I have a standard design that handles up to 7 pushbutton switches (as in
membrane switch) on just ONE PIC analog input pin. It works very
well, maintains a good input impedance, so needs NO special analog
input processing.

I was asked to share the design with another PIC engineer, so I might as
well share it with anyone that wants it. It will be in ZIP form, and will
contain PDF schematics and ASCII TEXT code for the PIC16F876A
with easily ports to just about ANY PIC with  an  A/D converter..

Each switches contacts to GND  a point within a resistor array. The
A/D seems a voltage of 0.0V, 0.7V, 1.4V, 2.1V, 2.8V, 3.5V, and 4.2V.
5V means no switches are pressed. A small software window is built
around each value, so an amount of error is tolerated; for example,
rubber button switches normally measure 100 ohms when closed.

Please note that this works for 7 pins PER ANALOG PIN. That means
for the 8-pin analog switching of the PIC16F877A, up to 56 switches
can be handled with ease.

I read the switches every 1mS but that is because the design is a serial 
keypad.
For most applications, reading every 10-30mS should be fine.

If anyone wants a copy, just ask.

--Bob

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