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Using the MAX153 with the 6802

Discussion in 'General Electronics' started by Tim, Dec 4, 2003.

  1. Tim

    Tim Guest

    For an electronics class at my high school, we're building systems
    from scratch around the Motorola 6802 microprocessor (shh), and the
    eventual goal is building in an ADC and a DAC and using the system as
    a signal processor.

    The ADC that's been thrown at us is the MAX153... but the data sheet
    (at http://pdfserv.maxim-ic.com/en/ds/MAX153.pdf, 456K) is confusing
    as hell and nobody seems to know how to use it. (Er.)

    I'm trying to use the 153 in read mode (page 7, diagram on page 8).
    The problem I'm having with it is that it looks like it wants RD and
    CS held low even while the microprocessor waits for an interrupt,
    which seems like it requires an external addressable latch on the CS
    (chip select) and RD lines.
    The problem here is that using an external latch for CS (as opposed to
    just throwing it on the address decoder) is that CS controls the
    tri-state data lines. CS goes low before the microproc starts waiting
    for the interrupt, and remains low until the data comes off the bus,
    according to the data sheet. If we use an external latch for CS, we
    also have to buffer the data outputs to isolate them from the data bus
    so that the ADC doesn't interfere with the MPU's fetching the
    interrupt vector, etc. from ROM, or even resetting the CS line.

    I'd almost accept this as simple ugliness, but the data sheet
    advertises that it can be connected directly to a microprocessor data
    bus, requiring no external interface logic. Am I misinterpreting their
    marketing-speak? Is it expecting to live on an isolated bus segment?

    I'm sort of hoping that I'm misreading their assertations that RD must
    remain low through the entire transition and that CS must remain low
    for RD to be recognized... does that mean I can throw CS high and it
    won't notice RD changing states?

    I'd greatly appreciate it if anyone could shed some light on this for
    me. Thanks!
    Tim Smith
    TJHSST (http://www.tjhsst.edu)
     
  2. Robert Baer

    Robert Baer Guest

    Forget Maxim, as they do not want to make most of what they advertise;
    strictly vaporware - unless one is willing to toss tens of thousands of
    dollars at a given part type and wait 6-24 months....
    Many distributors refuse to carry Maxim due to this fact.
    ***
    According to how i read it, the microCPU should do the following:
    1) keep MODE low and WRbar/RDY high, 2) strobe CS low and while it is
    low: 3) strobe RD low and while it is low: 4) read status and/or data as
    required.
    I see absolutely *nothing* that STARTS the conversion, so maybe it
    always free-runs.
    And it is not clear if the interrupt line is strobed/gated by the RD
    line.
    (did not read the blah blah)
     
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