8th bit should be different for the odd and even characters. The parity can be determined by the setting of the parity bit for one of the two characters. If the 8th bit is always a 1, this is the same as a 7-bit character, no parity and at least 2 stop bits or a 7-bit character with force 1 parity. If the 8th bit is always a zero, then either the character is a 7-bit character with force 0 parity, or the character is a 8-bit character with no parity.
• Case 3– This case is the same as 7-bit character with force 0 parity. The 8th bit of the byte will always be zero.
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• Case 4–This case assumes a 8-bit character with the 8th bit of the character equal to a 0 (ASCII character codes define the 8th bit as zero). If the parity is either even or odd, then after receiving an odd and an even character, a framing error (FE) interrupt should have been generated for one of them (the interrupt is generated when the parity bit is zero). The user can determine the parity by which character generated a FE interrupt (if the odd character did, then the parity is odd). If a framing error occurs on every character, then the character is 8-bits with force 0 parity. If no framing error occurs, than this is the same as Case 5.
• Case 5– This case is not supported, because it can not be differentiated from 7-bit force 0 parity and 8-bit no parity. If the 9th bit is a 1, then it will be interpreted as a stop bit.
1.5.7 AutoBaud Reception Error Handling Procedure
The AutoBaud controller reports reception error conditions using the AutoBaud Command
Descriptor. Three types of errors are supported:
• Carrier Detect Lost during reception
When this error occurs and the channel is not programmed to control this line with software, the channel terminates reception, sets the Carrier Detect lost (CD) bit in the
Command Descriptor, and generates the CCR interrupt, if enabled. CCR is bit 3 of the
SCCE register.
• Overrun Error
When this error occurs, the channel terminates reception, sets the overrun (OV) bit in the
Command Descriptor, and generates the CCR interrupt, if enabled.
• End Of Table Error
When this error occurs, the channel terminates reception, sets the end of table (EOT) bit in the Command Descriptor, and generates the CCR interrupt, if enabled.
Any of these errors will cause the channel to abort reception. In order to resume AutoBaud operation after an error condition, the M68000 should clear the status bits and issue the
Enter_Baud_Hunt command again.
1.5.8 AutoBaud Transmission
The autobaud package supports two methods for echoing characters or transmitting characters. The two methods are automatic echo and smart echo.
1.5.8.1
Automatic Echo
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This method uses the SCC hardware to automatically echo the characters back on the TxD pin.
The automatic echo is enabled by setting the DIAG bits in the SCM to ‘10’. On Revision C and later of the MC68302, the transmitter should not be enabled. The hardware echo is done automatically. The CD pin needs to be asserted in order for the characters to be transmitted back.
On SCC1, the external CD pin must be tied low. On SCC2 and SCC3, either the external CD pin must be tied low or the CD pins should be left configured as general purpose input pins (the CD signal to the SCC is then connected to ground internally).
Using the automatic echo, the receiver still autobauds correctly and performance is not affected. The SCC echos the received data with a few nanoseconds delay.
1.5.8.2
Smart Echo
This method requires addition hardware and software to implement. The user must provide two clock sources. One clock source is the sample clock which is input on RCLK and cannot be divided down. The BRG is used to divide the second clock down to provide the clock used for transmit. The second clock can be either the system clock or a clock connected to TIN1. The TIN1 and RCLK pins can be connected to each other externally.
After the first character is received, the user must take the following steps:
1. Determine the baudrate from the returned NOM_START value and program SCON to (input frequency/baudrate)-1, where the input frequency is either the system clock or the clock on
TIN1.
2. Program the DSR to $FFFF. The DSR will need to be programmed back to $7FFF before the
Enter_Baud_Hunt command is issued again.
