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Multiplexing

Discussion in 'Electronic Design' started by Tanty, Jul 24, 2003.

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  1. Tanty

    Tanty Guest

    I'm building a digital rpm meter. The input is a pulse train related to the
    sparks of one of the high tension cables comming off the distributor. I'm
    using a motorola HC08 (JK3). I need time between pulses to get the frequency
    but at the same time multiplex four 7-segement digits. any pulse will need
    to interrupt the chip which will also disturb any cycle I have running to
    MUX the digits. How can i implement that in code? Its driving me out my
    mind.
    Any ideas?
    thanx
    Tanty
     
  2. Sure, don't use code at all. Use the excellent hardware peripherals on
    the Moto micros. Use the input capture mode on the timer to measure
    the pulse widths. Use an interrupt to time the display mux, either off
    the second timer module or, if the frequency is high enough, off the
    free running timer overflow flag, and save the second timer for
    something more critical.

    Best regards,
    Spehro Pefhany
     
  3. Hello Tanty,

    Your interrupt should just increment a counter and exit. You don't need to
    calculate on every pulse. Periodically you calculate dP/dt and reset the
    counter (for example read the accumulated pulses P every 10 seconds and then
    rpm=6P) then clear the counter (i.e P=0). Interrupts should not 'disturb'
    your mux routine. Perhaps you can explain in more detail what the perceived
    problem is. I see none here.



    Regards,

    Patrick
     
  4. John Fields

    John Fields Guest

    ---
    If you're using LEDs, get yourself four 8-bit serial-in parallel-out
    shift registers with output latches and use them to update the displays
    whenever you're not doing anything else. All you'll need will be 3 IO's;
    serial data out, shift clock and latch clock. Check out Allegro's
    A6275's, you only need four resistors to set the segment current. (one
    for each chip) In lieu of that you could use something like an HC595,
    but you'd need a godawful lot of resistors and there are limits to the
    total current you can sink or source from the chip, so your display
    brightness might suffer.

    If you're Using LCDs, National's got a nice chip for that. Three input
    lines and a whole bunch (28 anyway)of parallel outputs as well as a
    backplane driver. The part number escapes me but their website is easy
    to get around in, so if you're using LCD's you might want to give them a
    try.
     
  5. Jim Thompson

    Jim Thompson Guest

    Typical digital overkill. Why aren't you using an F-to-V converter
    chip, readily available, and cheap? Then relegate your uP to the
    A-to-D and display functions.

    ...Jim Thompson
     
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