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TCAN4550-Q1: Crystal options - range or specific values

Darren Jenkins
Genius 11826 points
Part Number: TCAN4550-Q1

TCAN4550-Q1 team,

The datasheet section 9.1.1 says the crystal or clock should be a minimum of 20 MHz, while 40 MHz is recommended for CAN FD.  Is it the case that 20 and 40 Mhz are the only options, or can other frequencies be used within that range?

The customer doesn't need to reach FD rates, just 1 Mbps max.

I noticed the Watchdog register only allows a choice or 20 or 40 MHz, but the precision of the watchdog is probably not so important. 

Thanks,

Darren

  • 0
    TI__Guru 62920 points
    Hi Darren,

    That's a good question. 20 MHz and 40 MHz are the most commonly used values, but I don't know of anything in the controller design that would restrict the usage of other frequencies as long as they could be scaled to provide an integer number of time quanta at the desired bit rate. I'll look into this in detail when I'm back at work on Monday, though, and get back to you then.

    What oscillator frequency did you have in mind?

    Regards,
    Max
  • 0 in reply to Miguel Robertson
    TI__Genius 11826 points

    Thanks Max.

    The clock or crystal is 38.4 Mhz. 

    Now that you mention it, this could be problem just because of the integer number of clock periods per bit period.  250 kbps, 500 kbps, 1 Mbps are the desired rates and these don't work out well.

    I'll still be interested in hearing what you find out in case the difference in frequency is acceptable.

    Thanks,

    Darren

  • 0 in reply to Darren Jenkins
    TI__Guru 62920 points
    Hi Darren,

    Just to follow up - you're right that 38.4 MHz would be problematic for most "standard" CAN rates since it doesn't correspond to a configuration that could give an integer number of time quanta per bit. However, I can confirm that in general there is not a restriction on using other frequencies between 20 MHz and 40 MHz. The timers used for the watchdog also use this clock, though, so their durations may differ from the datasheet descriptions (they would scale with clock rate).

    Regards,
    Max