38.3.3.7 Recommendations for Bit Time Configuration
The following recommendations need to be considered when configuring the bit time:
- Select the highest available CAN
clock frequency:
- Short TQ leads to high resolution to select the sample point.
- Use 20 MHz or 40 MHz for SYSCLK.
- Select the lowest NBRP and
DBRP:
- Low BRP leads to short TQ.
- NSYNC and DSYNC will be short and reduce the quantization error.
- The receiving node can synchronize more accurately to the transmitting node.
- Set NBRP equal to DBRP:
- Identical TQ in both phases prevents quantization errors during Bit Rate Switching.
- Use the same Nominal Sample Point
(NSP) and Data Sample Point (DSP) in all nodes on the CAN FD network:
- Different sample points in the different nodes lead to different lengths of the BRS and CRC delimiter bits and introduce phase errors when switching the bit rate.
- NSP need not be equal to the DSP.
- The SSP can be different in differing CAN FD nodes.
- Select the largest possible NSJW
and DSJW:
- Maximizes the oscillator tolerance.
- Allows the receiving nodes to quickly resynchronize to the transmitting nodes.
- Enable automatic TDC for DBR of 1
Mbps and higher:
- Automatic TDC measurement compensates for transmitter delay variations.