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CD74HC123

Chris Renick
Prodigy 30 points
Other Parts Discussed in Thread: CD74HC123

Hi all; I have a couple of questions about this part. I hope someone can help.

   1) Can the output pulse width be shorter (in time) than the input pulse used to trigger it? I want to use a 10 second pulse to trigger this part for an output pulse width of 50mS.

   2) I want to trigger this 50mS output on both the rising and falling edge of the input pulse. Is this as simple as connecting the input pulse to both /A and B inputs? Is there some other way to accomplish this?

Thanks very much, Chris

  • 0
    TI__Guru* 96661 points

    Hi Chris,

    1.  Yes - the output is triggered on the chosen edge of the input (trailing edge on /A and leading edge on B).  As long as the input does not trigger again during the output pulse, it will be the selected length regardless of how long the input is left on.

    2.  Based on the given truth table, for the rising edge to trigger properly /A must be L while B is rising.  Since /A is also rising, I can't be completely certain how the chip will act in that circumstance based on the information I currently have.  I will research this further and get back to you within 24 hours.

    Can you provide me with any further details regarding the specific application you are using this for?  A block diagram (or even better, a schematic) would be very helpful.

  • 0 in reply to Emrys Maier
    TI__Guru* 96661 points
    Chris,
    I wasn't able to get all of the tests done that I wanted to today. From what I've seen so far, I don't think connecting both inputs together will work, but let me do some more tests next week. You will hear back from me again Monday or Tuesday.
  • 0 in reply to Emrys Maier
    TI__Guru* 96661 points

    Chris,

    After more testing, I have found that the CD74HC123  will not work with both inputs connected to the same signal.  In order to have an ouput that contains both a leading and trailing edge triggered pulse of the same length, I would recommend using both monostable multivibrators in the CD74HC123 and running the outputs into an OR gate.