Keypad Scanning with PIC MCU

muahaha · Joined: 11 Oct 2005 Posts: 4

How can i scan a keypad only using two pins of MCU and what is the best values of resistors and capacitor used for a keypad which has got 12 keys. Maybe somebody send a chart for this circuit. Also i have problems on C programming part. Please help me! Thanks for your interest...

Ttelmah · Guest

Is there an ADC available on one or both pins?. If so is it being used for anything else?. If not, are the inputs schmidt trigger types or normal logic types?. How often do you need to 'scan' the keys?. What timer resources are available?. Could one of the inputs connect to a interrupt, or CTC input?.

Best Wishes

treitmey · Joined: 23 Jan 2004 Posts: 1094 Location: Appleton,WI USA

Google search. "one pin keypad"

analog
http://www.elecdesign.com/Articles/Index.cfm?ArticleID=3560
freq
http://www.elecdesign.com/Articles/ArticleID/1869/1869.html

Ttelmah · Guest

This is what I suspect he is thinking about. Which is why I asked if an ADC is available on the pins. However there are some major 'caveats', because of the charge time of the ADC (which shoots through the roof as the source impedance rises, if accuracy is to be maintained), if the ADC is being used for anything else (second question I asked), this could cause problems. If an ADC is not available, there is a way to do it, by generating a crude 'time reading' based on the discharge of a capacitor, being charged by one pin, and read by the other. Hence the questions about timing resources, and inputs.

Best Wishes