HT1647 datasheet(7/17 Pages) HOLTEK

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HT1647

Rev. 1.40

November 9, 2010

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Functional Description

System Oscillator

The HT1647 system clock is used to generate the time

base/Watchdog Timer (WDT) clock frequency, LCD

driving clock, and tone frequency. The clock source

may be from an on-chip RC oscillator (32kHz), a crystal

oscillator (32.768kHz), or an external 32kHz clock by

the S/W setting. The configuration of the system oscilla-

tor is as shown. After the SYS DIS command is exe-

cuted, the system clock will stop and the LCD bias

generator will turn off. That command is available only

for the on-chip RC oscillator or for the crystal oscillator.

Once the system clock stops, the LCD display will be-

come blank, and the time base/WDT loses its function

as well.

The LCD OFF command is used to turn the LCD bias

generator off. After the LCD bias generator switches off

by issuing the LCD OFF command, using the SYS DIS

command reduces power consumption, thus serving as

a system power down command. But if the external

clock source is chosen as the system clock, using the

SYS DIS command can neither turn the oscillator off nor

carry out the power down mode. The crystal oscillator

option can be applied to connect an external frequency

source of 32kHz to the OSCI pin. In this case, the sys-

tem fails to enter the power down mode, similar to the

case in the external 32kHz clock source operation. At

the initial system power on, the HT1647 is at the SYS

DIS state.

Display Memory

- RAM Structure

The static display RAM is organized into 512

´4 bits and

stores the display data. The contents of the RAM are di-

rectly mapped to the contents of the LCD driver. Data in

the RAM can be accessed by the READ, WRITE and

READ-MODIFY-WRITE commands. The following is a

mapping from the RAM to the LCD patterns.

O S C I

O S C O

C r y s t a l O s c i l l a t o r

3 2 7 6 8 H z

E x t e r n a l C l o c k S o u r c e

3 2 k H z

O n - c h i p R C O s c i l l a t o r

3 2 k H z

S y s t e m

C l o c k

System Oscillator Configuration

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