Righto. I'll leave it as it is while I get my other chip working, then I
---John Fields wrote:
You have what's called a "static" display, and the pins marked "com"
are connected to the backplane, which is the transparent electrode
which forms one side of the capacitor comprising each segment and the
backplane. MOST static LCD displays aren't designed to be driven by
DC, and what will happen is that the ITO will plate out of either the
backplane or the segment and migrate to the opposite electrode, with
the eventual result that the capacitor will be destroyed and the digit
will become unreadable. The proper way to drive static LCDs is with
square wave AC; the backplane and the segment being driven in phase
when the segment isn't supposed to be displayed, and out of phase when
it is, like this for the segment to be off:
__ __ __ __ __ __
SEGOFF__| |__| |__| |__| |__| |__|
__ __ __ __ __ __
BP __| |__| |__| |__| |__| |__|
and like this for it to be on:
__ __ __ __ __ __
SEGON |__| |__| |__| |__| |__| |__
__ __ __ __ __ __
BP __| |__| |__| |__| |__| |__|
This task _can_ be accomplished with a ľC and a longish shift register
By EXORring the backplane and the segments at about 30Hz., but it's
often done with a device like a National MM5483 so all you have to do
is shift data into it and strobe the output registers without any
extra software overhead.
http://cache.national.com/ds/MM/MM5483.pdf
If you've got a 3-1/2 digit seven-segment display, that's 23 segments,
and the 5483 can drive 32 segments and a backplane, so that leaves you
with nine extra segment drivers for decimal points, annunciators, or
the colon if you're doing a clock. Best of all, Digi-Key's got them
in 40-pin DIP packages for about five bucks.
I see (oops)... What I don't understand though, if "com" is for the
backplane - what about ground? :-\
---
There _is_ no hard ground required, it's like this:
INn------------------------------------------+
|
INb----------------------+ |
| |
INa---+ | |
| | |
+--A +--A +--A
EXOR Y------+ EXOR Y------+ EXOR Y------+
INBP>-+--B | +--B | +--B |
| | | | | |
+------------|-----+------------|------+ |
|COM a| b| n|
+-|------------|------------------|-------------------|----+
| | | | | |
| | [SEGa] [SEGb] [SEGn] |
| | | | | |
| +------------+------------------+---------//--------+ |
| DISPLAY GLASS |
+----------------------------------------------------------+
Where the EXORs are part of the external drive circuitry, the "DISPLAY
GLASS" is the display itself, and SEGa, SEGb, and SEGn represent the
inherent capacitances between the various segment electrodes and the
backplane.
Righto. I'll leave it as it is while I get my other chip working, then I
might play about with changing it as you suggest!