3. Set the ENT bit in the mode register.
4. Program the transmit character BD as show in Table 6 on page 15.
Table 6. Transmit Character BD
15 14 13 12 11 10
R CL PE PM
9 8 7 6 5 4 3
CHAR
2 1 0
R (ready bit)
0 = Character is not ready
1 = Character is ready to transmit
CL (character len)
0 = 7 bits + parity or 8 bits with no parity
1 = 8 bits + parity
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PE (parity enable)
0 = no parity
1 = parity
PM (parity mode)
0 = even parity
1= odd parity
The AutoBaud controller issues a Tx interrupt after each character is transmitted.
1.5.9 Reprogramming to UART Mode or another protocol
The following steps should be followed in order to switch the SCC from AutoBaud to UART mode or to another protocol.
• Disable the SCC by clearing ENR and ENT.
• Issue the Enter_Hunt_Mode command.
• Initialize the SCC parameter RAM (specifically, the Rx and Tx internal states and the words containing the Rx and Tx BD#s) to the state they were at after reset and initialize the Protocol Specific Parameter Area for the new protocol.
• Re-enable the SCC with the new mode.
NOTE
This initialization sequence is for REV B or later parts (mask
2B14M or later).
1.6 AutoBaud Programming Example
The following program is a simple working example that shows how to initialize the autobaud controller and handle the interrupts. This was run on a 68302 ADS board with a 16.67 MHz system clock and a 1.8432 MHz clock on TIN1 for the sampling clock.
***************************** EQU TABLE *****************************
* The following three values are application dependent
BASE
INIT
EQU
EQU
$0700000
$0030300
; This is set according to the value in BAR
; Initialization Routine
INT_VEC EQU
LOOKUP EQU
LK_SZ EQU
RXCHARS EQU
$0031000
$0030200
15
$4000
; Interrupt Vector for SCC1
; Look up table for baud rate
; Number of entries in the lookup table
; Pointer to receive buffer for characters
NOMSP EQU
RXINTPT EQU
TXINTPT EQU
BASE+$4A0
BASE+$488
BASE+$494
; Nominal speed reported for SCC1
; Pointer to internal receive pointer
; Pointer to internal transmit pointer
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CARRT
* Commands
RSTCP
STPTX1
RSTX1
ENHM1
SCON1
SCM1
DSR1
SCCE1
SCCM1
ENRON
ENTON
EQU
EQU
EQU
EQU
EQU
ENBHM1 EQU
CR
BAR
MER
SCR
GIMR
IPR
IMR
ISR
PACNT
PBCNT
EQU
EQU
SIMODE EQU
EQU
EQU
EQU
EQU
EQU
EQU
EQU
EQU
EQU
EQU
EQU
EQU
EQU
EN_SCC EQU
EQU
EQU
ENTOFF EQU
$0F
$81
$01
$11
$21
$071
BASE+$0860
$0F2
$0F8
$0F4
BASE+$0812
BASE+$0814
BASE+$0816
BASE+$0818
BASE+$081E
BASE+$0824
BASE+$08B4
*** SCC Parameter Table ***
ST_BD EQU 0
SS_BD
LN_BD
EQU
EQU
1
2
PT_BD
SZ_BD
EQU
EQU
4
$08
READY
EMPTY
WRAP
EQU
EQU
EQU
$07
$07
$05
* SCC1 Parameter *
FCR_1 EQU
MRBLR_1 EQU
MIDL_1 EQU
BRKCR_1 EQU
PAREC_1 EQU
FRMEC_1 EQU
NOSEC_1 EQU
BRKEC_1 EQU
UADR1_1 EQU
UADR2_1 EQU
RCCR_1 EQU
