66.5.1.4.1 Description

The MCAN protocol unit has implemented a delay compensation mechanism to compensate the delay, thereby enabling transmission with higher bit rates during the CAN FD data phase independent of the delay of a specific CAN transceiver.

To check for bit errors during the data phase, the delayed transmit data is compared against the received data at the secondary sample point. If a bit error is detected, the transmitter will react to this bit error at the next following regular sample point. During arbitration phase the delay compensation is always disabled.

The transmitter delay compensation enables configurations where the data bit time is shorter than the transmitter delay, it is described in detail in the new ISO11898-1. It is enabled by setting bit MCAN_DBTP.TDC.

The received bit is compared against the transmitted bit at the SSP. The SSP position is defined as the sum of the measured delay from the MCAN’s transmit output CANTX through the transceiver to the receive input CANRX plus the transmitter delay compensation offset as configured by MCAN_TDCR.TDCO. The transmitter delay compensation offset is used to adjust the position of the SSP inside the received bit (for example, half of the bit time in the data phase). The position of the secondary sample point is rounded down to the next integer number of CAN core clock periods.

MCAN_PSR.TDCV shows the actual transmitter delay compensation value. MCAN_PSR.TDCV is cleared when MCAN_CCCR.INIT is set and is updated at each transmission of an FD frame while MCAN_DBTP.TDC is set.

The following boundary conditions have to be considered for the delay compensation implemented in the MCAN:

  • The sum of the measured delay from CANTX to CANRX and the configured delay compensation offset MCAN_TDCR.TDCO has to be less than 6 bit times in the data phase.
  • The sum of the measured delay from CANTX to CANRX and the configured delay compensation offset MCAN_TDCR.TDCO has to be less or equal 127 CAN core clock periods. In case this sum exceeds 127 CAN core clock periods, the maximum value of 127 CAN core clock periods is used for delay compensation.
  • The data phase ends at the sample point of the CRC delimiter, that stops checking of receive bits at the SSPs.