Attention!
This feature is experimental. The implementation might change in future versions.
The CAN protocol, specified in ISO 11898, allows the bitrate, sample point and number of samples to be optimized for a given application. These parameters, known as bit timings, can be adjusted to meet the requirements of the communication system and the physical communication channel.
These parameters include:
- tseg1: The time segment 1 (TSEG1) is the amount of time from the end of the sync segment until the sample point. It is expressed in time quanta (TQ).
- tseg2: The time segment 2 (TSEG2) is the amount of time from the sample point until the end of the bit. It is expressed in TQ.
- sjw: The synchronization jump width (SJW) is the maximum number of TQ that the controller can resynchronize every bit.
- sample point: The sample point is defined as the point in time within a bit where the bus controller samples the bus for dominant or recessive levels. It is typically expressed as a percentage of the bit time. The sample point depends on the bus length and propagation time as well as the information processing time of the nodes.
Bit Timing and Sample Point
For example, consider a bit with a total duration of 8 TQ and a sample point at 75%. The values for TSEG1, TSEG2 and SJW would be 5, 2, and 2, respectively. The sample point would be 6 TQ after the start of the bit, leaving 2 TQ for the information processing by the bus nodes.
Note
The values for TSEG1, TSEG2 and SJW are chosen such that the sample point is at least 50% of the total bit time. This ensures that there is sufficient time for the signal to stabilize before it is sampled.
Note
In CAN FD, the arbitration (nominal) phase and the data phase can have different bit rates. As a result, there are two separate sample points to consider.
Another important parameter is f_clock: The CAN system clock frequency
in Hz. This frequency is used to derive the TQ size from the bit rate.
The relationship is f_clock = (tseg1+tseg2+1) * bitrate * brp.
The bit rate prescaler value brp is usually determined by the controller
and is chosen to ensure that the resulting bit time is an integer value.
Typical CAN clock frequencies are 8-80 MHz.
In most cases, the recommended settings for a predefined set of common bit rates will work just fine. In some cases, however, it may be necessary to specify custom bit timings. The :class:`~can.BitTiming` and :class:`~can.BitTimingFd` classes can be used for this purpose to specify bit timings in a relatively interface agnostic manner.
:class:`~can.BitTiming` or :class:`~can.BitTimingFd` can also help you to produce an overview of possible bit timings for your desired bit rate:
>>> import contextlib >>> import can ... >>> timings = set() >>> for sample_point in range(50, 100): ... with contextlib.suppress(ValueError): ... timings.add( ... can.BitTiming.from_sample_point( ... f_clock=8_000_000, ... bitrate=250_000, ... sample_point=sample_point, ... ) ... ) ... >>> for timing in sorted(timings, key=lambda x: x.sample_point): ... print(timing) BR: 250_000 bit/s, SP: 50.00%, BRP: 2, TSEG1: 7, TSEG2: 8, SJW: 4, BTR: C176h, CLK: 8MHz BR: 250_000 bit/s, SP: 56.25%, BRP: 2, TSEG1: 8, TSEG2: 7, SJW: 4, BTR: C167h, CLK: 8MHz BR: 250_000 bit/s, SP: 62.50%, BRP: 2, TSEG1: 9, TSEG2: 6, SJW: 4, BTR: C158h, CLK: 8MHz BR: 250_000 bit/s, SP: 68.75%, BRP: 2, TSEG1: 10, TSEG2: 5, SJW: 4, BTR: C149h, CLK: 8MHz BR: 250_000 bit/s, SP: 75.00%, BRP: 2, TSEG1: 11, TSEG2: 4, SJW: 4, BTR: C13Ah, CLK: 8MHz BR: 250_000 bit/s, SP: 81.25%, BRP: 2, TSEG1: 12, TSEG2: 3, SJW: 3, BTR: 812Bh, CLK: 8MHz BR: 250_000 bit/s, SP: 87.50%, BRP: 2, TSEG1: 13, TSEG2: 2, SJW: 2, BTR: 411Ch, CLK: 8MHz BR: 250_000 bit/s, SP: 93.75%, BRP: 2, TSEG1: 14, TSEG2: 1, SJW: 1, BTR: 010Dh, CLK: 8MHz
It is possible to specify CAN 2.0 bit timings using the config file:
[default]
f_clock=8000000
brp=1
tseg1=5
tseg2=2
sjw=1
nof_samples=1
The same is possible for CAN FD:
[default]
f_clock=80000000
nom_brp=1
nom_tseg1=119
nom_tseg2=40
nom_sjw=40
data_brp=1
data_tseg1=29
data_tseg2=10
data_sjw=10
A :class:`dict` of the relevant config parameters can be easily obtained by calling
dict(timing) or {**timing} where timing is the :class:`~can.BitTiming` or
:class:`~can.BitTimingFd` instance.
Check :doc:`configuration` for more information about saving and loading configurations.
.. autoclass:: can.BitTiming
:class-doc-from: both
:show-inheritance:
:members:
:member-order: bysource
.. autoclass:: can.BitTimingFd
:class-doc-from: both
:show-inheritance:
:members:
:member-order: bysource