On Sun, Aug 22, 2004 at 02:01:35PM +0000, iain duncan wrote:
> I am a newbie to pics and electronics, though I have no problems with
> assembly programming.

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

> I get much more confused by what's going on
> outside the pic. I would like to interface four 4x4 keypads to one chip
> that will also do midi output, which i guess means multiplexing of some
> sort,

It does.

> is this likely to be a terribly difficult project?

Not really. It'll require some external hardware I'd guess off the top of my
head.

>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.

>
> 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.

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.

BAJ

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