Hi all.

I am trying to find a convenient way to (interactively) set the contrast 
on a "standard" character LCD display, and I wanted to avoid using a 
mechanical Pot., so I had the hair-brained idea that I could use the 
comparator's voltage reference to specify the contrast voltage for the 
LCD display. Basically, the contrast voltage is "guaranteed" to be 
between the Vss and Vdd, and so is Vref, so can I use the resistor 
ladder in the PIC to set the Vref (and allow it to be changed 
dynamically when the LCD is in sub-zero temps for e.g. ....).

I have searched for specs on both "typical" LCD character displays, and 
on the PIC itself, but the LCD specs do not give the current 
requirements of Vo, and apart from the pin maximum sink/source current, 
I have not found a current max for the Vref. I am using the PIC18F4320, 
and there is the statement (page 228/229 of datasheet) that "The RA2 pin 
can be used as a simple D/A output with limited drive capability", but I 
can not find anywhere that indicates what the drive capability actually 
is. Will I need to create the Op-Amp "buffer" as described in that same 
section? Table 26.3 indicates the  Voltage Reference Specifications, but 
nowhere does it indicate what the max supply current is....

Additionally, I can't seem to find the input current requirements for 
the LCD display. Initial investigation seems to indicate that it is only 
used for potential, and no current actually flows... based on the 
references: http://www.pacificdisplay.com/lcd_cont_adj.htm and 
http://www.microchip.com/stellent/groups/picmicro_sg/documents/devicedoc/en023491.pdf 
Essentially, the LCD voltage (Vo) is applied accross the LCD 'pixel', 
which acts as a capacitor, and thus no significant current flows.

I guess I could just measure the current in to the pin, but I would like 
third-party confirmation that I could just plug the Vref of the PIC to 
the Vo of the LCD module, and get an easy "Dynamic" non-mechanical 
contrast adjuster.

While I am on the subject, I also thought I could connect the backlight 
of the LCD module via a resistor and MOSFET to the PWM output of the CCP 
module, and thus effectively have dynamically adjustable "brightness" as 
well. More duty-cycle = more brightness.

Is there anything in here I have got horribly (or even a little) wrong?

Rolf
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