38.6.2.4 Transceiver Delay Compensation

During the data phase of a CAN FD transmission only one node is transmitting, all others are receivers. The length of the bus line has no impact. When transmitting via pin CAN_TX the CAN receives the transmitted data from its local CAN transceiver via pin CAN_RX. The received data is delayed by the CAN transceiver’s loop delay. In case this delay is greater than TSEG1 (time segment before sample point), a bit error is detected. In order to enable a data phase bit time that is even shorter than the transceiver loop delay, the delay compensation is introduced. Without transceiver delay compensation, the bit rate in the data phase of a CAN FD frame is limited by the transceivers loop delay.

Description

The CAN’s protocol unit has implemented a delay compensation mechanism to compensate the transmitter 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 of transmitting nodes, the delayed transmit data is compared against the received data at the Secondary Sample Point SSP. If a bit error is detected, the transmitter will react on 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 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 CAN’s transmit output CAN_TX through the transceiver to the receive input CAN_RX plus the transmitter delay compensation offset as configured by TDCR.TDCO. The transmitter delay compensation offset is used to adjust the position of the SSP inside the received bit (e.g. 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 mtq.

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

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

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

    Transmitter Delay Compensation Measurement

If transmitter delay compensation is enabled by programming DBTP.TDC = ‘1’, the measurement is started within each transmitted CAN FD frame at the falling edge of bit FDF to bit res. The measurement is stopped when this edge is seen at the receive input CAN_TX of the transmitter. The resolution of this measurement is one mtq.

Figure 38-2. Transceiver delay measurement

To avoid that a dominant glitch inside the received FDF bit ends the delay compensation measurement before the falling edge of the received res bit, resulting in a too early SSP position, the use of a transmitter delay compensation filter window can be enabled by programming TDCR.TDCF. This defines a minimum value for the SSP position. Dominant edges of CAN_RX, that would result in an earlier SSP position are ignored for transmitter delay measurement. The measurement is stopped when the SSP position is at least TDCR.TDCF AND CAN _RX is low.