As mentioned in Section 4.2.1, when using the charge pump, software controls the V_LCD of the display. The V_LCD allows the user to easily adjust contrast in software. Changing the V_LCD adjusts all of the other LCD voltages, regardless of internal or external biasing, because all of the other voltages are divided down from V_LCD.

When using an external bias resistor ladder for generating the bias voltages, the sizing of R impacts contrast. If contrast is not even across all segments, reduce the size of the resistors in the bias ladder at the tradeoff of additional current consumption (see Section 4.2 on biasing).

The different biasing modes and the particular LCD used also impacts contrast ratio. Seen in Table 4-3, the contrast ratio is represented as V_RMS,ON/V_RMS,OFF, or the RMS voltage from the waveforms for a segment that is on, divided by the RMS voltage from the waveforms for a segment that is off. The higher the contrast ratio, the greater the difference in appearance of an on-segment versus an off-segment. Table 4-3 shows that there is better or worse contrast depending on the bias configuration and muxing of the display – since these settings affect the characteristics of the output waveforms. As shown in Table 4-3, higher MUX rates tend to have lower contrast ratios, therefore performance is more sensitive to any tradeoffs that affect contrast. These ratios mean that a more sensitive LCD glass with a better threshold, or other factors to provide better contrast (such as higher V_LCD, smaller bias resistors, or the techniques from Driving Large LCDs), is needed for the desired LCD performance.

A typical approach to determine the V_LCD for good contrast, is to use the threshold voltage when the contrast is 10% and with the V_RMS,OFF/V_LCD ratio from the user's guide table to calculate a recommended V_LCD using Equation 1.

The V_th,10% is a characteristic of the fluid used in the LCD display which varies with the display. The display information provided by the manufacturer typically lists a visual threshold voltage for 10%.

Some configurations trade off a reduced contrast ratio for a reduction of the full-scale LCD voltage V_LCD used. For example, on some modules 1/3 bias give a better contrast, but the 1/3 bias mode may require a higher V_LCD. See the LCD module-specific section in the MSPM0 L-Series 32MHz Microcontrollers technical reference manual for more information pertaining to the particular LCD module and contrast ratio. User's guides have tables like Table 4-3 with information specific to that module's muxing and bias options.