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Philips LPC UART & CAN baud rates with 1 XTAL

Discussion in 'Embedded' started by StanV, Jan 31, 2006.

  1. StanV

    StanV Guest

    Hi all,

    Anyone some advice or experience on calculating the Xtal freq for a
    LPC22xx ARM7.

    I just don't find a correct value to generate stnadard UART baudrates
    (9600, 19200, 38400..) and standard CAN-bus speeds (125,250,500,1000)

    At this moment i have a 17.456MHz Xtal and this is working fine for the
    UART.

    For the CAN-bus, a multiple of 8Mhz should work fine.

    I tried to get to a compromis by changing PLL and Peripheral clock
    speeds, but it seems that there is no value that would exactly meet
    both requirments.

    Any suggestions on how to find the right frequency ?

    Thanks

    Stan
     
    StanV, Jan 31, 2006
    #1
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  2. I use 8MHz and 16MHz crystals in many of my designs and the baudrates
    generated with that clock are close enough to the desired values. I have
    never had problems up to 57600.
    The largest error occurs at 8MHz and 57600, where it is 3.5% off.

    Meindert
     
    Meindert Sprang, Jan 31, 2006
    #2
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  3. If you want to use CAN, you have to use a multiple of of 8 MHz or a
    multiple of 12 MHz. The UART will be limited afaik to 38400 with 60
    MHz CPU frequency (Crystal 12 *5 PLL)

    What is needed is what Philips has introduced on the LPC214x devices, a
    so called fractional baudrate generator.

    Hopefully this will be implemented in the CAN devices as well because
    it would enable 115200 and all CAN baudrates with a 12 MHz crystal.

    An Schwob
     
    An Schwob in the USA, Feb 1, 2006
    #3
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