AN3114 datasheet(12/43 Pages) STMICROELECTRONICS

12 / 43 page

LCD controller drive signals

AN3114

12/43

Doc ID 16829 Rev 3

2.3

Quadruplex LCD drive

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

LCD segments, which other side is connected to one, two or four backplanes. Consequently,

only (S/4)+4 MCU pins are necessary to drive an LCD with S LCD segments (pixels). For

example, to drive an LCD with 112 LCD segments (28 ×4), only 32 I/O ports are required

(28 I/O ports to drive the segment lines and 4 I/O ports to drive the backplanes).

Four different voltage levels have to be generated on the common lines: 0, VDD/3, 2VDD/3

and VDD. The Segment line voltage levels are also 0, VDD/3, 2VDD/3 and VDD. The LCD

segment is inactive if the RMS voltage applied is below the LCD threshold voltage Vth, and

is active if the LCD RMS voltage is above the threshold. Figure 7. shows typical backplane,

Segment lines and LCD waveforms. The intermediate voltage VDD/3 and 2VDD/3 are

required for backplane voltages. When a backplane or COM is active (0 V during an odd

frame and VDD during an even frame), the others are made inactive by applying to them

2VDD/3 during an odd frame and VDD/3 during an even frame.

Figure 6.

Basic LCD segment lines connection in quadruplex mode

S11

COM1

ai14762

COM2

S14

COM3

COM4

S12

S13

AN3114 Datasheet(PDF) 12 Page - STMicroelectronics