RCH1_1
RCH6_1
RCH8_1
EQU
EQU
EQU
BASE+$0882
BASE+$0884
BASE+$0886
BASE+$0888
BASE+$088A
$0C
$08
$04
$0FFFB
BASE+$0480
BASE+$0482
BASE+$049C
BASE+$04A0
BASE+$04A2
BASE+$04A4
BASE+$04A6
BASE+$04A8
BASE+$04AA
BASE+$04AC
BASE+$04AE
BASE+$04B0
BASE+$04BA
BASE+$04BE
; ASCII for Carriage Return
; Reset CP
; STOP TRANSMIT SCC1
; RESTART TRANSMIT SCC1
; ENTER HUNT MODE SCC1
; ENTER BAUD HUNT MODE SCC1
; Command Register
; Base Address Register
; UCODE Option Enabling Register
; System Control Register
; Global Interrupt Mode Register
; Interrupt Pending Register
; Interrupt Mask Register
; In-Service Register
; PORT A CONTROL
; Serial Interface Mode Register
; SCC1 Configuration Register
; SCC1 Mode Register
; SCC1 DSR Register
; SCC1 Event Register
; SCC1 Mask Register
; ENR and ENT bits in SCM
; For Setting ENR
; For Setting ENT
; For Clearing ENT
; Status and Control in BD
; Status in BD
; Data Length in BD
; Buffer pointer in BD
; Size of BD = 8 bytes
; Ready bit in the 1st byte of TX BD
; Empty bit in the 1st byte of RX BD
; Wrap bit in the 1st byte of BD
; RFCR and TFCR for SCC1
; Max Rx Buffer Length
; Max IDLE char (received)
; Break counter (transmitted)
; RX Parity error
; RX Framing error
; RX Noise counter
; RX Break condition counter
; UART Address char 1
; UART Address char 2
; RX Control Char
; CONTROL char 1
; L word to hold ptr to char buffer
; CONTROL char 8
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* Autobaud special
ABCR1_1 EQU
ABCR2_1 EQU
RxPTR EQU
TxBD EQU
RXBD_01 EQU
TXBD_01 EQU
RXBD_71 EQU
BASE+$04AA
BASE+$04AC
BASE+$04BA
BASE+$04BE
BASE+$0400
BASE+$0440
BASE+$0438
; User defined Char 1 (a/A)
; User defined Char 2 (a/A)
; Pointer to external Rx data buffer
; Transmit character descriptor
; 1st RXBD of SCC1
; 1st TXBD of SCC1
; 7th RXBD of SCC1 for Autobaud command
* The followings are application dependent, for this example
BD_CNT EQU $08 ; Number of BDs used
SZ_BF
N_DATA
EQU
EQU
$10
$0A
; Size of buffer = 16 bytes
; Number of data to be sent in a buffer
******************************* Data Used ******************************
DC.W
DC.W
DC.W
DC.W
DC.W
DC.W
DC.W
DC.W
DC.W
DC.W
DC.W
DC.W
DC.W
DC.W
DC.W
ORG
DC.W
DC.W
DC.W
DC.W
DC.W
DC.W
DC.W
DC.W
DC.W
DC.W
DC.W
DC.W
DC.W
DC.W
DC.W
96
269
128
403
192
806
384
1613
768
3226
1536
6451
3072
17594
8378
32
101
48
134
64
161
77
202
16
50
24
67
LOOKUP
17
8
34
;115200
;57600
;38400
;28800
;19200
;14400
;12000
;9600
;7200
;4800
;2400
;1200
;600
;300
;110
; Semaphore for character reception RXFLAG DC.W
0
******************** M68302 Initialization Code *****************
* Register Initialized values in ADS board before execution
* USP=00080000 ISP=000040000 (Stack pointer not used)
* This program assumes that the AutoBaud microcode package has
* already been downloaded into the internally dual-port RAM.
