Jinx wrote...

>> David Harris wrote...
>
>>CHALLENGE: can you scan a keyboard, and run an LCD
>> display with only 8 pins/bits =3D 1 port -- without extra active
>> hardware?
>
>Yes, with a 4-bit LCD (7 i/o lines needed) and a k/b matrix
>of 4 rows as o/p (ie driven) and 4 columns as i/p. Flip between
>port as o/p for the LCD and port as mixed i/o for the k/b. If the
>4 row drivers are also the LCD data lines, the s/w should be
>able to avoid any unwanted writes to the LCD because of
>unwanted conditions on RS, R/W and E caused by key presses.
>Might take a little work to idiot-proof but I'm sure it could be done

I briefly considered an arrangement like this on one of my projects but
rejected it because of three problems which appeared insoluble:

=46irst is the possibility of LCD data corruption if a key happens to get
pressed while an LCD write operation is in progress. =20

Second, under the same circumstances two PIC output pins will end up
getting shorted together (by the depressed key) while they're both
outputs, and if they are in opposite states a large I/O pin current will
flow.  While this probably won't damage the PIC it will cause a high
drain on the power supply, which isn't acceptable in my applications. =20

The third problem is that the LCD E line can end up getting toggled
during keypad scanning, leading to a false LCD write.

There seemed no way to absolutely, positively prevent these three
problems from occurring, so I gave up on this approach.

Dave

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