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TCAN4550-Q1: Crystal and CAN speed

Dylan You
Intellectual 2630 points
Part Number: TCAN4550-Q1
Other Parts Discussed in Thread: TCAN4550

Dear Teams,

My customer is using TCAN4550. They are wondering whether they can used 8MHz crystal, instead of 40MHz crystal? Also, they want to use CAN with speed rate 1M, but not CAN-FD with 2.0B.

Could you help give some suggestions or feedback?

Thanks!

Dylan

  • 0
    TI__Guru 53555 points

    Hi Dylan,

    While using a lower-speed clock is certainly possible for TCAN4550, doing so will limit the achievable CAN rates of the device. The oscillator input is used by the device to determine where to sample each CAN bit. The CAN protocol specifies that there must be at least 4 samples in every bit-time (the more samples per bit-time, the more reliable the system). With an 8MHz crystal, the theoretical maximum CAN rate would be 2Mbps, so 1Mbps would also be possible. However, with a smaller crystal-to-datarate ratio, it is more likely that the system will encounter issues with delay, crystal tolerances, and synchronisation. 
    Please see section 4 of the TCAN4550 software user's guide for more info on bit-timing configurations. 

    The only other aspects of device operation that would change with a slower clock rate would be a limited SPI rate (less dramatic than the CAN limit) and different timer references (accommodations can be done with simple calculation). 

    Let me know if you have any more questions.

    Regards,
    Eric Schott