MC68336/376
CAN 2.0B CONTROLLER MODULE (TouCAN)
MOTOROLA
USER’S MANUAL
Rev. 15 Oct 2000
13-13
At the end of a successful transmission, the value of the free-running timer (which was
captured at the beginning of the identifier field on the CAN bus), is written into the time
stamp field in the message buffer. The code field in the control/status word of the mes-
sage buffer is updated and a status flag is set in the IFLAG register.
13.5.3.1 Transmit Message Buffer Deactivation
Any write access to the control/status word of a transmit message buffer during the
process of selecting a message buffer for transmission immediately deactivates that
message buffer, removing it from the transmission process.
While a message is being transferred from a transmit message buffer to a serial mes-
sage buffer, if the user deactivates that transmit message buffer, the message will not
be transmitted.
If the user deactivates the transmit message buffer after the message is transferred to
the serial message buffer, the message will be transmitted, but no interrupt will be
requested and the transmit code will not be updated.
If a message buffer containing the lowest ID is deactivated while that message is
undergoing the internal arbitration process to determine which message should be
sent, then that message may not be transmitted.
13.5.3.2 Reception of Transmitted Frames
The TouCAN will receive a frame it has transmitted if an empty message buffer with a
matching identifier exists.
13.5.4 Receive Process
The receive process includes configuring message buffers for reception, the transfer
of received messages by the TouCAN from the serial message buffers to the receive
message buffers with matching IDs, and the retrieval of these messages by the user.
The user should prepare/change a message buffer for frame reception by executing
the following steps.
1. Write the control/status word to hold the receive buffer inactive (code = %0000).
2. Write the ID_HIGH and ID_LOW words.
3. Write the control/status word to mark the receive message buffer as active and
empty.
NOTE
Steps 1 and 3 are mandatory for data coherency while preparing a
message buffer for reception.
Once these steps are performed, the message buffer functions as an active receive
buffer and participates in the internal matching process, which takes place every time
the TouCAN receives an error-free frame. In this process, all active receive buffers
compare their ID value to the newly received one. If a match is detected, the following
actions occur: