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TCAN4550: OSC1 with 8MHz crystal oscillator for 1M CAN

Part Number: TCAN4550

Hi team,

We can see from the DS that TCAN4550 need 20MHz crystal oscillator for 2M CAN FD and 40MHz crystal oscillator for 5M CAN FD.

And also TCAN4550 have classic CAN backwards compatible.

So could you please kindly help check if customer can use 8MHz crystal oscillator for classic 1M CAN?

Thanks,

Brian

  • Hi Brian,

    While it is not recommended to use a clock frequency less than 20MHz, it may be possible to communicate with a 1Mbps Classic CAN frame.  The minimum number of time quantum (tq) in a Classic CAN bit is 5tq and each tq is equal to one cycle of the 8MHz clock.  Therefore there would be 8tq available per bit for a 1Mbps rate and an 8MHz clock which is greater than the minimum number needed, so a Classic CAN 1Mbps rate is possible.

    However, you should note that other timing parameters in the TCAN4550 will also be affected by this slower clock such as the watchdog timer, the Sleep Wake Error (SWE) Failsafe timer, etc.  If a clock frequency other than 20MHz or 40MHz is used, then the 20MHz or 40MHz frequency closest to the actual clock frequency used should be selected in the CLK_REF  in the Modes of Operation and Pin Configuration Register (0x0800[27]).  Then the timer values will need to be scaled according to the difference between the actual clock frequency and the selected 20MHz or 40MHz clock selected.

    Industry standard clock frequencies for CAN are 20MHz, 40MHz, and 80MHz and it is recommended that all devices on a CAN bus have the same bit timing configuration settings and an equivalent number of time quantum. Bit sampling errors can occur when nodes have different settings.  Using an 8MHz clock may be possible but it also may result in some communication errors and should be thoroughly tested.

    Regards,

    Jonathan

  • Hi Jonathan,

    Thanks a lot for your detailed explanation.

    And could you please kindly check can they use 24MHz clock to communicate with a 1Mbps Classic CAN frame?

    Thanks,

    Brian

  • Hi Brian,

    CAN bit timing requires an integer number of clock cycles (called time quantum or "tq" for short) to allocated for a single bit.  Allocating more or less tq will increase or decrease the bit width, which in turn determines the bit rate.  Therefore 24MHz clock cycles or tq would be used to create a 1Mbps data bit similar to how an 8MHz clock would use 8 clock cycles or tq for a 1Mbps data bit.

    As long as there is an integer number of clock cycles for the bit rate, and the clock is at least 5x the bit rate (because there is a minimum of 5tq per CAN bit), the bit width should be achievable.

    However, it is still recommended that one of the industry standard common frequencies of 20MHz, 40MHz, or 80MHz be used to help ensure interoperability with other devices on a CAN bus.

    Regards,

    Jonathan