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Help with ttl power-on reset pulse

Discussion in 'General Electronics Discussion' started by Titus, Aug 19, 2016.

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  1. Titus

    Titus

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    Aug 19, 2016
    Untitled-2.png Hello everybody, I need a power-on reset circuit for ttl 5v levels.
    It needs to send a 500 ms high (+5v) pulse when initially powered on and then stay low
    Thank you in advance!

    View attachment 28667
     
    Last edited: Aug 19, 2016
  2. Titus

    Titus

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    0
    Aug 19, 2016
    I've tried many examples that I found online but none worked as intended
    Tried with resistors and caps, also tried one with a diode but still can't make it work.
    On power on, it randomly latches but I need to set it to a known state at power on / startup
     
    Last edited: Aug 20, 2016
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    You need a capacitor to ground and a pull up resistor with a time constant of about 0.5 seconds.

    Be aware that this pulse will have a slow rise time and you should probably apply this to an input with a Schmitt trigger.

    Alternatively you can square it up with a standalone Schmitt trigger (typically an inverter as well)
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Oh, it's a good idea not to reply to your own question before anyone else because it appears as if the question has been answered (at least for those of us who look for threads that nobody has replied to)
     
  5. Titus

    Titus

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    Aug 19, 2016
    untitled.png
    Hi Steve, thank you for your answer.
    I ended up using a 555 for the reset pulse, I know is a little over the top and could be done in a more simple way without using a 555, but it seems to work very good for me.
    This is like a programmer that is not very knowledgeable and uses 100 lines of code to execute a certain task when the same task could be done with 20 lines if you really know what you are doing.
    I've used the remaining gates on the IC to be able to have an external source for the reset as well as the pulse from the 555 and the last gate as an inverter.
    The timing with this circuit is about 110 ms
     
  6. AnalogKid

    AnalogKid

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    Jun 10, 2015
    Here is the same circuit without the 555. The added resistor R3 adds hysteresis to the R-C input to eliminate possible noise bursts at the output. It does not affect logic action caused by the pin 11 input.

    ak
    POR-1-c.gif
     

    Attached Files:

    Last edited: Aug 24, 2016
    Titus likes this.
  7. Titus

    Titus

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    Aug 19, 2016
    Awesome! thank you!
     
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