
MOTOROLA
Chapter 16. CAN 2.0B Controller Module
16-15
TouCAN Operation
The TouCAN transmits no more than eight data bytes, even if the transmit length contains
a value greater than eight.
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 message
buffer is updated and a status flag is set in the IFLAG register.
16.4.3.1 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.
If the transmit message buffer is deactivated while a message is being transferred from it to
a serial message buffer, the message is not transmitted.
If the transmit message buffer is deactivated after the message is transferred to the serial
message buffer, the message is transmitted, but no interrupt is requested, and the transmit
code is not 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.
16.4.3.2 Reception of Transmitted Frames
The TouCAN receives a frame it has transmitted if an empty message buffer with a
matching identifier exists.
16.4.4
Receive Process
During the receive process, the following events occur:
The user configures the message buffers for reception
The TouCAN transfers received messages from the serial message buffers to the
receive message buffers with matching IDs
The user retrieves these messages
The user should prepare or 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 = 0b0000)
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