Jason Wolfson wrote:
--------8<--------
>>Does anyone have any clever, low parts count
>>methods of multiplexing 3 pushbuttons onto
>>2 port pins?
--------8<--------

Then Philip Martin replied:

--------8<--------
>Easy,
>
>Switch one connects to pin one.
>Switch two connects to pin two.
>
>Switch three connects to both pin one and two, each connection via an
>IN4148 diode.
--------8<--------

The problem with this (which may not be a problem, depending on the
application) is that you can't distinguish between switch 3 being pressed
vs. switches 1 & 2 being pressed simultaneously. Without some kind of
lockout circuitry in the encoder, there's no way to do this with just a
diode/resistor matrix alone.

That being said, you might be able to do something using diodes & pullup
resistors in conjunction with time multiplexing code to sort out which
switch (or combination) is being pressed. It would go something like this:

B0, B1 are the I/O pins, SW1-SW3 are the switches. SW1 is connected between
B0 & B1 with its diode D1 in series, and SW2 its diode D2 are connected
antiparallel. SW3 is connected to both B0 & B1 through two steering diodes,
D3 & D4:



                       Vcc
  SW3                   |
     __I__              R1
                 +      |                           |
 +---o   o---o----D3----o--------o----------o-------| B0
 |           |                   |          |       |
 R3          |             SW1 | o    SW2 | o       |
 |           |                -|         -|         |
Gnd          |                 | o        | o       |
             |                   |          |       |
             |         Vcc       |+         |       |
             |          |        D1         D2      |
             |          R2       |          |+      |
             |   +      |        |          |       |
             +----D4----o--------o----------o-------| B1
                                                    |





To read the status of the switches, you must set the TRIS & data bits so as
to cycle B0 and B1 through the following three states:


     B1         B0               Meaning
---------------------------------------------------------

State 0
   out low    in high        Neither SW2 nor SW3 pressed
   out low    in low         SW2 OR SW3 pressed

State 1
   in high    out low        Neither SW1 nor SW3 pressed
   in low     out low        SW1 OR SW3 pressed

State 2
   in low     in low         SW3 pressed
   in high    in high        No switch pressed



Just a little more work will allow you to figure exactly which switch is
pressed.

Note: I haven't tested this, but it seems as if it should work. No doubt
others will spot any errors or shortcomings.

--BN


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