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4-MULTIPLEXED LCD WITH A STANDARD ST62

1, ϕ6 and ϕ5, ϕ7 and ϕ3. Changing the levels applied during phases ϕ0 and ϕ4 does not

change the DC value nor the RMS of

S∆1, S∆2 and S∆3 voltages. It does not change the DC

value of

S∆0 voltage, but affects its RMS, as explained in Table 1.

Table 1. How to switch one segment on and off in 4-multiplexed drive

Note that even if a segment is OFF, its RMS voltage is not zero. As a result, contrast is not as

good as in direct drive. In addition, there is a risk of cross-talk (or ghosting): if segment voltage

(

S∆i) frequency is too high, a segment can become opaque even though the RMS voltage is

below the threshold. So make sure that the driving frequency (considering the whole cycle, i.e.

the 8 phases) is under 2000 Hz typically.

The intermediate voltage

VDD/2 is only required for the backplane voltages. The ST62 I/O pins

connected to the backplanes are configured by software to output mode for 0 or

Vdd levels or

to high impedance input mode for

VDD/2. This intermediate voltage is defined by two equal-

valued resistors externally connected to the I/O pin.

By using an MCU with flexible I/O pin configuration such as a standard ST62, 4-multiplexed

LCD drive can be made with only 8 additional resistors.

S waveform

Segment S0

Other segments

RMS

state

RMS

state

H during

0, L during ϕ4

(

√3/4).V

OFF

only depend on the rest

S waveform

L during

0, H during ϕ4

(

√7/4).V

AN1447 Datasheet(PDF) 5 Page - STMicroelectronics