AN1048D datasheet(6/21 Pages) STMICROELECTRONICS

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LCD drive signals

AN1048

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2.3

Quadruplex LCD drive

In a quadruplex LCD drive, four backplanes are used. Each LCD pin is connected to four

LCD segments, with each segment connected on the other side to one of the four

backplanes. Thus, only (S/4)+4 MCU pins are necessary to drive an LCD with S segments.

For example: to drive an LCD with 128 segments (32 x4), only 36 I/O ports are required (32

I/O ports to drive the segments, 4 I/O ports to drive the backplanes).

Three different voltage levels have to be generated on the common lines: 0, VDD/2, VDD.

The Segment line voltage levels are 0 and VDD only. The LCD segment is inactive if the

RMS voltage is below the LCD threshold voltage and is active if the LCD RMS voltage is

above the threshold. Figure 6 shows typical Backplane, Segment and LCD waveforms. The

intermediate voltage VDD/2 is only required for Backplane voltages. The MCU I/O pins

selected as “Backplanes” are set by software to output mode for 0 or VDD levels and to the

high impedence input mode for VDD/2. The VDD/2 voltage is defined by two resistors of

equal value, externally connected to the I/O pins. When one backplane or COM is active, the

other ones are neutralized by applying VDD/2 to them.

Figure 5.

Basic LCD Segment Connection in Quadruplexed Mode

COM1

COM2

S11 S12

COM3

COM4

S13

S14

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