CAN Samplepoint

The direct influence is quite straight forward: the later you sample, the longer the cable can be. But there are other effects that can disfavour late sampling, most prominently clock frequency differences.

The quoted percentages of bit times suggest you have different t_qu times on your two types of devices (75% --> 8 t_qu = 1 us, 60% and 73% are incompatible with that). That could be making things worse. Try to do look at this in terms of actual bit segment times (t_SEG1, t_SEG2, and keep an eye on t_SJW), instead of somewhat meaningless percentages.

Any useful textbook on CAN will have more details for you.

In our book only the principle of the timing is explained. The meaning of late sampling and the size of the SJW is not handled.

In all of our controllers we use quite exact oscillators for the timing. So we could set SJW smaller? Currently SJW is set to 375ns, even bigger than TSEG2 (250ns)! Which factors should also be considered for SJW?

Regards,

Wolfgang Kopp

CAN is a relatively low speed bus that can be physically quite large. ISO 11898 is the relevant standard for signaling.

As the bus grows significantly, the transition time of the signals grows. At the receiver, one needs (quite reasonably) to have the differential signal valid. For some arbitrary bus length, with some fixed rise/fall time, there will be a minimum time after the transmission before the signal at the receiver has reached valid levels. As the frequency of operation is increased, the sample point position, as a percentage of bit time, wil increase. Indeed, the limiting factor on speed is the issue of the signal at the receiver not having sufficient time to become valid.

So it is no surprise that as the frequency of operation increases, the sample point as a percentage of bit time also increases.

If you read the datasheet for a typical CAN bus driver, you'll get this basic information.

As a sample (there are a number of vendors at the physical layer) try:

Cheers

PeteS

Then you must have selected a useless one ;-). The one edited by Wolfhard Lawrenz (3-7785-2780-0) explains a good deal more about this.

That doesn't make sense. t_SJW may not be longer than t_SEG2. One of two uses of SJW is to change the lengt of SEG2 to (t_SEG2-t_SJW). But a negative length of the SEG2 is clearly nonsensical --- it would mean you sample outside the bit time.