© 2004 Microchip Technology Inc. DS70070B-page 23-55
Section 23. CAN
CAN Module
23
23.7.6 Transmit Boundary Conditions
The module handles transmit commands which are not necessarily synchronized to the CAN bus
message framing time.
23.7.6.1 Clearing TXREQ bit as a Message Starts
The TXREQ bit can be cleared just when a message is starting transmission, with the intent of
aborting the message. If the message is not being transmitted, the TXABT bit will be set,
indicating that the Abort was successfully processed.
When the user clears the TXREQ bit and the TXABT bit is not set two cycles later, the message
has already begun transmission.
If the message is being transmitted, the abort is not immediately processed, at some point later,
the TXnIF interrupt flag or the TXABT bit is set. If transmission has begun the message will only
be aborted if either an error or a loss of arbitration occurs.
23.7.6.2 Setting TXABT bit as a Message Starts
Setting the ABAT bit will abort all pending Transmit Buffers and has the function of clearing all of
the TXREQ bits for all buffers. The boundary conditions are the same as clearing the TXREQ bit.
23.7.6.3 Clearing TXREQ bit as a Message Completes
The TXREQ bit can be cleared when a message is just about to successfully complete transmis-
sion. Even if the TXREQ bit is cleared by the Data bus a short time before it will be cleared by
the successful transmission of the message, the TXnIF flag will still be set due to the successful
transmission.
23.7.6.4 Setting TXABT bit as a Message Completes
The boundary conditions are the same as clearing the TXREQ bit.
23.7.6.5 Clearing TXREQ bit as a Message Loses Transmission
The TXREQ bit can be cleared when a message is just about to be lost to arbitration or an error.
If the TXREQ signal falls before the loss of arbitration signal or error signal, the result will be like
clearing TXREQ during transmission. When the arbitration is lost or the error is set, the TXABT
bit will be set, as it will see that an error has occurred while transmitting, and that the TXREQ bit
was not set.
If the TXREQ bit falls after the arbitration signal has entered the block, the result will be like
clearing TXREQ during an inactive transmit time. The TXABT bit will be set.
23.7.6.6 Setting TXABT bit as a Message Loses Transmission
The boundary conditions are the same as clearing the TXREQ bit.
23.7.7 Effects of a Reset
Upon any Reset the CAN module has to be initialized. All registers are set according to the reset
values. The content of a transmitted message is lost. The initialization is discussed in Section
23.5.5 “Configuration Mode”.
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