NXP Semiconductors MPC5777C - Page 91

3.5.1.4 Transmitter 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
M_CAN_Tx the M_CAN receives the transmitted data from its local CAN transceiver
via M_CAN_Rx pin. The received data is delayed by the transmitter 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 transmitter delay, the
delay compensation is introduced. Without transmitter delay compensation, the bit rate in
the data phase of a CAN FD frame is limited by the transmitter delay.
3.5.1.4.1 Description
The M_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.
The following figure describes how the transceiver loop delay is measured.
arbitration phase
data phase
m_can_tx
m_can_rx
Delay
Start Stop
BRS DLCEDL
E
S
I
r0
transceiver loop delay
data phase
arbitration phase
transceiver delay compensation offset
secondary sample point position (
TEST.TDCV)
(FBTP.TDCO)
Measurement
Figure 3-49. Transceiver delay measurement
Within each CAN FD frame, the transmitter measures the delay between the data
transmitted at pin M_CAN_Tx and the data received at pin M_CAN_Rx. The
measurement is done once, at the falling edge of bit EDL to bit r0. The delay is measured
in M_CAN clock periods.
A secondary sample point position is calculated by adding a configurable transceiver
delay compensation offset FBTP[TDCO] to the measured transceiver delay. The
transceiver delay compensation value TEST[TDCV] is the sum of the measured
transceiver delay and the transceiver delay compensation offset. The transceiver delay
Chapter 3 Modular CAN (M_CAN)
MPC5777C Reference Manual Addendum, Rev. 1, 12/2015
Freescale Semiconductor, Inc. 91