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championx
Joined: 28 Feb 2006 Posts: 151
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4x4 keypad and noise |
Posted: Fri Aug 14, 2015 5:31 am |
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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. |
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asmboy
Joined: 20 Nov 2007 Posts: 2128 Location: albany ny
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Posted: Fri Aug 14, 2015 6:52 am |
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post a schematic of you circuit for keypad and PIC |
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championx
Joined: 28 Feb 2006 Posts: 151
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asmboy
Joined: 20 Nov 2007 Posts: 2128 Location: albany ny
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Posted: Fri Aug 14, 2015 8:33 am |
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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. |
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championx
Joined: 28 Feb 2006 Posts: 151
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Posted: Fri Aug 14, 2015 8:37 am |
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Thanks for your help asmboy, sorry, but what do you mean with "termination resistor" ? |
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ezflyr
Joined: 25 Oct 2010 Posts: 1019 Location: Tewksbury, MA
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Posted: Fri Aug 14, 2015 9:49 am |
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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! |
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asmboy
Joined: 20 Nov 2007 Posts: 2128 Location: albany ny
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Posted: Fri Aug 14, 2015 11:03 am |
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Quote: | Termination resistors |
pull the PIC INPUT port ( rows ) LOW
unless pulled UP when the scan driven output(column) PIN
goes HIGH .
the "Termination resistors" maintain a sure LOW state for the ROW detect lines when the ROW inputs is read while ONE COLUMN at a time is pulled high during the scan.
w/o these resistors - the lines float and can appear HIGH due to environmental coupling etc. |
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ezflyr
Joined: 25 Oct 2010 Posts: 1019 Location: Tewksbury, MA
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Posted: Fri Aug 14, 2015 2:40 pm |
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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! |
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temtronic
Joined: 01 Jul 2010 Posts: 9173 Location: Greensville,Ontario
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Posted: Wed Oct 21, 2015 1:35 pm |
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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 |
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