MC68HC55 Datasheet, PDF (32/40 Pages) Freescale Semiconductor, Inc – Two-Channel CMOS ASIC Device

MC68HC55 Technical Data
When the previous message was eight bits and the current message is
16 bits, the response message (which is also eight bits) finishes before
the current message frame and the CRC bits look like data bits 7 through
4 in the new 16-bit message format. Since the CRC validation of this
8-bit message response is not reliable, this 8-bit response should not be
used.
When the previous message was 16 bits and the current message is
eight bits, the response message (which is also 16 bits) cannot finish
before the current message frame. The last four data bits and the four
CRC bits are lost. Bits 7 through 4 of the 16-bit response message look
like the CRC bits of an 8-bit response and almost certainly would not be
correct. Since the response is incomplete and the CRC check is not
valid, this response is not useful.
The 16- to 8-bit message size transition normally only occurs after
setting up the addresses of the DSI bus peripherals. During address
setup, a message with address 0000 is sent to attempt to set the
address of the next peripheral on the daisy-chained bus. Before any
peripherals have been assigned an address, their bus switches are
opened so the addressing message only goes to the first peripheral in
line. As each peripheral gets an address, it closes its bus switch so the
next address assignment command can reach the next peripheral in line
on the bus. Each peripheral responds to an address assignment only
once (during the next message after the command that set its address).
When the master MCU fails to receive a response, it knows it has passed
the last peripheral, and that the (16-bit) address assignment command
that received no response will have no response either. At this point, the
master will begin sending 8-bit messages and the first such message
frame will have no meaningful response associated with it.
The first message after reset is also a special case because there was
no previous message and therefore there will be no meaningful
response during the first message transfer.
Technical Data
32
MC68HC55
Functional Description