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Using different numbers of TQ but same frequency on CAN bus?

Discussion in 'Embedded' started by Mike Noone, Feb 24, 2007.

  1. Mike Noone

    Mike Noone Guest

    Hi - I have a system that consists of one Atmel SAM7X256 connected via
    CAN to 6 Microchip MCP2515 chips. The MCP2515s have 20MHz clocks, and
    the SAM7X256 has a 55MHz clock. I have the CAN bus running at 1MHz
    right now, but the chips aren't perfectly matched. Specifically, the
    SAM7X256 uses 11TQ, while the MCP2515 uses 10TQ. I had to do this as
    otherwise I'd have to have fractional baudrate divisors, which
    obviously is not possible. Communication is perfect without any CAN
    errors whatsoever - but I'm just wondering - is this allowed within
    spec? Or am I just getting lucky? I have the propagation segment on
    the SAM7X256 1TQ longer than on the MCP2515. Is this the best part to
    lengthen? These are my settings:

    syncseg = 1TQ
    PS1 = 3TQ
    PS2 = 3TQ
    propseg = 3TQ

    syncseg = 1TQ
    PS1 = 3TQ
    PS2 = 3TQ
    propseg = 4TQ


    Mike Noone, Feb 24, 2007
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  2. Mike Noone

    Jack Klein Guest

    We've been doing this for quite some time, without any problems, using
    a master with external CAN controllers with 8 MHz clock (8 TQ at 500
    KHz), and the on-chip CAN controllers on slave DSPs that wind up using
    20 TQ. Never had a problem.

    If you're not having any problems, I wouldn't change anything.

    Jack Klein
    Home: http://JK-Technology.Com
    FAQs for
    comp.lang.c http://c-faq.com/
    comp.lang.c++ http://www.parashift.com/c++-faq-lite/
    Jack Klein, Feb 26, 2007
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  3. If the bit times are exactly the same, I think you should have no
    problems caused by clock mis-match. I've often used nodes with
    different clocks and different CANbus "time quanta" but I've almost
    always been able to match bit times precisely.

    (Bus length and speed are still as constrained as always.)

    My belief is that the only testing that counts for much is testing
    that forces a lot of bus contention (arbitration). The fact that
    nodes can hear each other when the packets don't collide never
    convinces me I have a functional bus. I create a test case with a
    heavily loaded bus, and make sure there are many messages whose IDs
    are the same for the first many bits. If THAT runs without retries, I
    trust my cabling, transceivers, PropSeg, SynchSeg, sample point, bit
    times etc.

    Rick Corey
    (Am I paranoid _enough_?)
    richard.corey, Feb 26, 2007
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