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'how can I only uses 5 cpu I/O pin for 16 keyboard '
2000\03\26@072141 by

Wing Kent Kwan appears to have written:

> how can I only uses 5 cpu I/O pin for 16 keyborad keys input?
>  My means is No use IC to do it!
>  only use the diodes to do it? but I don't know how to connected it?

Fairly simple.  Each key goes to ground, each CPU line has a pull-up.
Each diode in the matrix goes anode to CPU pin, cathode to key.

Ten keys have two diodes each to a pair of CPU pins, there are 5c2 or
10 (= 5 x 4 / 2) such combinations.  Ten more keys could have three
diodes each to CPU pins (5c3 = 5 x 4 x 3 / 3 x 2 .. consider it as each
*not* going to 2 CPU pins out of the five).

Remember you will need a *good* debounce routine for this, it is
*zero*-key rollover IOW two keys pressed simultaneously will quite
possibly show as a third, different key.

You will probably want to save on diodes by using all two-diode and
only six three-diode key encodings, total 38 diodes.  I feel it is
somewhat more reliable than a single-line encoding.  Of course if you
allowed anywhere from one to five diodes per key, you can encode 31
keys.

1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20
A  x  x  x  x                    x  x  x  x  x  x
B  x           x  x  x           x  x  x           x  x  x
C     x        x        x  x     x        x  x     x  x     x
D        x        x     x     x     x     x     x  x     x  x
E           x        x     x  x        x     x  x     x  x  x

Frankly, I think it would be simpler and easier to use a shift
register or counter...
--
Cheers,
Paul B.

I have to wonder sometimes when someone asks this type of question.

1) Are they just sooooo new that they don't know what kind of trouble they
are getting themself into?

2) They have LOTS of diodes laying around and too broke to afford the IC's?

3) They want us to do their homework for them??

"Paul B. Webster VK2BZC" wrote:

> Wing Kent Kwan appears to have written:
>
> > how can I only uses 5 cpu I/O pin for 16 keyborad keys input?
> >  My means is No use IC to do it!
> >  only use the diodes to do it? but I don't know how to connected it?
>
>   Fairly simple.  Each key goes to ground, each CPU line has a pull-up.
> Each diode in the matrix goes anode to CPU pin, cathode to key.

nICE one
why not use another pic? ok. not as cheap as the diodes but cheaper than you
could emulate a PC keyboard put infrared link as well,but hell if you are
into pic's then they are the only thing to use...........

{Original Message removed}

On Sun, 26 Mar 2000 09:15:59 -0800, you wrote:

>I have to wonder sometimes when someone asks this type of question.
>
>1) Are they just sooooo new that they don't know what kind of trouble they
>are getting themself into?
>
>2) They have LOTS of diodes laying around and too broke to afford the IC's?
>
>3) They want us to do their homework for them??
4) They haven't figured out that you can almost always share pins with
other functions for this type of app.

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