HT47C20 datasheet(18/56 Pages) HOLTEK

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HT47C20

January 18, 2000

Real time clock - RTC

The real time clock (RTC) is operated in the

same manner as the time base that is used to

supply a regular internal interrupt. Its

time-out period ranges from fs/28 to fs/215 by

software programming . Writing data to RT2,

RT1 and RT0 (bits 2, 1, 0 of RTCC;09H) yields

various time-out periods. If the RTC time-out

occurs, the related interrupt request flag (RTF;

bit 6 of INTC0) is set. But if the interrupt is en-

abled, and the stack is not full, a subroutine call

to location 0CH occurs. The real time clock

time-out signal can also be applied to be a clock

source of timer/event counter, so as to get a lon-

ger time-out period.

RT2 RT1 RT0

RTC Clock Divided

Factor

000

001

010

210

011

211

100

212

101

213

110

214

111

215

Power down operation - HALT

The HALT mode is initialized by the HALT in-

struction and results in the following.

· The system oscillator will turn off but the

WDT oscillator or RTC oscillator keeps run-

ning (if the WDT oscillator or the real time

clock is selected).

· The contents of the on-chip RAM and regis-

ters remain unchanged.

· The WDT will be cleared and recounted again

(if the WDT clock comes from the WDT oscil-

lator or the real time clock oscillator).

· All I/O ports maintain their original status.

· The PD flag is set and the TO flag is cleared.

· LCD driver is still running by mask option (if

the WDT OSC or RTC OSC is selected).

The system can leave the halt mode by means of

an external reset, an interrupt, an external fall-

ing edge signal on port A or a WDT overflow. An

external reset causes a device initialization and

the WDT overflow performs a ²warm reset².Ex-

amining the TO and PD flags, the reason for

chip reset can be determined. The PD flag is

cleared when the system power-up or executing

the CLR WDT instruction and is set when the

HALT instruction is executed. The TO flag is set

if a WDT time-out occurs, it causes a wake-up

that only resets the PC and SP, the others main-

tain their original status.

The port A wake-up and interrupt methods can

be considered as a continuation of normal exe-

cution. Each bit in port A can be independently

selected to wake up the device by mask option.

Awakening from an I/O port stimulus, the pro-

gram will resume execution of the next instruc-

tion. If awakening from an interrupt, two

sequences may happen. If the related interrupt

is disabled or the interrupt is enabled but the

stack is full, the program will resume execution

at the next instruction. If the interrupt is en-

abled and the stack is not full, the regular inter-

rupt response takes place.

If an interrupt request flag is set to ²1² before

entering the halt mode the wake-up function of

the related interrupt will be disabled.

f s /2

D i v i der

f s

8 t o 1

Mu x .

P r e sca l e r

RT 2

RT 1

RT 0

RT C I n t e r r u p t

f s /2 ~ f s /2

Real time clock

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