Both of you are on the right track. There are a
couple of ways this can be done, depending on how many segments you want to
drive.
Method #1
Connect all pins of the LCD
to the PIC including the backplane. When generating your code keep in mind that
you must always keep the outputs switching. LCD's must be driven by an AC
voltage, DC will over time leave the segment permanently on. The drive frequency
should be 60 to 100 Hz, just enough for persistence of vision and no noticeable
flicker. Drive the backplane at a constant frequency. If you want a segment on
then drive it with the same constant frequency out of phase with the backplane
signal. If you want the segment off drive the segment with an in phase signal to
the backplane. The way this works is that when both the backplane and segment
are in phase the voltage across the segment is zero. When they are out of phase
the voltage is 5V or whatever the supply voltage is.
Method #2
Use XOR gates. Drive the backplane and one XOR input of each
gate with a 60 to 100Hz frequency. This can be easily generated from a 555 or a
PIC. Connect the outputs of each XOR gate to each segment pin on the LCD.
Driving the second input of the XOR gate with a simple high for 'on' and low for
'off' will turn the respective segment on or off. If you need to know how this
works draw it out as a timing diagram and work out each pulse with the XOR truth
table.
There have been some good app. notes written on the first
method. One of the best is from Microchip, AN563.
Hope this helps everyone out.
James
> netQ wrote:
> Hi. I want to use a 3 1/2 segment static drive
LCD for data display with
> a PIC16F84. I've found a stable square wave to
drive the segments and
> it's generated by a 4047 (1 KHZ square output,
standard and inverted (Q & 'Q))
> I thought that in order to let the
PIC control a segment, I should let it
> control AC current flowing to any
pin (except for pins 1 & 40 which correspond
> to the
backplane).
> Apparently, if I want a segment off it must be supplied with
the same AC current
> of the backplane (let's say Q) and if I want to turn
it on, inverted AC
> current ('Q) should be supplied.
> Could
anybody suggest a way to let the pic switch a segment on or off given
the
> above?
> Do you people know of an IC that would let the
PIC multiplex this type of
> signal?
>> Unless I am
misunderstanding the problem a garden-variety 2-input XOR ought to
>>
provide what you want, yes? Or am I way off base?
>>
Michael