Driving LCD 7-seg passive displays with PIC

I have a passive LCD with 8 7-seg digits plus point and some other
symbol. It has 23 connections.
I wish to drive it with a PIC that has LCD module built-in but I have
seen that all PICs capable to drive LCD have only 4 commons. I
supposed that was needed 1 common for each digit but probably I have
to use bias plus multiplex. In this case how can I do? I have not
found any application notes and info on internet. Thanks.

 

[Robert Monsen]

On Wed, 17 Sep 2008 06:05:36 -0700 (PDT), cyberbige@gmail.com wrote:

I have a passive LCD with 8 7-seg digits plus point and some other
symbol. It has 23 connections.
I wish to drive it with a PIC that has LCD module built-in but I have
seen that all PICs capable to drive LCD have only 4 commons. I
supposed that was needed 1 common for each digit but probably I have
to use bias plus multiplex. In this case how can I do? I have not
found any application notes and info on internet. Thanks.

You can use a port expander. Microchip has at least one of these, and
there are app notes on how to use them. You can also use serial in -
parallel out shift registers to drive these things. That is probably
easier to control. Using these, you can drive all the bits with 3 GPIO
pins. You will need 3 of them.

http://www.allaboutcircuits.com/vol_4/chpt_12/4.html

Regards,
Bob Monsen