Post by thread:Keyboard decoders and design philosophy

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