Hello,
Am 02.11.21 um 16:36 schrieb Oliver Hartkopp:
> Hi Matthias,
>
> On 02.11.21 16:03, Matthias Weißer wrote:
>
>> we recently had a case here where one member of a CAN bus couldn't
>> receive
>> frames with data content of only zeros:
>>
>> $ cansend can0 123#0000000000000000
>>
>> After some investigation we found the root cause to be a slight
>> difference
>> (about 1%) in actual bitrates of the two members. The one with showed the
>> RX errors had a sjw value of 1 and a lot of time quanta (40) due to
>> the 40MHz
>> CAN clock.
>>
>> This leads to a build up of phase error (as sjw is not able to
>> compensate for
>> enough of the bitrate difference) which at some point leads to a framing
>> error due to missing a stuff bit. Playing around with the sample point
>> can
>> improve or worsen the behavior.
>>
>> We can fix this quite easily by specifying a higher sjw value.
>
> You can specify the sjw value with the ip command (for CAN FD there is
> also a dsjw). And IIRC you can set it to the max. value for your CAN
> controller if you define sjw to be 4.
>
> Best regards,
> Oliver
>
> $ ip link help can
> Usage: ip link set DEVICE type can
> [ bitrate BITRATE [ sample-point SAMPLE-POINT] ] |
> [ tq TQ prop-seg PROP_SEG phase-seg1 PHASE-SEG1
> phase-seg2 PHASE-SEG2 [ sjw SJW ] ]
This usage description is not totally correct! It is also possible, to
increase "sjw" together with the bitrate, e.g.:
# ip link set can0 type can bitrate 250000 sjw 2
# ip -d link show can0 | grep sjw
tq 250 prop-seg 6 phase-seg1 7 phase-seg2 2 sjw 2
Here is the code for it:
# cat dev.c
...
/* check for sjw user settings */
if (!bt->sjw || !btc->sjw_max) {
bt->sjw = 1;
} else {
/* bt->sjw is at least 1 -> sanitize upper bound to sjw_max */
if (bt->sjw > btc->sjw_max)
bt->sjw = btc->sjw_max;
/* bt->sjw must not be higher than tseg2 */
if (tseg2 < bt->sjw)
bt->sjw = tseg2;
}
Wolfgang