ORG INIT
MOVE.W
#$2700,SR
; PC=00030300
; SR=2700, mask off interrupts
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LP00
MOVE.W
#$0700,BAR
LEA.L
$40000,A7
MOVE.W
#$0001,MER
MOVE.B
#RSTCP,CR
BTST.B
BNE.B
#0,CR
LP00
MOVE.L
#0,SCR
*** Setups for interrupt ***
MOVE.W
#$0A0,GIMR
MOVE.W
#0,IMR
MOVE.W
#$FFFF,IPR
*** Set up Serial Interface Connection ***
MOVE.W
#$0008,PBCNT
* Set up SIMODE = NMSI Normal operation
MOVE.W
#$0,SIMODE
; BAR=0700
; Load initial stack pointer
; Enable Ucode for autobaud
; Reset CR of communication processor
; Nothing special for this example
; Normal mode, v7-v5=5
; Mask off all for now
; Clear IPR
; FOR TIN1 INSTEAD OF PB3
***************** SCC1 Initialization for Autobaud ********************
* Interrupt Vector: SCC1 interrupt handler is at INTSCC1
* v7-v5=5, v4-v0=$0d => vector=$ad => Exception vector = ($ad<<2) = $2b4
MOVE.L
#INTSCC1,$02B4
************************* begin autobaud *****************************
MOVE.W
#$4000,SCON1 ; Set initial divider
MOVE.W
#$7FFF,DSR1 ; Initialize DSR to look for START bit
* Set SCC1 Mode for autobaud - BISYNC mode, s/w operation, REVD and RBCS set.
MOVE.W
#$1133,SCM1
***************************** end autobaud ***************************
*
*
* Set up Parameter RAM
MOVE.W
#0,FCR_1
MOVE.W
#$0A,MRBLR_1
MOVE.W
#$4,MIDL_1
; Clear RFCR and TFCR
; MAX BUFFER length
; MAX_IDL
* The following must be executed before the STOP Transmit command is issued
MOVE.W
#$1,BRKCR_1 ; BRKCR = 1.Only one break char. sent if
STOP TRANSMIT command is executed.
* Init counters
MOVE.W
#$0,PAREC_1
MOVE.W
#$0,FRMEC_1
MOVE.W
#$0,NOSEC_1
MOVE.W
#$0,BRKEC_1
* Put 'a' and 'A' into user defined characters.
MOVE.W
MOVE.W
#$61,ABCR1_1
#$41,ABCR2_1
* Set default SCC mode: Uart, 8-char, no parity, 1 stop, s/w operation
MOVE.W
#$013D,D_SCM_1
* UART Address characters not used in normal operation.
MOVE.W
#$8000,RCH1_1
; ASCII for 'a'
; ASCII for 'A'
MOVE.W
#$0000,RCH8_1 ; no software echoing in this example
* No control characters are initialized to cause special interrupts.
* The flow-control facility (XON-XOFF) is not used.
MOVE.B
#$FF,SCCE1 ; clear SCCE1
MOVE.B
#$08,SCCM1 ; only interrupt for RX control character
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**********************************************************************
*** Prepare Buffer Descriptors ***
* Clear M68000 data registers
CLR.L
D0
CLR.L
CLR.L
D1
D2
CLR.L
CLR.L
CLR.L
D3
D4
D5
* SCC1 Rx Buffer Descriptors Initialization values before execution.
* SCC1 Autobaud Descriptors Initialization values before execution:
* 00700430 0000 0008 0002 0000
LEA.L
LEA.L
RXBD_71,A0
RCH6_1,A1
; A0 points to autobaud command descriptor
; A1 points to the LOOKUP table for autobaud
MOVE.W
#$0,ST_BD(A0) ; Clear all Status Bits
MOVE.W
#LK_SZ,LN_BD(A0) ; Initialize Lookup Table Size
MOVE.L
MOVE.L
#LOOKUP,PT_BD(A0) ; Load pointer to Lookup table
#RXCHARS,(A1) ;Load the pointer to rx characters
*
*
* Special function for Auto baud Mode
* At this time the SCCs are still disabled.
* If autobaud is entered dynamically, one should first do the followings
* before issuing ENTER BAUD HUNT for the SCC that is going to run autobaud:
* (1) STOP TRANSMIT, (2) disable ENT and ENR, and (3) ENTER HUNT MODE.
