HT1647 datasheet(9/24 Pages) HOLTEK

9 / 24 page

HT1647

April 21, 2000

90%

50%

10%

GN D

t r

WR , R D

Cl o c k

t CL K

t f

t CL K

V DD

Figure 1

50 %

V A L I D DA T A

50 %

GN D

W R , R D

Cl o c k

t su

t h

V DD

Figure 2

GN D

50 %

FI R S T

Cl o c k

LA S T

Cl o c k

GN D

WR , R D

Cl o c k

t su 1

t h1

t CS

V DD

Figure 3

Functional Description

System oscillator

The HT1647 system clock is used to generate

the time base/Watchdog Timer (WDT) clock fre-

quency, LCD driving clock, and tone frequency.

The source of the clock 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 os-

cillator is as shown. After the SYS DIS com-

mand is executed, the system clock will stop

and the LCD bias generator will turn off. That

command is, however, available only for the

on-chip RC oscillator or for the crystal oscilla-

tor. Once the system clock stops, the LCD dis-

play will become 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 gen-

erator switches off by issuing the LCD OFF

command, using the SYS DIS command re-

duces power consumption, 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 oscilla-

tor off nor carry out the power down mode. The

crystal oscillator option can be applied to con-

nect an external frequency source of 32kHz to

the OSCI pin. In this case, the system 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.

OSC I

OSC O

C r y s t al Os c i l l at or

3276 8H z

E x t e r n a l C l o ck S o u r ce

32k H z

On- c h i p R C Os c i l l at or

32k H z

S yst e m

Cl o c k

System oscillator configuration

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