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Bit Timing Configuration
========================
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.
.. figure:: images/bit_timing_light.svg
:align: center
:class: only-light
.. figure:: images/bit_timing_dark.svg
:align: center
:class: only-dark
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:
.. code-block:: none
[default]
f_clock=8000000
brp=1
tseg1=5
tseg2=2
sjw=1
nof_samples=1
The same is possible for CAN FD:
.. code-block:: none
[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
|
