HT82K68A datasheet(9/30 Pages) HOLTEK

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HT82K68A

Rev. 1.10

November 20, 2001

a low to high transition of the PC2. When PE0 is output

low, the external interrupt is triggered by a high to low

transition of PC2.

During the execution of an interrupt subroutine, other in-

terrupt acknowledgements are held until the RETI in-

struction is executed or the EMI bit and the related

interrupt control bit are set to 1 (if the stack is not full). To

return from the interrupt subroutine, a RET or RETI in-

struction may be invoked. RETI will set the EMI bit to en-

able an interrupt service, but RET will not.

Interrupts occurring in the interval between the rising

edges of two consecutive T2 pulses, will be serviced on

the latter of the two T2 pulses, if the corresponding inter-

rupts are enabled. In the case of simultaneous requests,

the following table shows the priority that is applied.

These can be masked by resetting the EMI bit.

No.

Interrupt Source

Vector

External interrupt 1

04H

Timer counter overflow

08H

The timer counter interrupt request flag (T0F), external

interrupt request (EIF) enable timer counter bit (ET0I),

enable external interrupt bit (EEI) and enable master in-

terrupt bit (EMI) constitute an interrupt control register

(INTC) which is located at 0BH in the data memory. EMI,

ET0I and EEI, are used to control the enabling/disabling

of interrupts. These bits prevent the requested interrupt

from being serviced. Once the interrupt request flags

(T0F) are set, they will remain in the INTC register until

the interrupts are serviced or cleared by a software in-

struction.

It is suggested that a program does not use the

²CALL

subroutine

² within the interrupt subroutine. Because in-

terrupts often occur in an unpredictable manner or need

to be serviced immediately in some applications, if only

one stack is left and enabling the interrupt is not well

controlled, once the

²CALL subroutine² operates in the in-

terrupt subroutine it will damage the original control se-

quence.

Oscillator configuration

There are two oscillator circuits in HT82K68A. Both are

designed for system clocks; the RC oscillator and the

Crystal oscillator, which are determined by mask op-

tions. No matter what oscillator type is selected, the sig-

nal provides the system clock. The HALT mode stops

the system oscillator and resists the external signal to

conserve power.

If an RC oscillator is used, an external resistor between

OSC1 and VDD is needed and the resistance must

range from 51k

W to 1MW. The system clock, divided by

4, is available on OSC2, which can be used to synchro-

nize external logic. The RC oscillator provides 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 suit-

able for timing sensitive operations where accurate os-

cillator frequency is desired.

If the Crystal oscillator is used, a crystal across OSC1

and OSC2 is needed to provide the feedback and phase

shift needed for oscillator, no other external components

are needed. Instead of a crystal, the resonator can also

be connected between OSC1 and OSC2 to get a fre-

quency reference, but two external capacitors in OSC1

and OSC2 are required.

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

tor, 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 for a

period of approximately 78

ms. The WDT oscillator can

be disabled by mask option to conserve power.

Watchdog Timer

- WDT

The WDT clock source is implemented by a dedicated

RC oscillator (WDT oscillator) or instruction clock (sys-

tem clock divided by 4), decided by mask options. This

timer is designed to prevent a software malfunction or

S y s t e m

c l o c k / 4

8 - b i t C o u n t e r

D T P r e s c a l e r

7 - b i t C o u n t e r

8 - t o - 1 M U X

D T T i m e - o u t

S 0 ~ W

S 2

M a s k

O p t i o n

S e l e c t

D T

O S C

Watchdog Timer

Crystal Oscillator

RC Oscillator

OSC1

OSC2

OSC1

OSC2

SYS/4

(NMOS Open

Drain Output)

System oscillator

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