AN2687 datasheet(7/22 Pages) STMICROELECTRONICS

7 / 22 page

AN2687

LCD drive signals

Doc ID 14296 Rev 1

7/22

LCD drive signals

2.1

Quadruplex LCD drive

In a quadruplex LCD drive, four backplanes (common lines) are used. Each LCD pin is

connected to four LCD segments, whose other side is connected to one of the four

backplanes (refer to Figure 3). 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 × 4), 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.

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-impedance input mode for VDD/2. The VDD/2 voltage is determined by two resistors of

equal value, externally connected to the I/O pins as shown in Figure 5. When one backplane

or COM is active, the other ones are neutralized by applying VDD/2 to them.

Figure 3.

Basic LCD segment connection in quadruplexed mode

S11

COM1

ai14762

COM2

S14

COM3

COM4

S12

S13

AN2687 Scheda tecnica(PDF) 7 Page - STMicroelectronics