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MCU modem

Discussion in 'Electronic Design' started by terry, Nov 27, 2004.

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  1. terry

    terry Guest


    Recently I have a thinking to implement a baud 9600 modem using ADC
    and DAC and MCU. I know the implementation involves to encode and
    decode by QAM. Is it possible to do it? How fast the MCU needs to do

  2. Tim Wescott

    Tim Wescott Guest

    Only if the original signal is QAM. I did a quick web search and found
    V32 in the same sentence with "trellis coding" -- which ain't QAM. You
    need to find the specs for the modem you want to talk to and make sure
    you understand them.
    Yes, but you have to know what you're doing.
    I have decoded 400 baud MSK (a close relative to QAM) with a 68HC11
    running at 8MHz (about 2MIPs), using 98% of the available processor
    power. If you scale from there you'd need about 24MIPs for your
    decoding -- but the 68HC11 is a very inefficient machine. If you use a
    DSP chip you can probably do it with half that many MIPs, but you want
    to have your SW app using no more than 50% of the available processor time.

    So say a 25MHz DSP chip, or a 50MHz 16-bitter if _all_ you're doing is
    the modem.
    You're welcome.
  3. Joerg

    Joerg Guest

    Hi Terry,
    It depends on the kinds of math the uC can do. As Tim said, the classic
    uCs have a hard time with signal processing but some newer versions run
    16 bits internally and the 'leather seat' versions contain stuff like HW
    multipliers. An example is the MSP430. Check TI's code examples. I
    believe they had modem apps for situations much faster than 9600 baud.
    This could also save you much of the grunt work in writing the code and
    you can buy proto boards from Olimex at pretty low cost.

    Regards, Joerg
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