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
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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
77
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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
77
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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
197
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
1,328
305
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.

Sep 7, 2013
77
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9. THE_RB AAC Fanatic!

Feb 11, 2008
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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.