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Ideal crystal frequencies

Discussion in 'Electronic Design' started by Paul E. Schoen, Nov 16, 2006.

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  1. As a tangent from the post on low frequency crystals, there were several
    frequencies that were recommended.

    When I was designing 4 MHz Z80 CPU circuits, I used a 3.6864 MHz crystal,
    which divided down nicely to standard baud rates of 115200 to 300, and also
    could generate 100 Hz for 0.01 second resolution timers, and multiples of
    50 or 60 Hz for power line frequency synthesis or sampling. The frequencies
    are multiples of 2, 3, and 5.

    For my newer 20 MHz PIC applications, I use 14.7456 MHz which is just 4
    times higher and also can be used for precise timing of baud rates and such
    as listed above.

    Of course, other applications may have different needs, but I think these
    frequencies are ideal for pretty much everything I need to do. I might also
    consider 18.432 MHz which divides down to an exact 1.0 kHz for mSec timing.

    All of these are standard values, and are available from Jameco for $0.31
    each. At that price, it would not be worthwhile to take apart a TV, but
    modem boards have crystals that are of similar multiples, and almost
    everyone has some blown modems. The crystal in the one I found in my junk
    pile is 22.1184 MHz.

  2. PeteS

    PeteS Guest

    Interestingly, the PXA series from Intel (ARM cores) specify a 3.6864MHz
    crystal (for the very reason you cite, no doubt) which is then multipled
    as necessary, and gives a memory frequency of 99.x MHz (not quite 100).

    Quite amusing when doing timing analysis on the memory bus.


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