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The optimum sample point value depends on the bit symmetry. The more
asymmetric the bit is, the more the sample point would be located
towards the end of the bit. On the contrary, if the transceiver only
has a small asymmetry, the optimal sample point would be slightly
after the centre of the bit.
For NRZ encoding (used by Classical CAN, CAN FD and CAN XL with TMS
off), the optimum sample points values are above 70% as implemented in
can_calc_sample_point_nrz().
When TMS is on, CAN XL optimum sample points are near to 50% or
60% [1]. Add can_calc_sample_point_pwm() which returns a sample point
which is suitable for PWM encoding. We crafted the formula to make it
return the same values as below table (source: table 3 of [1]).
Bit rate (Mbits/s) Sample point
-------------------------------------
2.0 51.3%
5.0 53.1%
8.0 55.0%
10.0 56.3%
12.3 53.8%
13.3 58.3%
14.5 54.5%
16.0 60.0%
17.7 55.6%
20.0 62.5%
The calculation simply consists of setting a slightly too high sample
point and then letting can_update_sample_point() correct the values.
For now, it is just a formula up our sleeves which matches the
empirical observations of [1]. Once CiA recommendations become
available, can_calc_sample_point_pwm() should be updated accordingly.
[1] CAN XL system design: Clock tolerances and edge deviations edge
deviations
Link: https://www.can-cia.org/fileadmin/cia/documents/publications/cnlm/december_2024/cnlm_24-4_p18_can_xl_system_design_clock_tolerances_and_edge_deviations_dr_arthur_mutter_bosch.pdf
Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Link: https://patch.msgid.link/20251126-canxl-v8-14-e7e3eb74f889@pengutronix.de
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.