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Need 2 ms hardware timer

Discussion in 'General Electronics' started by David Evennou, Jun 20, 2007.

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  1. I am looking to implement a 2 ms hardware timer function. Condition is the
    Input goes from High to Low and stays Low indefinitely. Output should be a
    2m positive pulse.
    I looked at the 555 timer, but the trigger input must be a negative pulse
    for the timer to work properly. This is a hardware implemented fail-safe
    function to monitor the output of the MPU just in case the firmware get
    lost. The internal MPU watchdog will not do the job that I need.

    TIA,
    David
     
  2. neon

    neon

    1,325
    0
    Oct 21, 2006
    just tie pin 2 hi with 10k add a series cap if any transition gors kow pin 2 will follow triggering the lm555 as oneshoot
     
  3. Jerry R

    Jerry R Guest

    Take a look at a 74121 (TI, etc). It has both L-H and H-L inputs.

    ....jerry
     
  4. Hmm... 74121 and the like act on edges but do not wait "indefinitely" before
    providing the output pulse. So you have to define how long "indefinitely"
    lasts and need two timers: One to act on the edge and wait "indefinitely",
    the second to produce the 2ms pulse when "indefinitely" is over and the
    input is still low.

    petrus bitbyter
     
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