4x4 keypad and noise

championx · Joined: 28 Feb 2006 Posts: 151

Hi all! I'm testing a circuit that have a 4x4 matrix keyboard. The thing is when I put my hand near the traces, it detects some keypress, it seems that is a hardware design problem.. (no caps and no pullup/pulldown).

Is there something that i can do to reduce this problem? adding capacitors, or resistors, or something on the firmware that i can add to eliminate this?

thanks.

asmboy · Joined: 20 Nov 2007 Posts: 2128 Location: albany ny

post a schematic of you circuit for keypad and PIC

championx · Joined: 28 Feb 2006 Posts: 151

This is basically the keyboard:

https://electrosome.com/wp-content/uploads/2012/06/4x4-Matrix-Keypad.png

Where the row and col pins are directly connected to the pic pins.

Today I put some pulldown resistors to the input pins (ROWS) and it really got better. But, what do you think?

thanks.

asmboy · Joined: 20 Nov 2007 Posts: 2128 Location: albany ny

depending on your scan rate and dwell time -you can always reduce
stray signal by using input termination resistors.

if such sensitivity is extreme - i would use 5.1k or even 2.2k resistors for that task. capacitors are less good of an idea.

championx · Joined: 28 Feb 2006 Posts: 151

Thanks for your help asmboy, sorry, but what do you mean with "termination resistor" ?

ezflyr · Joined: 25 Oct 2010 Posts: 1019 Location: Tewksbury, MA

Hi,

Have you spent any time searching the forum archives? Here is a recent thread that will probably help to resolve your issue: http://www.ccsinfo.com/forum/viewtopic.php?t=54037&highlight=keypad

BTW, it's not surprising that you have noise. Your row and column inputs are 'floating', so when you scan them it's very likely that they may pick up some noise!

Termination resistors??? I don't get that solution!
_________________
John

If it's worth doing, it's worth doing in real hardware!

asmboy · Joined: 20 Nov 2007 Posts: 2128 Location: albany ny

ezflyr · Joined: 25 Oct 2010 Posts: 1019 Location: Tewksbury, MA

Hi,

Sorry, my comment was poorly worded. I wasn't questioning that particular solution - I understand it - but, rather I was unfamiliar with that terminology as applied in this particular application!
_________________
John

If it's worth doing, it's worth doing in real hardware!

temtronic · Posted: Wed Oct 21, 2015 1:35 pm

You should never, ever be able to 'false trigger' a keypad. It's a sure sign of bad things to come ! EMI in any form an kill a PIC so you must follow proper techniques in interfacing a keypad to a PIC.

There's lots of info on this site as well as tons on the 'Net'.

Jay