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Rev. 1.00
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Standard 8051 8-Bit Flash MCU
HT85F2260/HT85F2270/HT85F2280
Interrupts
Interrupts
Once an interrupt subroutine is serviced, all the other interrupts must be blocked by clearing
the EAL bit using the application program. This will prevent any further interrupt nesting from
occurring. However, if other interrupt requests occur during this interval, although the interrupt
will not be immediately serviced, the request flag will still be recorded. If an interrupt requires
immediate servicing while the program is already in another interrupt service routine, the EAL bit
should be set after entering the routine, to allow interrupt nesting. If the stack is full, the interrupt
request will not be acknowledged, even if the related interrupt is enabled, until the Stack Pointer is
decremented. If immediate service is desired, the stack must be prevented from becoming full.
When an interrupt request is generated, it takes several instruction cycles before the program jumps
to the interrupt vector.
Wait fo� � s�stem clocks
Main
P�og�am
ISR Ent��
…
…
Enable Bit Set ?
Main
P�og�am
RETI
(it will set EAL a�tomaticall�)
A�tomaticall� Disable Inte���pt
Clea� EAL & Req�est Flag
Inte���pt Req�est o�
Inte���pt Flag Set b� Inst��ction
N
Y
Interrupt Flowchart