Keyboard matrix problem.

S

SgtB

Member
Hello!
I've run into a problem with a 4x5 matrix on a 40X2 I've built. I'm pulling the four columns high one at a time and reading the value of each of the five rows into a ptr. My problem is that every button works fine, but if I push two buttons on the same row, the voltage drops from 5v to .8v and turns off everything on that row. What can I do to fix this? I'm a little foggy on where the sink is. Is it because the output isn't strong enough, or is it in how the inputs are on the silicon? Can I use diodes somehow, or would it be better to use sink detection instead with pullups? Is it a simple programming change I can make? Thanks in advance. Here's the relative code snippet.
EDIT: I have a 10k pull down on each input if that makes a difference.

Code: 
'Set symbols for button rows and columns
Symbol Col1 = A.0    <These are set high one at a time
Symbol Col2 = A.1
Symbol Col3 = A.2
Symbol Col4 = A.3
'********************
Symbol Row1 = pinA.5   <These are read as inputs one at a time.
Symbol Row2 = pinA.6
Symbol Row3 = pinA.7
Symbol Row4 = pinC.0
Symbol Row5 = pinC.1
'********************

ReadButtons:
'Read the current state of each button into scratchpad byes 1-20. 
'Compare them to previous values in 21-30.
'If they are different determine note on or off, and store cv in pv.

ptr = 1
high col1
 @ptrinc = row1 
 @ptrinc = row2
 @ptrinc = row3
 @ptrinc = row4
 @ptrinc = row5

low col1
high col2
 @ptrinc = row1
 @ptrinc = row2
 @ptrinc = row3
 @ptrinc = row4
 @ptrinc = row5
low col2
high col3
 @ptrinc = row1
 @ptrinc = row2
 @ptrinc = row3
 @ptrinc = row4
 @ptrinc = row5
low col3
high col4
 @ptrinc = row1
 @ptrinc = row2
 @ptrinc = row3
 @ptrinc = row4
 @ptrinc = row5
low col4

-SNIP-
 
D

Dippy

Moderator
Assuming I have right end of stick...

When you press 2 buttons on same row you are forming an intermittent short circuit between +V and Gnd on the PIC output driver.
Remember, High is PIC Output-on+V and Low is PIC Output-on+Gnd.

I think that if you tri-state the O/P pins instead of just saying Low it'll sort it.

Draw it out and put in the current paths.... I'm trying to do it in my head which is fairly thick.
 
S

SgtB

Member
Ahhh, so obvious! Thanks. What command would you replace low with? Set it as an input? I may just slap a diode on the top of each column.
 
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