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latching AND gate

Discussion in 'Electronic Basics' started by Pete, Apr 12, 2004.

  1. Pete

    Pete Guest

    Having problems controlling a AND gate that I want to latch.

    Have two inputs I1 and I2

    Output to go High when I1 and I2 are high, but once Output is high remain
    High until I2 falls to low. (regardless of I1)

    If I try to connect the output of the gate to I1 (to provide the high input
    when output goes high) strange things happen?

    What am I doing wrong?
     
  2. You need more than one gate to perform the AND function and the ORing
    of the output with one of the inputs to form a latch.

    +---+
    I1 ---| |
    |AND|---+--- Output
    +---| | |
    | +---+ |
    | +---+ |
    | | |---+
    +---|OR |
    | |--- I2
    +---+

    Depending on the logic family, you may be able to get away with making
    one of the gates with diodes.
     
  3. CFoley1064

    CFoley1064 Guest

    Subject: latching AND gate
    You're not doing anything wrong -- you need a latch circuit of some kind to go
    with your AND gate.

    However, if you've only got a CMOS AND gate, and you're willing to suffer the
    embarassment of building a cheesy MML (Mickey Mouse Logic) circuit, try this
    (view in fixed font or M$ Notepad):

    Latching AND Gate ([email protected]$)
    ___ ___
    I1 o--|___|--o---|___|--.
    22K | 22K |
    ___ | __ |
    I2 o--|___|--o---| | |
    | 22K |& |---'
    '-------------|__|

    1/4 4081


    Created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de (but not his
    fault)

    Remember this circuit will latch as you described, but it is slow, doesn't have
    noise immunity, and biases the gate input in the linear region, which means it
    will use a lot of power. BUT it might get you through. Just don't tell anyone
    you got it from me.

    Good luck
    Not Chris
     
  4. John Fields

    John Fields Guest

    ---

    +---+
    I2 ---| |
    |AND|---+--- Output
    +---| | |
    | +---+ |
    | +---+ |
    | | |---+
    +---|OR |
    | |--- I1
    +---+

    :)
     
  5. Thanks. I'll take all the help I can get. :)
     
  6. Seth Koster

    Seth Koster Guest

    Why not send the output to an OR gate along with I1 and feed that
    to the AND gate in place of I1?
     
  7. Seth Koster

    Seth Koster Guest

    Sorry, did not see previous responses.
     
  8. Soeren

    Soeren Guest

    __
    I2 O----------| \
    | )--+--O Out
    I1 O--|>|--+--|__/ |
    | |
    +---[R]---+

    The source at the input I1 may be a factor in determining the size of
    the resistor (but 10..100k will probably work).
     
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