HT47C20L datasheet(16/55 Pages) HOLTEK

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HT47C20L

January 18, 2000

Oscillator configuration

The HT47C20L provides one 32768Hz crystal

oscillator for real time clock and system clock.

The 32768Hz crystal oscillator still work at halt

mode. The halt mode stop the system clock and

T1 and ignores an external signal to conserve

power. The real time clock comes from 32768Hz

crystal and still works at halt mode.

A 32768Hz crystal across OSC1 and OSC2 is

needed to provide the feedback and phase shift

needed for the oscillator, no other external

components are needed.

Watchdog timer - WDT

The clock source of the WDT (fs) is implemented

by a 32768Hz crystal oscillator. The timer is

designed to prevent a software malfunction or

sequence jumping to an unknown location with

unpredictable results. The watchdog timer can

be disabled by mask option. If the watchdog

timer is disabled, all the executions related to

the WDT result in no operation.

The ²HALT² instruction is executed, WDT still

counts and can wake-up from halt mode due to the

WDT time-out.

The WDT overflow under normal operation will

initialize ²chip reset² and set the status bit TO.

Whereas in the halt mode, the overflow will ini-

tialize a ²warm reset² only the PC and SP are re-

set to zero. To clear the contents of WDT, three

methods are adopted, external reset (a low level

to RES), software instruction, or a HALT instruc-

tion. The software instructions are of two sets

which include CLR WDT and the other set - CLR

WDT1 and CLR WDT2. Of these two types of in-

struction, only one can be active depending on the

mask option -²CLR WDT times selection op-

tion².Ifthe ²CLR WDT² is selected (i.e., CLR

WDT times equal one), any execution of the CLR

WDT instruction will clear the WDT. In case

²CLR WDT1² and ²CLR WDT2² are chosen (i.e.

CLR WDT times equal two), these two instruc-

tions must be executed to clear the WDT; other-

wise, the WDT may reset the chip because of the

time-out.

The WDT time-out period ranges from

fs/215~fs/216. The ²CLR WDT² or ²CLR WDT1²

and ²CLRWDT2² instruction only clear the last

two-stage of the WDT.

Multi-function timer

The HT47C20L provides a multi-function timer

for the WDT, time base and real time clock but

with different time-out periods. The

multi-function timer consists of a 7-stage di-

vider and an 8-bit prescaler, with the clock

source coming from the 32768Hz. The

multi-function timer also provides a fixed fre-

quency signal (fs/8) for the LCD driver circuits,

and a selectable frequency signal (ranges from

fs/22 to fs/29) for buzzer output by mask option.

3276 8H z

( s t i l l w or k s at hal t m ode)

S yst e m c l o ck

( S t o ps a t h al t m o de )

( s t o ps at hal t m ode)

C r yst a l O sci l l a t o r

OS C 1

OS C 2

32768Hz crystal

3276 8H z C r y s t al O S C

D i v i der

P r e sca l e r

W D T C l ear

CK T

T i m e - o ut R es et

f s /2

~ f s /2

f s /2

f s

Watchdog timer

HT47C20L Datasheet(PDF) 16 Page - Holtek Semiconductor Inc

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