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.

 

The least number of pins is one.

 

Hi, thanks for the reply.

one? how can I output 256 combinations with 1 input??

 

one? how can I output 256 combinations with 1 input??

you can't.... what is the output and what is the input?

How is the demux applied to your microcontroller?

 

You can't do it with zero pins. Hence the least is one pin.
You can send the information out serially via one pin.

 

You can't do it with zero pins. Hence the least is one pin.
You can send the information out serially via one pin.

...but not with an input pin.....

 

...but not with an input pin.....

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);

 

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.

 

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?

 

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.