HT49C50 datasheet(16/52 Pages) HOLTEK

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HT49C50

August 18, 1999

viced. Once the interrupt request flags (RTF,

TBF, T0F, T1F, EIF1, EIF0) are all set, they re-

main in the INTC1 or INTC0 respectively until

the interrupts are serviced or cleared by a soft-

ware instruction.

It is recommended that a program not use the ²CALL

subroutine² within the interrupt subroutine. It¢s

because interrupts often occur in an unpredictable

manner or require to be serviced immediately in

some applications. At this time, if only one stack is

left, and enabling the interrupt is not well con-

trolled, operation of the ²call² in the interrupt

subroutine may damage the original control se-

quence.

Oscillator configuration

The HT49C50 provides two oscillator circuits

for system clocks, i.e., RC oscillator and crystal

oscillator, determined by mask option. No mat-

ter what type of oscillator is selected, the signal

is used for the system clock. The HALT mode

stops the system oscillator and ignores external

signal to conserve power.

Of the two oscillators, if the RC oscillator is

used, an external resistor between OSC1 and

VSS is required, and the range of the resistance

should be from 51kW to 1MW. The system clock,

divided by 4, is available on OSC2 with

pull-high resistor, which can be used to syn-

chronize external logic. The RC oscillator pro-

vides the most cost effective solution. 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.

On the other hand, if the crystal oscillator is se-

lected, a crystal across OSC1 and OSC2 is

needed to provide the feedback and phase shift

required for the oscillator, and no other exter-

nal components are required. A resonator may

be connected between OSC1 and OSC2 to re-

place the crystal and to get a frequency refer-

ence, 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

32.768kHz 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

oscillator circuit in order to get a stable fre-

quency.

The RTC oscillator circuit can be controlled to

oscillate quickly by setting the ²QOSC² bit (bit

4 of RTCC). It is recommended to turn on the

quick oscillating function upon power on, and

turn it off after 2 seconds.

The WDT oscillator is a free running on-chip

RC oscillator, and no external components are

required. Although the system enters the

power down mode, the system clock stops, and

the WDT oscillator still works with a period of

approximately 78ms. The WDT oscillator can be

disabled by mask option to conserve power.

OS C 3

OS C 4

RTC oscillator

C r yst a l O sci l l 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

f SY S /4

VD D

System oscillator

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