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AN2687 Datasheet(PDF) 7 Page - STMicroelectronics

No. de Pieza. AN2687
Descripción  STM8S20xxx LCD software driver
Descarga  22 Pages
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Fabricante  STMICROELECTRONICS [STMicroelectronics]
Página de inicio  http://www.st.com
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AN2687 Datasheet(HTML) 7 Page - STMicroelectronics

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AN2687
LCD drive signals
Doc ID 14296 Rev 1
7/22
2
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
S1
COM1
ai14762
S2
S3
COM2
S14
COM3
COM4
S12
S13


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