I haven't seen the electronics magazine article on 1 pin keypad, but
here is my answer to Ashley Barnett on basically similiar topic. I have
a working version of this on a proof of concept PCB that handles I.R.
coms, 128k bit EEprom(I2C), async serial, 4-bit LCD, piezo drive. Using
a 16C74 this leaves: 7 spare analog inputs, 9 digital I/O.
I designed it to run with a DS1307Z SMD I2C Real Time Clock also except
no DS1307Z from my supplier in 10 months

G.Daniel Invent Design wrote:
>
> ashley barnett wrote:
> >
> > Keypad Scanning Question, help reqd.
> >
> > I have a 3x4 keypad connected to portB of a pic16c74 and written the basic c
ode
> > which will jump to the interrupt vector whenever any key is pressed. I then
will
> > read portB and move the value to another register. thats the code so far,
> >
> > What I want to do are the following.
> >
> > a) discover which key was pressed
> > b) use each key to call a different subroutine
> >
> > My problem is how to implement the key descrimination as there is no compare
> > command
> ????????
> movlw   test_value1
> subwf   portb,w
> bz      test_value1_code
> the above is a very simple structure that you can repeat for each key,
> you can compact by reading test values out of a table read.
>
> , I think I need to XORLW ? but what then etc,
> > I also need to implement a table read, don't I ?
> >
> > Any help and direction appreciated
> >
> > Email to  - ashsparky@clara.net - thanx for your time
>
> That's a lota pins tied up with your keypad, are you sure you don't have
> a spare A2D pin ? ; use it to measure a value from keypad as follows:
>
> +5---------1---2---3    each key will give
>      |     |   |   |    a different value.
>    [1k]    |   |   |
>      |     |   |   |    by testing values,
>      ------4---5---6    you can determine
>      |     |   |   |    key thresholds &
>    [1k]    |   |   |    then perform a
>      |     |   |   |    computed jump.
>      ------7---8---9
>      |     |   |   |    these computed jump locations
>    [1k]    |   |   |    can have GOTO hash,GOTO star,GOTO zero etc.
>      |     |   |   |
>      ------*---0---#
>            |   |   |
>           [5k][5k][5k]
> PICpin_____|___|___|
>                |
>               [1k]
> 0volts_________|