MC68HC58 Motorola, MC68HC58 Datasheet - Page 64

• Manufacturer Part Number: MC68HC58
• Description: Data Link Controller
• Manufacturer: Motorola

4.5 Block Mode Operation
4-20
MOTOROLA
The DLC has the capability of transmitting and receiving message frames which ex-
ceed the frame length specified by J1850. These “block mode” messages are typically
used in a production or diagnostic environment, and not during normal operation.
When operating in block mode, the DLC can transmit or receive message frames con-
taining an unlimited number of data bytes.
The design of the DLC receiver allows it to receive block mode frames into the RxFIFO
buffer in a manner identical to normal J1850 frames. As each data byte is received
from the J1850 bus, it is placed in the RxFIFO buffer. Throughout reception of the
block mode frame, the status byte indicates to the host MCU the status of the RxFIFO
buffer. Also, if interrupts are enabled, the DLC generates a host MCU interrupt each
time the number of data bytes in the RxFIFO buffer exceeds 12.
As long as the host MCU retrieves the received frame bytes quickly enough to prevent
the RxFIFO buffer from overflowing, the DLC continues to place received data bytes
into the RxFIFO buffer. Once the EOD symbol is received from the J1850 bus, the
CRC byte of the frame is verified, and the appropriate completion code is placed in the
RxFIFO buffer. Following this, the status byte indicates that a complete message
frame is in the RxFIFO buffer, and a host MCU interrupt, if enabled, is generated. This
indicates to the host MCU that the block mode message frame is complete.
If the RxFIFO buffer does overflow, any remaining bytes of the frame are lost, and a
completion code is placed in the RxFIFO buffer, indicating that an RxFIFO buffer over-
run occurred.
The DLC can transmit a message frame in block mode by completely filling the TxFIFO
buffer with data bytes in a manner identical to a normal transmission, except that no
byte is accompanied by the “load as last byte of transmit data” command. Once the
TxFIFO buffer is completely filled with data bytes, it automatically disables the termi-
nate auto retry feature and begins normal J1850 bus access procedures. As long as
the host MCU continues to supply data bytes without latching a “load as last byte of
transmit data” command, the DLC continues to transmit the bytes in the block mode
frame. When the last data byte of the block mode frame is transferred to the DLC, it
should be accompanied by the “load as last byte of transmit data” command. This in-
dicates to the DLC that the end of the block mode frame has been reached.
After the last data byte has been transmitted, the DLC appends the CRC byte to the
frame. After the complete frame has been transmitted, the status byte shows the
TxFIFO buffer to be empty.
When the transmitter is operating in block mode, the status byte indicates “TxFIFO al-
most full” when the first frame byte has been sent out onto the J1850 bus, and “TxFIFO
contains some data” when the second byte is sent. If a DLC is configured for interrupt
operation, an interrupt of the host MCU is generated when the TxFIFO buffer becomes
half empty. As the TxFIFO buffer is refilled, status bytes and the interrupt line reflect
the changing state of the TxFIFO buffer. Once the “load as last byte of transmit data”
command is latched, the status byte shows the TxFIFO buffer to be full until transmis-
sion of the block mode message is completed.
DATA LINK CONTROLLER OPERATION
TECHNICAL DATA
MC68HC58