canfd-bittiming

A small C99 library that finds CAN FD bit timings for Bosch M_CAN-style controllers, and encodes them into the NBTP, DBTP and TDCR registers, including transmitter delay compensation. No dependencies, no heap, one header. It is the C version of the calculator on moutazm.com/projects/can-tools, and the tests check that both agree.

$ ./canfd-bt 40 500 80 2000 75
4 valid settings, best:
  nominal  brp   1  tq  80 = 1 +  63 +  16  sjw  16  sample point 80.00 %
  data     brp   1  tq  20 = 1 +  14 +   5  sjw   5  sample point 75.00 %
  TDC      on, TDCO = 15 clock cycles
  M_CAN    NBTP 0x1E003E0F  DBTP 0x00800D44  TDCR 0x00000F00
  Linux    ip link set can0 up type can bitrate 500000 sample-point 0.800 dbitrate 2000000 dsample-point 0.750 fd on

What it does

For a CAN clock, a nominal and a data bit rate and two target sample points it:

1. finds every prescaler for which the bit time is a whole number of time quanta (integer arithmetic, no floating-point guessing), within the M_CAN limits (the same values the Linux m_can driver uses): nominal prescaler 1–512, TSEG1 2–256, TSEG2 2–128; data prescaler 1–32, TSEG1 1–32, TSEG2 1–16;
2. splits each bit time into sync + TSEG1 + TSEG2 so the sample point is closest to the target; SJW = TSEG2;
3. ranks the combinations: sample-point error (0.1 % steps), then the lower data prescaler (TDC needs 1 or 2, and a short time quantum resynchronises better), then the lower nominal prescaler. Optionally only equal prescalers in both phases (CiA 601-3 recommendation);
4. computes the TDC secondary-sample-point offset TDCO = data BRP × (1 + TSEG1) and says whether TDC is allowed (ISO 11898-1: data prescaler 1 or 2 only);
5. encodes the result into the M_CAN registers (fields hold value − 1, TDCO capped at 127).

Use

#include "canfd_bittiming.h"

cfd_setting best[3];
size_t n = cfd_solve(40000000u, 500000u, 0.80, 2000000u, 0.75, true, best, 3);
if (n) {
    bool tdc = cfd_tdc_possible(&best[0].data);
    uint32_t nbtp = cfd_mcan_nbtp(&best[0].nominal);
    uint32_t dbtp = cfd_mcan_dbtp(&best[0].data, tdc);
    uint32_t tdcr = cfd_mcan_tdcr(&best[0].data);
}

cfd_solve returns how many valid settings exist and writes the best max (up to 64) into your buffer, best first. Pass NULL, 0 to only count.

Build and test

make            # libcanfd_bittiming.a + the canfd-bt command-line tool
make test       # unit tests

CI builds with gcc and clang, -std=c99 -Wall -Wextra -Wpedantic -Wshadow -Wconversion -Werror, and runs the tests again under AddressSanitizer and UndefinedBehaviorSanitizer. The reference cases in tests/test_bittiming.c (prescalers, time quanta, segments, SJW, register values, TDCO, number of solutions) were generated by the website's JavaScript, so a change in either one shows up as a failing test.

License

MIT