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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.

  2. neon


    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.

  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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