About GLCD Contrast
:: General Informations collected on the Internet , moved from the bitmap2lcd forum ::
Application note about LCD contrast Control >> PDF =>About LCD contrast Control
More about Negative Contrast Voltages :
Some modules just need a potentiometer or a positive voltage for contrast adjustment. Some models have it just programmable in registers.
Some other like some T6963c controller based modules without on module charge pump can need an external negative voltage ! Check the data sheet !
You can for example take the negative voltage out of the MAX232 when using one…
You can use a MC34063 for generating the negative voltage.
They are manufactured by ST Micro, Texas Instruments, and On Semiconductor.
Another possible solution is to use two charge pumps ICL7660 or MC7660 or TC7660 in cascade…
PDF ==>See Microchip Appnote for TC7660
Driving the contrast voltage from a PWM output is a recommended solution => Button contrast control. If an on board temperature sensor is present, you can even have an automatic contrast/temperature adjustment when not using an extended temperature grade device.
LCD CONTRAST ADJUSTMENT WITH TEMP COMPENSATION
Due to the contrast versus voltage versus temperature characteristics of liquid crystal fluids, and the sensitive nature of display drive voltage during multiplex operation, it may become necessary to compensate the LCD drive voltage for applications where the display is subjected to wide temperature excursions. For a typical Liquid Crystal fluid, with a negative temperature coefficient, an under voltage condition with diminished display contrast will result at low temperatures, and a “ghosting” or overdrive condition will occur at high temperatures, if no compensation techniques are employed. So if your application requires good contrast over a large temperature range, you may need to consider temperature compensation for your contrast adjustment circuitry.
When using an LCD module, one solution is to order an automatic temperature compensation option ( if available)