Demultiplexer

Discussion in 'General Electronics Chat' started by Dritech, Feb 8, 2013.

  1. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Hi all,

    I want a demultiplexer with 8 outputs to create a combination of 256 (2^8). The reason for this demultiplexer is to use the least pins possible from the microcontroller.

    Now my question is, what is the least number of input pins that can be used?

    Thanks in advance.
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,415
    3,354
    The least number of pins is one.
     
  3. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Hi, thanks for the reply.

    one? how can I output 256 combinations with 1 input??
     
  4. tshuck

    Well-Known Member

    Oct 18, 2012
    3,531
    675
    you can't.... what is the output and what is the input?

    How is the demux applied to your microcontroller?
     
  5. MrChips

    Moderator

    Oct 2, 2009
    12,415
    3,354
    You can't do it with zero pins. Hence the least is one pin.
    You can send the information out serially via one pin.
     
  6. tshuck

    Well-Known Member

    Oct 18, 2012
    3,531
    675
    ...but not with an input pin:confused:.....
     
  7. MMcLaren

    Well-Known Member

    Feb 14, 2010
    759
    116
    It's not clear, but, I thought the OP meant one input pin on the demultiplexer...

    Anyway, you should be able to do it any number of ways using one I/O pin. For example, as in the second drawing below where a single I/O line drives the 74HC164 clock and data lines (it takes approximately 100 microseconds to load the shift register with the RC values shown);

    [​IMG]
     
    Last edited: Feb 8, 2013
  8. crutschow

    Expert

    Mar 14, 2008
    12,977
    3,221
    You can use one μC output pin to serially output the mux address and one μC input pin to sense the mux output. Of course that will require a serial-to-parallel converter (shift-register) to generate the parallel address for the mux.

    To simplify the timing and synchronization you may want to have a serial address output and a serial clock output, requiring two outputs from the microcontroller.
     
  9. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Thanks for the replies.

    Lets assume I am going to use common I/O pins and want to use a demultiplexer without and clocks, what will be the minimum numbers of input pins?
     
    Last edited: Feb 8, 2013
  10. MrChips

    Moderator

    Oct 2, 2009
    12,415
    3,354
    I have 4-digit LED displays with sign and decimal points.
    I can connect as many of these to a microcontroller using a single pin.
     
Loading...