Multiplexer latch

Discussion in 'The Projects Forum' started by jismagic, Sep 20, 2013.

  1. jismagic

    Thread Starter Member

    Sep 7, 2013
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    i have a 3 input binary and 8 possible output combinations.

    i want to latch the output for various inputs.

    1)Set input to be 000, 8 outputs to be low.
    2)Set input to be 010, 1st line to go high and rest of the 7 outputs to remember previous state..
    3)Set input to be 001, 0th line to go high and rest of the 7 outputs to remember previous state

    and so on............

    any ideas? I found multiplexer with latch.. but not sure if that is the right one to pick..

    jis
     
  2. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    If I understand your question right it is flawed in concept. You seem to be saying that 000 clearns all "8" output latches, then 001 (etc) sets 1 output pin, and the other output pins stay in their previous state?

    For a start, you only have 7 output pins, not 8 (as you have 000 = clear, and xxx = 7 pin addresses).

    Then, you have input pin change contention where it will be practically impossible to go from one input to another. You cant input 001 to latch a value, then after that input 010 to latch another value, without having a transition! If you put 000 between transitions it clears your latches (fail) and if you don't have a safe state between transitions, then you go through undefined states;

    000 (clears latches)
    001 (sets pin1)
    0xx (two pins MUST change state, causes unwanted outputs or clearing!)
    010 (sets pin 2)

    I think you need to explain more about WHAT this circuit needs to do, and WHY you need to do it!
     
  3. jismagic

    Thread Starter Member

    Sep 7, 2013
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    Flawed thinking..Sorry!!


    Idea : To generate 8 digital out from 3 digital out of micro pin.

    Logic : The 8 numbers can be generated from 3 bits.

    Problem : How to set/reset multiple outputs using 3 bits..

    Your ideas pls...
     
  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    An octal multiplexer will always have one output 'on'. It accepts a binary 3-bit input and that generates a single 'hi' pin on a bank of 8 pins. See 74151 chip
     
  5. jismagic

    Thread Starter Member

    Sep 7, 2013
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    How do i remember my previous selection?
    as in 001 set ouput 1
    next, 010 set output 2
    But how do i keep output 1 enable when we set output 2..
     
  6. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
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    If you want to latch various combinations of 8 bits, while using only 3 bits, then consider using a shift register. Your 3-bits would be data, clock and latch. This method also allows adding more shift registers for even more bits of output.
     
  7. panic mode

    Senior Member

    Oct 10, 2011
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    why multiplex, if you hsve microcontoller, just use SIPO register like 74HC595, and if you need more than 8 outputs, just add another chip (and another...)...
     
    GopherT likes this.
  8. jismagic

    Thread Starter Member

    Sep 7, 2013
    77
    0
  9. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    1,305
    That page is still multiplexing. Forget multiplexing, it's the wrong thing.

    What you have is 3 PIC output pins, and you need full control of 8 output pins. That needs a serial-in parallel-out "shift register" (as PanicMode said).

    Try these links (found easily by googling "PIC and shift register");

    http://lucidtronix.com/tutorials/2

    http://arduino.cc/en/Tutorial/ShiftOut

    There are a lot of examples to google. :)
     
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