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'Keyboard decoders and design philosophy'
1999\01\09@130545 by Michael Mullen

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I usually use a '138 and a '151 (3-8 decoder, 8-3 mux) to scan up to a 64 key
keyboard.  Use the output of the mux as an input to see if a key is pressed.
Operation is simple:
for i=0 to 63
       port =i
       if input low then
               //key has been pressed, do something
       endif
next i

Uses 7 pins, but the 'ls (or hc or whatever) chips are cheap and multiple
sourced.  Can also prefix the address pins with a 6 bit counter, then you can
use 3 wires.  Clear counter, clock counter, check input.

Philosophic note:

There is no "best" keyboard encoder -- or usually, anything other function.
The above is a low cost hardware solution, rapid machine execution, and
trivial programming.  Shift registers save pins, but require more programming
and longer execution times.  Also harder for a newbie to debug.  I am building
things in 100 quantities, and adding a $.05 part to save an hour of
programming or design time is about a wash.  If you're doing this for fun,
designing stuff for 100k/year production rates, or building life-support
equipment, you get different answers.  I have designed stuff for
geosynchronous weather satellites (production rates 1/year) and video games (1
million/year), and believe me, you get different answers for the same problem.

Mike Mullen

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