AN594 datasheet(3/28 Pages) STMICROELECTRONICS

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Direct LCD Drive

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Duplexed LCD drive

For duplexed drive, two backplanes are used

instead of one. Each LCD pin is connected to

two LCD segments, each one connected on

the other side to one of the two backplanes.

Thus, only (S/2)+2 MCU pins are necessary

to drive an LCD with S segments.

Three different voltage levels have to be gen-

erated on the backplanes : 0, VDD/2 and VDD.

The “Segment” 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 voltage. Figure 4 shows

typical Backplane, Segment and LCD wave-

forms.

The intermediate voltage VDD/2 is only re-

quired for the “Backplane” voltages. The

ST62 I/O pins selected as “Backplanes” are

set by software to output mode for 0 or Vdd

levels and to high impedance input mode for

VDD/2. This voltage Vdd/2 is defined by two

equal valued resistors externally connected

to the I/O pin.

By using an MCU with flexible I/O pin config-

uration such as the ST6215x or ST6265x,

duplexed LCD drive can be made with only 4

additional resistors.

Figure 3. Basic LCD segment connec-

tion in duplexed mode

AN594 数据表(PDF) 3 Page - STMicroelectronics