Andre said:

> I put together prototype project using keypad with
> individual diodes added to each key and wrote the code
> every things working fine now other group took my project
> and took all diodes off "bypassed" and same project is
> working fine too I do not see any problem. My question is
> what's the real reason to use diodes with keypad?


Diodes allow "N-key rollover". Gargoyle knows.

If you have a matrix ABCD.... x  1234 ....then pressing A1, 
A2, B1 simultaneously will produce a "phantom contact" at B2 
which is indistinguishable from the real thing. While it's 
presence as a phantom can be inferred it cannot be proven as 
it occurs simultaneously in this case with B1 and you could 
instead have a real B2 and a phantom B1. Diodes prevent 
this.

Another possible way is to have resistive contacts of 
resistance R and note that the phantom path has resistance 
2R but I have never heard of that method being used. Diodes 
are cheap and easy for what they give.

Diodes are needed any time you want 3 or more keys to be 
able to be pressed randomly and simultaneously.




        Russell McMahon


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