3.3 Charge Pump with External Resistor Ladder (LCDVSRC<3:0> = 1001)

By setting the LCDVSRC<3:0> bits of the LCDVCON2 register to ‘1001’, voltage will be supplied to the LCD panel from the charge pump with external resistors. This mode of operation offers the flexibility of the internal LCD charge pump, along with the ability to use a custom designed external resistor ladder that allows the user to meet the specific drive requirements of their glass in applications where the internal resistor ladder is not sufficient. When configured to operate in this mode, the internal LCD charge pump will generate VLCD3, the highest of the LCD bias voltages. All other LCD bias voltages are generated from VLCD3 using an external resistor ladder that is connected to the various VLCDx pins on the PIC® microcontroller. The internal resistor ladder is bypassed in this mode, and any changes to the LCD Contrast Control bits (LCDCST) will not alter the contrast of the display.

While operating within this mode, contrast can be controlled by altering the output of the LCD charge pump (VLCD3) by programming the BIAS<2:0> bits. Additionally, contrast can be controlled by adjusting the characteristics of the external resistor ladder being used. This mode is valid for the following LCD panel bias types: Static (two discrete levels), 1/2 Bias (three discrete levels), and 1/3 Bias (four discrete levels).