> Drive the output LOW to test for keys on a given row or column.  You can
> tell that a switch is pressed if the input is LOW.  The pullup resistor
will
> cause the input to quickly return high after the output is turned off or
the
> switch is released.

This is what I did and it seem to fix the problem.  I make the Pins on the
OUTPUT like 0111 1011 1101 1110.  Then when I connect to another Input Pin I
get a result that is LOW only at the time I'm on the scan for that button
and as soon as I release it goes back to HIGH cause if the internal pull
ups.

> Do NOT drive any of the outputs high because they could be shorted to the
> output which is driven low if more than one key is pressed in the matrix.
> Load the output latch with a "0" for each output bit and leave it that
way.
> Set the corresponding TRIS bit to "0" to drive a row or column output.
Set
> the TRIS bit back to a "1" to stop driving the row or column output.

This is what I can't understand?  How will I know which pin is which if I
don't set the other 3 high when I'm checking my inputs.  I realize the
situation if I press 2 keys on the same ROW at the same time I short out the
Outputs.  Is this that bad?  How else could I do it?

> If you can replace the 270 Ohm resistors with larger values (10k would be
> nice) then the circuit would consume less power.  The inputs would still
> return high within a microsecond or two unless you have more than a
coupple
> hundred pF capacitance on them.

Noted for future designs.

Darren King


> Darren King wrote:
> >I'm trying to read a Keyboard Matrix that is connected so that I scan 4
> >input lines and pulse 4 output lines and when I pulse and check I find
the
> >button that is pressed.  I wrote an E-Mail about this before and I thank
> >everyone for that answer.  Now my problem is when I press a button
(Buttons
> >has a 270Ohm Pull up resistor) the BIT goes high, then when I let go of
the
> >button the BIT stays high.  Then after about a second or 2 it fades off.
> >This wasn't a good result.
> >
> >This Buttons do not have an Normal Open Pull down resistor so I guess the
> >just float when disconnected.  I tried to activate the internal pull ups.
> >This didn't help.  This just made all the bits high all the time.  I've
> been
> >Thinking of Toggling the TRISB state after a read to reset the Pins
states,
> >but there must be an easier way.