Functional Description
Generally, the IPT is CAN controller-specific, but may not be longer than 2 tq. The IPC length is the lower
limit of the programmed length of Phase_Seg2. In case of a synchronization, Phase_Seg2 may be
shortened to a value less than IPT, which does not affect bus timing.
23.3.16.2.1 Calculation of the Bit Timing Parameters
Usually, the calculation of the bit timing configuration starts with a desired bit rate or bit time. The resulting
bit time (1 / Bit rate) must be an integer multiple of the CAN clock period.
NOTE:
8 MHz is the minimum CAN clock frequency required to operate the DCAN at a bit rate of 1
MBit/s.
The bit time may consist of 8 to 25 time quanta. The length of the time quantum t
q
is defined by the baud
rate prescaler with t
q
= (Baud Rate Prescaler) / CAN_CLK. Several combinations may lead to the desired
bit time, allowing iterations of the following steps.
First part of the bit time to be defined is the Prop_Seg. 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 expandible
CAN bus systems. The resulting time for Prop_Seg is converted into time quanta (rounded up to the
nearest integer multiple of t
q
).
The Sync_Seg is 1 t
q
long (fixed), leaving (bit time – Prop_Seg – 1) t
q
for the two Phase Buffer Segments.
If the number of remaining t
q
is even, the Phase Buffer Segments have the same length, Phase_Seg2 =
Phase_Seg1, else Phase_Seg2 = Phas 1.
The minimum nominal length of Phase_Seg2 has to be regarded as well. Phase_Seg2 may not be shorter
than any CAN controller’s Information Processing Time in the network, which is device dependent and can
be in the range of [0…2] t
q
.
The length of the synchronization jump width is set to its maximum value, which is the minimum of four (4)
and Phase_Seg1.
The oscillator tolerance range necessary for the resulting configuration is calculated by the formulas given
in
If more than one configurations are possible to reach a certain Bit rate, it is recommended to choose the
configuration which allows the highest oscillator tolerance range.
CAN nodes with different 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 have to 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.
The resulting configuration is written into the bit timing register:
Tseg2 = Phase_Seg2-1
Tseg1 = Pha Prop_Seg-1
SJW = SynchronizationJumpWidth-1
BRP = Prescaler-1
23.3.16.2.2 Example for Bit Timing at High Baud Rate
In this example, the frequency of CAN_CLK is 10 MHz, BRP is 0, the bit rate is 1 MBit/s.
3913
SPRUH73H – October 2011 – Revised April 2013
Controller Area Network (CAN)
Copyright © 2011–2013, Texas Instruments Incorporated