First I'd like to apologize for not putting [PIC] in the subject. I
thought it got there automatically, won't happen again.

> As I always say, learn PIC assembly so you can converse with folks, then
> look to higher level languages.

Yup, I'm doing that. I have the Predko book and the Katzen book.
Actually I like the assembly part. The hard part for me is that the PIC
books naturally assume that you've already done a fair bit of
electronics. ; )

>>Can anyone tell
>>me what other parts I will need to get more than one keypad into the
>>pic, or point me at any good resources for this? Is there anyone on here
>>who might have made a keypad-to-midi controller?
>
> It'll be an interesting device for sure.

If I get it working properly I'll put the design online. The nice part
is that I just scored 20 free keypads by asking around, and someone had
tons of old credit scanners with a 4x4 keypad.

>>Also, will it be possible in such a situation to write the firmware so
>>that it can accept multiple simultaneous keypresses across the pads, or
>>is that reall hard?
>
> That'll make it harder.

For a controller, it would make it more useful, but I suppose I can live
without that feature until it works at least.

> OK let's talk about organization. You can organize your keypads in 3
> configurations:
>
> 1) 4 individual keypads.
> 2) A single 8x8 keypad.
> 3) A single 4x16 keypad.
>
> Each will require a different amount of I/O
>
> 1) The 4 individual keypads would require 32 I/O lines
> 2) The 8x8 would require 16 I/O lines.
> 3) The 4x16 would require 20 I/O lines.
>
> In each case a decoder and/or multiplexer can be used to reduce the lines to
> the corresponding power of 2. So for the cost of a chip, you can greatly
> reduce the number of lines required.
>
> Personally I'd organize in the 8x8 wiring the 4 keypads in a square. Then
> I'd drive the rows with the outputs of a 74HC138 decoder, and read the outputs
> from the columns using a 74HC151 multiplexer. It'll take 7 PIC I/O pins to do
> the job: 3 to drive the HC138, 3 to drive the HC151, and one input to read the
> HC151.

Thanks so much for the tips. If anyone knows of an example of the above
somewhere, preferably both assembly and schematics, that would be lovely.

Thanks
Iain

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