37.5.1.7 Disabled Automatic Retransmission

According to the CAN Specification (see ISO 11898-1, 6.3.3 Recovery Management), the CAN provides means for automatic retransmission of frames that have lost arbitration or that have been disturbed by errors during transmission. By default automatic retransmission is enabled. To support time-triggered communication as described in ISO 11898-1, chapter 9.2, the automatic retransmission may be disabled through the CCCR.DAR bit (CCCR <6>).

Frame Transmission in DAR Mode

In DAR mode all transmissions are automatically cancelled after they started on the CAN bus. A Tx Buffer’s Tx Request Pending bit (TXBRP.TRPx) is reset after successful transmission, when a transmission has not yet been started at the point of cancellation, has been aborted due to lost arbitration, or when an error occurred during frame transmission.

• Successful transmission:
  • The Corresponding Tx Buffer Transmission Occurred bit (TXBTO.TOx) is set
  • The Corresponding Tx Buffer Cancellation Finished bit (TXBCF.CFx) is not set
• Successful transmission in spite of cancellation:
  • The Corresponding Tx Buffer Transmission Occurred bit (TXBTO.TOx) is set
  • The Corresponding Tx Buffer Cancellation Finished bit (TXBCF.CFx) is set
• Arbitration lost or frame transmission disturbed:
  • The Corresponding Tx Buffer Transmission Occurred bit (TXBTO.TOx) is not set
  • The Corresponding Tx Buffer Cancellation Finished bit (TXBCF.CFx) is set

In case of a successful frame transmission, and if storage of Tx events is enabled, a Tx Event FIFO element is written with Event Type ET = 10 (transmission in spite of cancellation).