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.