HT47C20 datasheet(16/56 Pages) HOLTEK

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HT47C20

January 18, 2000

Oscillator configuration

There are two oscillator circuits in the HT47C20.

Both are designed for system clocks; the RC oscil-

lator and the crystal oscillator, which are deter-

mined by mask option. No matter what oscillator

type is selected, the signal provides the system

clock. The halt mode stops the system oscillator

and ignores an external signal to conserve power.

The OSC1 and OSC2 are at the same level

when the system enters the power down mode.

If an RC oscillator is used, an external resistor

between OSC1 and OSC2 is need and the resis-

tance must range from 51kW to 1MW. The RC

oscillator provides the most cost effective solu-

tion. However, the frequency of the oscillation

may vary with VDD, temperature and the chip

itself due to process variations. It is, therefore,

not suitable for timing sensitive operations

where accurate oscillator frequency is desired.

If a crystal oscillator is used, a crystal across

OSC1 and OSC2 is needed to provide the feed-

back and phase shift needed for oscillator, no

other external components are needed. Instead

of a crystal, a resonator can also be connected

between OSC1 and OSC2 to get a frequency ref-

erence, but two external capacitors in OSC1

and OSC2 are required.

There is another oscillator circuit designed for

the real time clock. In this case, only the

32768kHz crystal oscillator can be applied. The

crystal should be connected between OSC3 and

OSC4, and two external capacitors along with

one external resistor are required for the oscil-

lator circuit in order to get a stable frequency.

The RTC oscillator circuit can be controlled to

oscillate quickly by setting ²SAVE² bit (bit 4 of

RTCC) to ²0². After power on reset, the ²SAVE²

bit initial value is ²0² that is on the

quick-oscillate mode. It¢s recommended to turn

it off by setting the ²SAVE² bit ²1² after a period

ofabout2secondstoavoiddrainingextrapower.

The WDT oscillator is a free running on-chip

RC oscillator, and no external components are

required. Even if the system enters the power

down mode, the system clock is stopped, but the

WDT oscillator still works with a period of ap-

proximately 90ms. The WDT oscillator can be

disabled by mask option to conserve power.

Watchdog timer - WDT

The clock source of the WDT(fs) is implemented

by a dedicated RC oscillator (WDT oscillator) or

a instruction clock (system clock divided by 4)

or a real time clock oscillator (RTC oscillator),

decided by mask options. The timer is designed

to prevent a software malfunction or sequence

jumping to an unknown location with unpre-

dictable results. The watchdog timer can be dis-

abled by a mask option. If the watchdog timer is

disabled, all the executions related to the WDT

result in no operation.

If the clock source of WDT chooses the internal

WDT oscillator, the time-out period may vary

with temperature, VDD, and process varia-

tions. On the other hand, if the clock source se-

lects the instruction clock and the ²HALT²

instruction is executed, WDT may stop count-

ing and lose its protecting purpose, and the

logic can only be restarted by external logic.

When the device operates in a noisy environ-

ment, using the on-chip RC oscillator (WDT

OSC) is strongly recommended, since the HALT

can cease the system clock.

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

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

OS C 3

OS C 4

3276 8H z

RTC oscillator

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

R C O s c illa t o r

OS C 1

OS C 2

OS C 1

OS C 2

System oscillator

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