Bob,
how did you deal with the possibility of more than one switch being pushed
at a time? Did you just verify the math to make sure that there is no
overlap in the output voltage ranges?
- Marcel

On 8/14/06, Bob Axtell <engineer@neomailbox.com> wrote:
>
> 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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