ST62T42B datasheet(47/68 Pages) STMICROELECTRONICS

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ST62T42B/E42B

5.4 LCD CONTROLLER-DRIVER

On-chip LCD driver includes all features required

for LCD driving, including multiplexing of the com-

mon plates. Multiplexing allows to increase display

capability without increasing the number of seg-

ment outputs. In that case, the display capability is

equal to the product of the number of common

plates with the number of segment outputs.

A dedicated LCD RAM is used to store the pattern

to be displayed while control logic generates ac-

cordingly all the waveforms sent onto the segment

or common outputs. Segments voltage supply is

MCU supply independant, and included driving

stages allow direct connection to the LCD panel.

The multiplexing ratio (Number of common plates)

and the base LCD frame frequency is software

configurable to achieve the best trade-off con-

trast/display capability for each display panel.

The 32Khz clock used for the LCD controller is

derivated from the MCU’s internal clock and there-

fore does not require a dedicated oscillator. The

division factor is set by the three bits HF0..HF2 of

the LCD Mode Control Register LCDCR as sum-

marized in Table 18 for recommanded oscillator

quartz values. In case of oscillator failure, all seg-

ment and common lines are switched to ground to

avoid any DC biasing of the LCD elements.

Table 18. Oscillator Selection Bits

Notes:

The usage fOSC values different from those

defined in this table cause the LCD to operate at a

reference frequency different from 32.768KHz, ac-

cording to division factor of Table 18.

It is not recommended to select an internal

frequency lower than 32.768KHz as the clock su-

pervisor circuit may switch off the LCD peripheral

if lower frequency is detected.

Figure 24. LCD Block Diagram

MCU

Oscillator

fOSC

HF2 HF1 HF0

Division Factor

Clock disabled: Display off

1.048MHz

2.097MHz

4.194MHz

128

8.388MHz

256

DATA BUS

CONTROL

REGISTE R

LCD

RAM

SEGMENT

DRIVER

COMMON

DRIVE R

VOLTAGE

DIVIDER

CONTROLLER

CLOCK

SELECTION

VLCD

1/3

2/3

VLCD VLCD

BACKPLA NES

SEGMENTS

fint

OSC 32KHz

(When available)

VR02099

32KHz

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