>>
>>The "proper" approach would be pull-ups on the output lines.
>>But that's extra parts, and equal in parts count to your
>>approach of series resistors in the output lines.
>
> Or use PORTB with it's internal pullups.
>

I agree that we should be able to do it with no external resistors. If we
drive the cols and read the rows, the cols could have the latch register
holding 0x00 and TRIS holding 0xff. As each col is to be driven the
appropriate bit of the TRIS would be set low, causing that col output to
be pulled low. We'd then read the row on a portb input with internal
pull-up. If a key is down, we detect the row of the key on portb and the
col based on which TRIS bit we set low. If multiple keys are down, we
could have trouble figuring out which ones they are, but could throw out
multiple keys as invalid. Also, if keys in a couple cols are low, the use
of TRIS prevents us from getting an output high to output low short. Of
course we DO have floating inputs on the col lines that are not being
pulsed low at the time, but I expect that with a typical scan rate the
lines would tend to stay low based on their last being driven low. This
should prevent excess power dissipation due to an intermediate level on a
PIC digital input.

So, use of TRIS to simulate an open drain output (pull low, but not pull
high) is interesting.

Harold



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