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MPC5200 baud rate frequency

Discussion in 'Electronic Design' started by Didi, Mar 21, 2007.

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  1. Didi

    Didi Guest

    I have a Freescale MPC5200 board I designed a while ago which
    comes out of reset (I am writing the flash with a debug monitor
    which has served me over the years - I wrote it first for the 68020,
    I made a CPU32 version, then an MPC8240 version etc...), and the
    monitor says something through the UART (I use PSC_6).

    But the baud rate is totally off the mark.

    I made the other system say the same through its UART
    and, measuring the length (43 characters, enough to get an idea
    if speeds match) I see the new system is apr. 2.7 times slower
    than it should be.

    Great, I though I'd just write a 2.7 times lower value in the counter
    timer and see what happens. Well, the thing is, nothing happened.
    Same time (2.7 times what it should be) again. I just love that kind
    of thing when you make dramatic changes only to achieve no change.
    I am writing to the so called Counter Timer Upper and then Lower
    registers, treat them as 8 bit wide (i.e. I write using a .b access,
    stb) and I write the value of 215, which should yield a 19200 baud
    given the 132 MHz IPB clock. I already described the effect of
    changing the 215 to 79 (or sort of, I lost the precise number, it was
    not exactly 2.7, so it may have been 78 or 80, not that it matters).

    I am tired now and will reluctantly go to sleep (I hate doing that
    unsolved basic issues like that).
    Hopefully someone will have an idea what am I doing wrong which
    will help me tomorrow...


  2. jasen

    jasen Guest

    Not being familar with that hardware I see three possible mistakes.

    I hope the new day has brought you success.

    1> you were writing the correct value but later overwriting it
    2> you were writing the correct value but to the wrong location
    3> you were writing the correct value but with an incorrect setup

    It may be worth checking the errata in the hardware documentation.

  3. Didi

    Didi Guest

    Not being familar with that hardware I see three possible mistakes.
    Ah, it did, thanks. There had been more to it than just the baud rate,
    I rewrote the UART initialization code in a less ambitious manner
    (thus more
    reasonable for a first-time out of reset CPU type here) and it worked.

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