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4-MULTIPLEXED LCD WITH A STANDARD ST62

1.3 4-MULTIPLEXED LCD DRIVE

For 4-multiplexed drive, four backplanes are used instead of one. The LCD segments are

equally distributed between the four backplanes. They form groups of 4 segments, where

each segment is allocated to a different backplane. All the segment electrodes (or frontplane

electrodes) belonging to the same group are connected to a single external pin. Thus, a dis-

play using

N segments contains (N/4+4) external connections: N/4 pins driving groups of seg-

ment electrodes (S0, S1,…) and 4 “common” pins (COM0, COM1, COM2 and COM3). On the

rest of this document, the pins driving groups of segment electrodes are called “frontplane

pins”.

Figure 4. Connections inside a 4-multiplexed LCD

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

VDD/2 and VDD. The

frontplane voltage levels are 0 and

VDD only. Figure 5 shows typical backplane, frontplane and

segment voltage waveforms.

Each period is divided into 8 phases

0 to ϕ7. Like in direct drive, COM waveforms are applied

continuously, whereas

S waveforms depend on the required display. The logic level applied

on S during phase

4 is the negation of the one applied during phase ϕ0, and so on for ϕ5 and

COM3

backplanes

segment

electrodes

…

COM2

COM1

COM0

pins driving groups of

segment electrodes

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