piclist 1997\08\20\220747a >
Thread: PIC16C924 LCD question
face BY : John Payson email (remove spam text)

> Does anybody know if it possible to drive two separate LCD's from one '924.
> I would like to use separate 4 digit LCD's and tie there segments together
> but use the COM0 and COM1 backplane drivers to drive them separately.
> Is this possible? What should I be aware of and watch out for?

I see no problem with doing this.  The only caveats I can really see are

[1] Your contrast won't be as good as it would be with static drive.  Even
a 2:1 mux severely limits your contrast (often not objectionably so, but
with static drive the clear parts get no juice and the dark parts get max
juice; by comparison, with 2:1 mux your RMS_on:RMS_off ratio is only
sqrt(5):1(*) This may necessitate the addition of a contrast knob since
you will probably no longer be able to get totally dark blacks or totally
clear whites.
(*) You'll get a sqrt(5):1 RMS ratio whether you use a bias voltage of 1/2
or 1/3.  If you use a bias voltage of 1/(1+sqr(2)), however, you can get a
contrast ratio of up to sqrt(3+2*sqrt(2)):1 [about sqrt(5.8:1)]  The PIC
16C924, however cannot bias the display with such a voltage.

[2] Your displays will have to be 31 segments or less, each.

[3] It may be easier to wire things if you use two displays, each of which
is internally 2:1 or 3:1 multiplexed.  If you go 2:1, your net output will
be the same as if you use your existing displays, but you'll only have to
connect about half as many pins.  If you go 3:1, your contrast won't be
quite as good, but you'll have even fewer pins.
<199708210205.VAA11474@Kitten.mcs.com> 7BIT

In reply to: <199708202236.QAA160930@huey.cadvision.com>
See also: www.piclist.com/techref/io/lcd/pic.htm?key=lcd
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