SLAU723A October 2017 – October 2018 MSP432E401Y , MSP432E411Y
Usually, the calculation of the bit timing configuration starts with a required bit rate or bit time. The resulting bit time (1/bit rate) must be an integer multiple of the system clock period.
The bit time may consist of 4 to 25 time quanta. Several combinations may lead to the required bit time, allowing iterations of the following steps.
The first part of the bit time to be defined is Prop. Its length depends on the delay times measured in the system. A maximum bus length as well as a maximum node delay has to be defined for expandable CAN bus systems. The resulting time for Prop is converted into time quanta (rounded up to the nearest integer multiple of tq).
Sync is 1 tqlong (fixed), which leaves (bit time – Prop – 1) tqfor the two Phase Buffer Segments. If the number of remaining tqis even, the Phase Buffer Segments have the same length, that is, Phase2 = Phase1, else Phase2 = Phase1 + 1.
The minimum nominal length of Phase2 has to be regarded as well. Phase2 may not be shorter than the CAN controller's Information Processing Time, which is, depending on the actual implementation, in the range of [0 to 2] tq.
The length of the synchronization jump width is set to the least of 4, Phase1, or Phase2.
The oscillator tolerance range necessary for the resulting configuration is calculated by the formula given in Equation 9:
where
Maximum frequency tolerance must take into account Equation 10 and Equation 11:
where
If more than one configuration is possible, that configuration allowing the highest oscillator tolerance range should be chosen.
CAN nodes with different system clocks require different configurations to come to the same bit rate. The calculation of the propagation time in the CAN network, based on the nodes with the longest delay times, is done once for the whole network.
The CAN system's oscillator tolerance range is limited by the node with the lowest tolerance range.
The calculation may show that bus length or bit rate must be decreased or that the oscillator frequencies' stability has to be increased in order to find a protocol-compliant configuration of the CAN bit timing.