Hello, Heinz-Jürgen Oertel wrote: > Am Mittwoch, 11. August 2010 17:59:17 schrieb Marc Kleine-Budde: >> Bit timing parameters for flexcan with 24.576000 MHz ref clock >> nominal real Bitrt nom real SampP >> Bitrate TQ[ns] PrS PhS1 PhS2 SJW BRP Bitrate Error SampP SampP >> Error CAN_CTRL 1000000 40 8 8 8 1 1 983040 1.7% 75.0% >> 68.0% 9.3% 0x003f0007 1000000 81 4 4 3 1 2 1024000 2.4% >> 75.0% 75.0% 0.0% 0x011a0003 1000000 122 2 3 2 1 3 >> 1024000 2.4% 75.0% 75.0% 0.0% 0x02110001
> I was not aware that the clock is 24.576000 MHz, always thought 24.000000. The passage from the Datasheet you quoted said it's running at 24.576000 MHz > It is often the case that the crystal clock is chosen to get a perfect bit > rate for UARTS, not for CAN. But a UART frame, having only 10 bits, can live > with a much higher deviation from the nominal bit clock. > > I see that you typical use a sample point at 75%. > CANopen has a "Recommended location of sample point" at 87.5%. (CiA 301). I don't have access to the standard, but [1] lists a sample point of 75% for 1000 kbit/s and 80% for 800 kbit/s. All slower bitrates have the 87.5% sample point. cheers, Marc [1] www.can-cia.org/fileadmin/cia/files/icc/9/koppe.pdf -- Pengutronix e.K. | Marc Kleine-Budde | Industrial Linux Solutions | Phone: +49-231-2826-924 | Vertretung West/Dortmund | Fax: +49-5121-206917-5555 | Amtsgericht Hildesheim, HRA 2686 | http://www.pengutronix.de |
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