LCD drive signals

AN1048

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LCD drive signals

2.1

Single backplane LCD drive

In a single backplane drive, each LCD segment is connected to a segment line(Sx) and to

one backplane(common line) common to all the segments. A display using S segments is

driven with S+1 MCU output lines. The backplane is driven with a “COM” signal controlled

When switching a segment “ON”, a signal with opposite polarity to “COM” is sent to the

corresponding “Segment” pin. When the non-inverted signal “COM” is sent to the “Segment”

pin, the segment is “OFF”. Using an MCU, the I/O operates in output mode either at logic 0

or 1.

Figure 2.

LCD signals for direct drive

2.2

Duplexed LCD drive

In a duplexed drive, two backplanes are used instead of one. Each LCD segment line(Sx) is

connected to two LCD segments, each one connected on the other side to one of the two

backplanes or common lines(refer to Figure 3). Thus, only (S/2)+2 MCU pins are necessary

to drive an LCD with S segments.

Three different voltage levels have to be generated on the backplanes: 0, V

voltages. The ST7 I/O pins selected as “Backplanes” are set by software to output mode for

COM

S1=COM-S

-Vdd

+Vdd

S

S1=COM-S

S

O

F

F

O

N

+Vdd

+Vdd

+Vdd

+Vdd