ht82k68e-l Holtek Semiconductor Inc., ht82k68e-l Datasheet - Page 10

• Manufacturer Part Number: ht82k68e-l
• Description: Ht82k68e-l/ht82k68a-l -- Multimedia Keyboard Encoder 8-bit Mcu
• Manufacturer: Holtek Semiconductor Inc.

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
interrupts are enabled. In the case of simultaneous re-
quests, the following table shows the priority that is ap-
plied. These can be masked by resetting the EMI bit.
Once the interrupt request flags (T0F) are set, they will
remain in the INTC register until the interrupts are ser-
viced or cleared by a software instruction.
It is suggested that a program does not use the CALL
subroutine within the interrupt subroutine. Because
interrupts 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 CALL subroutine operates in
the interrupt subroutine it will damage the original con-
trol sequence.
Oscillator Configuration
There are two oscillator circuits in the microcontroller.
Both are designed for system clocks; the RC oscillator
and the Crystal oscillator, which are determined by
mask options. No matter what oscillator type is
selected, the signal 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 20k
by 4, is available on OSC2, which can be used to syn-
chronize external logic. The RC oscillator provides the
most cost effective solution. However, the frequency of
the oscillation may vary with VDD, temperature and the
Rev. 1.10
External interrupt 1
Timer counter overflow
Interrupt Source
to 39k . The system clock, divided
Vector
04H
08H
Watchdog Timer
10
chip itself due to process variations. It is, therefore, not
suitable for timing sensitive operations where accurate
oscillator 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 s. 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 se-
quence jumping to an unknown location with unpredict-
able results. The Watchdog Timer can be disabled by
mask option. If the Watchdog Timer is disabled, all the ex-
ecutions related to the WDT results in no operation.
Once the internal WDT oscillator (RC oscillator normally
with a period of 78 s) is selected, it is first divided by 256
(8-stages) to get the nominal time-out period of approxi-
mately 20ms. This time-out period may vary with temper-
ature, VDD and process variations. By invoking the WDT
prescaler, longer time-out periods can be realized. Writ-
ing data to WS2, WS1, WS0 (bit 2,1,0 of the WDTS) can
give different time-out periods. If WS2, WS1, WS0 are all
equal to 1, the division ratio is up to 1:128, and the maxi-
mum time-out period is 2.6 seconds.
HT82K68E-L/HT82K68A-L
System Oscillator
April 2, 2008