STM32f072 can bus crytal-less possible?

[KLausL]

Posted on February 21, 2017 at 11:04

Hello forum,

I am planning to design a new module for my can-bus based home automation system (

https://github.com/klaus-liebler/sensact

 ) with a STM32F072. The CAN bitrate ist 125kbps. It would be great if I could omit the crystal in my design. I think this should be possible, because timing of CAN is not _that_ critical . What do you think?

Thx!

Klaus

[AvaTar]

Posted on February 21, 2017 at 11:55

Home automation implies to me reduced temperature swing, so working on the internal RC oscillator might work out.

You can test the core clock deviation over temperature quite easily.

Aging would be another problem.

Not sure if omitting the quartz is such a great cost/effort reduction, though.

[T J]

Posted on February 21, 2017 at 14:38

apart from that aging problem, which should be a definite consideration, (thanks to AvaTar for the enlightenment)

Yes CANBUS at 250KHz works easily on my '091 solution without a crystal, with 8 byte payloads.

For a short CAN frame the frequency drift wont be a huge concern, but longer frames, the bit centers will drift away from the ideal center. and so you will notice many framing errors.

an 8byte payload frame would have approx 100 bits of data, a 1% frequency error would result in 100% frames errors.

add the crystal as insurance against this catastrophic probability in extreme temperature variations.

(Crystals only vary when the humidity changes significantly, but the variation is merely ppm, not a concern for this issue)

[AvaTar]

Posted on February 21, 2017 at 16:31

I personally would not consider a RC oscillator driven MCU for a protocol-based application (CAN, USB, Ethernet, even RS232) - for my private projects.

Thus I never investigated RC drift over temperature or time.

But I agree with you, a rising torrent of error frame would probably be the long-term result.

I would thoroughly investigate it before, to not produce typical 'China crap' [*].

Being able to confine the working temperature can delay this issues considerable (except probably for the initial warm-up to thermal equilibrium).

[*] = No offense to Chinese products.

I KNOW for a fact that several crappy 'Chinese Products' are actually designed in Europe or the US, specifically for the Chinese market.