New-Tech Europe Magazine | August 2017

However, this change increases the power consumption due to the increased current flowing through the resistors. As the LCD panel increases in size, the resistance value must be decreased to maintain the image quality. Sometimes adding capacitors in parallel to the resistance can reduce the distortion caused by charging and discharging currents. This effect is limited since, at some point, a large resistor and large capacitor cause a voltage level shift that negatively impacts the display quality. Adding a potentiometer allows external contrast control. Internal resistor biasing To avoid the trouble of adding external components and to save the use of up to three pins for voltage generation, PIC microcontrollers provide internal resistor biasing and internal contrast control. This mode does not use resistors but internal resistor ladders that are configured to generate the bias voltage. The internal resistor ladder can be used to divide the LCD bias voltage to two or three equally spaced voltages that will be supplied to the LCD segment pins. To create this, the reference ladder consists of three matched resistors. When in 1/2 bias mode, the middle resistor of the ladder is shorted out so that only two voltages are generated. This mode reduces the ladder resistance thus increasing current consumption. Conclusion Various MCUs with LCD controllers provide design flexibility and straightforward ways to drive the LCD glass. The internal biasing, contrast control and power-saving features in the LCD module eliminate the need for extra hardware. They let designers make the most of these features while maintaining display quality.

Fig. 2: LCD clock generation

Fig. 3: Type A versus type B waveforms

External resistor biasing The resistor ladder method is most commonly used for higher VDD voltages. This method uses inexpensive resistors to create the multi-level LCD voltages. Regardless of the number of pixels that are energised, the current remains constant. The resistance values are determined by the display quality and power consumption. Display quality is a function of the LCD drive waveform. Since the LCD panel is a capacitive load, the waveform is distorted due to the charging and discharging currents. This distortion can be reduced by decreasing the resistance value.

shows both types of waveforms for 1/3 MUX and 1/3 bias. The voltage applied across a particular pixel is the voltage on the COM pin minus the voltage on the SEG pin. If the resulting voltage is at or above the Von threshold then the pixel is visible. If it is at or below the Voff threshold then the pixel is not visible. The contrast of an LCD can be determined by calculating the discrimination ratio, which is the ratio between the RMS voltage of an on-pixel and the RMS voltage of an off-pixel. Segment mapping provides a simple and organised way to determine which pixels should be on or off.

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