The CAN communication protocol relies on synchronous data transmission, where the CAN controller continuously performs bit synchronization. However, different systems have varying requirements for this synchronization process. To meet these needs, it's essential to understand another key concept: the time period specification.
Time period specifications are tailored based on the demands of the data communication system. For instance, if you want to achieve the maximum bus length at a given bit rate or minimize latency (i.e., maximize bit rate) for a specific bus length, the retention time—also known as the phase buffer segment—must be minimized. When the buffer segment is set to its smallest value, only a phase error of |e|=1 can be corrected during one resynchronization. This makes the synchronization requirements very strict, which typically means using a high-precision quartz crystal with an error margin of less than 0.1%.
Figure 1 illustrates the timing specifications for the bit period, where the product of the bit rate and bus length is maximized. These standards are commonly used in industrial automation systems.
If the bit rate and bus length requirements are not as strict, the product of the two can be reduced, allowing for a longer time buffer segment. This enables correction of larger phase errors, such as |e|=4, during a single resynchronization. In such cases, a more cost-effective ceramic oscillator can be used. Figure 2 shows the timing specifications for automotive electronics, where the oscillator error is maximized.
Usually, the bit timing specification is first determined by the required bit rate. The bit time must be an integer multiple of the system clock period, expressed as tbit = n × tq, where n ranges from 4 to 25, and tq represents the time unit. One approach to setting the bit timing parameters involves first determining the length of the transmission segment, taking into account the maximum bus length and internal delay time. The round-trip delay is converted into the corresponding number of time units and rounded to the nearest multiple of tq. Since the synchronization segment is fixed at 1 tq, the remaining two phase buffer segments are calculated as (tbit - tprog_seg - tq). If the number of remaining time units m = (tbit - tprog_seg - tq) / tq is even, both buffer segments are equal in length; if odd, the second segment is extended by one tq.
It’s also important to consider the minimum nominal length of Phase_Seg2, as it cannot be shorter than the processing time of the CAN controller, which typically ranges between 0 and 2 tq. The sync jump width (SJW) should be set to its maximum value, which is Min{4, tPhase_Seg1 / tq}.
The allowable oscillator error is governed by the following two formulas:
Formula 1:
Df ≤ tSJW / (20 × tBit)
Where:
tBit = nominal bit time
tSJW = resynchronize jump width
Formula 2:
Df ≤ min{tPhase_seg1, tPhase_seg2} / (2 × (13 × tBit - tPhase_seg2))
These equations help ensure that the oscillator remains within acceptable tolerances, maintaining reliable communication across the CAN network.
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