Re: Queries on CAN frame work

[Marc Kleine-Budde <mkl@pengutronix.de>]

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On 13.01.2022 21:30:04, Marc Kleine-Budde wrote:
> > Recently we did few experiments with our CANFD network. Stuff error
> > observed with different Nominal and Dataphase prescaler configuration
> > values, which we get from the frame work.
> > 
> > For lower baud rates separate prescalers are working good, but when we
> > switch to high baud rates(like 4Mbps and 5Mbps) observed the BUSOFF
> > state(Due to stuff errors) on IP(Xilinx).
> > 
> > With shared prescaler(low prescalers) we are not seeing the issue..
> > 
> > In Iso spec also mentioned to avoid tolerance, we have to maintain
> > common TQ for Nominal and Data phase.

This is a known problem with the current arbitration and data bit rate
calculation. They are done independently of each other, resulting in not
common pre-scalers and thus not common TQ.

> > Spec: ISO 11898-1:2015(E)  page no :50
> > 
> > " If the same time quantum length is used in the nominal bit time and
> > in the data bit time and the positions of the sample points in the
> > nominal bit time are the same in all CAN nodes of a network, then
> > optimum clock tolerance is accomplished for networks using FD frames."
> > 
> > CIA:
> > https://can-newsletter.org/uploads/media/raw/f6a36d1461371a2f86ef0011a513712c.pdf
> > 
> > From above CAN-newsletter also suggested the same in recommendation 2 and 3.

I'll have a look at the newsletter and see what changes need to be done
to the existing algorithm.

> > Recommendation 2: Set the BRPA bit-rate prescaler equal BRPB
> > Recommendation 3: Choose BRPA and BRPD as low as possible
> > 
> > Is there any possibility in framework to use shared prescalars for
> > Nominal and Dataphase?.

Not yet.

> > One more question. How to calculate TDCO ?, is there any generic
> > formula for this ?.

Vincent (Cc'ed) can help you on this.

regards,
Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde           |
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