HT46R652 datasheet(13/47 Pages) HOLTEK

13 / 47 page

HT46R652

Rev. 1.00

December 19, 2006

During the execution of an interrupt subroutine, other

maskable interrupt acknowledgments are all held until

the

²RETI² instruction is executed or the EMI bit and the

related interrupt control bit are set both to 1 (if the stack

is not full). To return from the interrupt subroutine,a

²RET² or ²RETI² instruction may be executed. RETI sets

the EMI bit and enables an interrupt service, but RET

does not.

Interrupts occurring in the interval between the rising

edges of two consecutive T2 pulses are serviced on the

latter of the two T2 pulses if the corresponding interrupts

are enabled. In the case of simultaneous requests, the

priorities in the following table apply. These can be

masked by resetting the EMI bit.

Interrupt Source

Priority

Vector

External interrupt 0

04H

External interrupt 1

08H

Timer/Event Counter 0 overflow

0CH

Timer/Event Counter 1 overflow

10H

Time base interrupt

14H

Real time clock interrupt

18H

The EMI, EEI0, EEI1, ET0I, ET1I, ETBI, and ERTI bits

are all used to control the enable/disable status of the in-

terrupts. These bits prevent the requested interrupt from

being serviced. Once the interrupt request flags, RTF,

TBF, T0F, T1F, EIF1, EIF0 are set, they remain in the

INTC1 or INTC0 register respectively until the interrupts

are serviced or cleared by a software instruction.

It is recommended that a program should not use a

²CALL² instruction within the interrupt subroutine. This is

because interrupts often occur in an unpredictable man-

ner or require to be serviced immediately in some appli-

cations. During that period, if only one stack is left, and

enabling the interrupt is not well controlled, execution of

²CALL² in the interrupt subroutine may damage the

original control sequence.

Oscillator Configuration

The device provides three oscillator circuits for the sys-

tem clock, namely an RC oscillator, a crystal oscillator

and an RTC 32768Hz crystal oscillator, the choice of

which is determined by configuration option. When the

device enters the Power Down mode, the RC or crystal

oscillator will cease running to conserve power. The

32768Hz crystal oscillator, however, will keep running

when the device is in the Power Down mode. If the

32768Hz crystal oscillator is selected as the system os-

cillator, when the device enters the Power Down mode,

the system oscillator keeps running, but instruction exe-

cution will cease. Since the 32768Hz oscillator is also

designed for timing purposes, the internal timing func-

tions, RTC, time base and WDT, continue to operate

even when the system enters the Power Down mode.

If the RC oscillator is used, an external resistor con-

nected between pins OSC1 and VSS is required, whose

value should range from 24k

W to 1MW. The system clock,

divided by 4, can be monitored on pin OSC2 if a pull-high

resistor is connected. This pin can be used to synchro-

nise external logic. The RC oscillator provides the most

cost effective solution. However, as the frequency may

vary with VDD, temperature, and process variations, it is

therefore not suitable for timing sensitive operations

where an accurate oscillator frequency is desired.

If a crystal oscillator is selected, a crystal across OSC1

and OSC2 is needed to provide the feedback and phase

shift required for the oscillator, and no other external

components are required. A resonator may be connected

between OSC1 and OSC2 to replace the crystal and to

get a frequency reference, but two external capacitors

between OSC1 and OSC2 and ground are required.

The other oscillator circuit, which is a real time clock, re-

quires a 32768Hz crystal oscillator to be connected be-

tween OSC3 and OSC4.

The RTC oscillator circuit can be controlled to start-up

quickly by setting the

²QOSC² bit, which is bit 4 of

RTCC. It is recommended to turn on the quick start-up

function during power on, and then turn it off again after

2 seconds.

The WDT oscillator is a free running on-chip RC oscilla-

tor, which does not require external components. Al-

though when the system enters the Power Down mode

and the system clock stops, the WDT oscillator still oper-

ates with a nominal period of approximately 65

msat5V.

The WDT oscillator can be disabled by a configuration

option to conserve power.

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

R C O s c i l l a t o r

O S C 1

O S C 2

f S Y S / 4

O S C 2

3 2 7 6 8 H z C r y s t a l / R T C O s c i l l a t o r

O S C 3

O S C 4

V D D

4 7 0 p F

System Oscillator

Note: *32768Hz crystal enable condition: for WDT clock source or for system clock source.

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