LP1
MOVE.B
BTST.B
#ENBHM1,CR
#0,CR
BNE.B
ORI.W
LP1
#ENRON,SCM1 ; For SCC1 ENR=1, the transmitter will be automatically enabled by the microcode once it sets up the UART-autobaud-mode.
; Enter BAUD HUNT mode
; Wait for the command to complete
* Ready to accept interrupt*** Set IMR ***
MOVE.W
MOVE.W
JMP
#$2000,IMR
#$2000,SR
MAIN
; Allow SCC1 interrupts only.
; Unmask interrupt
; Go to Main routine for Tx and Rx
**************************** Main Routine *********************************
* Set up BD pointers
MAIN CLR.L
D5 ; COUNTER FOR OVER-RUN
* The following clears out the external data buffer for received characters
RxReady LEA.L
RXCHARS,A5
MOVE.W
#$20,D4
CLRLP MOVE.L
DBNE
#0,(A5)+
D4,CLRLP
; Load address of receive buffer
; Load counter for clear loop
; Clear out the memory
LEA.L
RXCHARS,A5 ; Load address of character buffer
*Wait for character to be received.
WAITF CMPI.W
#0,RXFLAG
BNE
BTST.B
RX_REC
#1,RXBD_71+1
BEQ
ADDI.W
WAITF
#1,D5
;COMPARE COUNTER
; CHECK OV
; INCREMENT OVERRUN COUNTER
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* This just returns to wait for the next character.
* At this point or in the interrupt handler, the user should put code
* that checks for character length and parity as well as the end of
* message.
*
RX_REC BRA WAITF
* When the end of message is detected, the user is responsible for
* the following:
* (1) STOP TRANSMIT, (2) disable ENT and ENR, (3) ENTER HUNT MODE,
* (4) write the Rx and Tx internal states back to their reset values,
* (5) write the Rx and Tx buffer numbers back to their rest values,
* (6) program the SCON register for the correct baudrate (returned at base + $4A0)
* (7) program the SCC mode to UART mode with the correct character length and parity
* (8) issue the RESTART Transmit and Enter Hunt Mode commands
* and (9) and enable the receiver and transmitter.
***************** SCC1 Interrupt handler ***************************
* This routine only handle the case when RX=1
*
ORG
INTSCC1 CLR.L
MOVE.B
ANDI.W
INT_VEC
D1
SCCE1,D1
#8,D1
; Interrupt Vector for SCC1
; Clear D1
; SCCE1 => D1
; Is CCR set?
CMPI.W
BNE.B
#0,D1
RXINT1
; If set
; Handle receiver's interrupt
*
* The handling of other events, e.g., CTS, CD IDL, BSY, is left to
* the users as desired.
*
EXINT1 MOVE.W
RTE
#$2000,ISR ; Clear SCC1 bit in ISR
************* Receiver portion of SCC1 interrupt routine *************
* This routine handles received (non-empty) BD: set data length = 0,
* clear status bits, set empty = 1, and update PRD
*
* Clear the identified events as soon as possible, so as not to lost
* events occur during the interrupt handling
RXINT1
*
MOVE.B
#8,SCCE1 ; Clear RX in SCCE1
* While Not-Empty continue to process the next Character, Else Exit.
*
NXPRD1 MOVE.L
CMP.L
A5,D1
TXINTPT.L,D1
; Load current pointer
; Check to see if a char was rx
BEQ
MOVE.B
CMP.B
BEQ
BRA
EXINT1
(A5)+,D1
#CARRT,D1
EXRX1
NXPRD1
; If no, then exit
; Load Character
; Is the char a carriage return
; If so, then set flag and exit
; Otherwise check for another char
EXRX1 MOVE.W
#1,RXFLAG
JMP EXINT1
; SET SEMAPHORE
; Exit receiver portion of the handler
**********************************************************************
END
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