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APPENDIX

Appendix 9. 7721 Group Q & A

7721 Group User’s Manual

17–68

Interrupt

If an interrupt request (b) occurs while executing an interrupt routine (a), is it true that the main routine

is not executed at all from when the execution of the interrupt routine (a) is completed until the

execution of the INTACK sequence for the next interrupt (b) starts?

(2)

If the next interrupt request (b) occurs immediately after sampling pulse is generated, the

microcomputer executes one instruction of the main routine before executing the INTACK

sequence for (b). It is because that the interrupt request is sampled by the next sampling

pulse .

Sampling for interrupt requests is performed by sampling pulses generated synchronously with the

CPU’s op-code fetch cycles.

(1)

If the next interrupt request (b) occurs before sampling pulse

for the RTI instruction is gener-

ated, the microcomputer executes the INTACK sequence for (b) without executing the main rou-

tine (not even one instruction). It is because that sampling is completed while executing the RTI

instruction.

Conditions:

q I is cleared to “0” by executing the RTI instruction.

q Iinterrupt priority level of interrupt (b) is higher than IPL of main routine.

q Interrupt priority detection time is 2 cycles of

φ.

Interrupt routine (a)

Main routine

INTACK sequence

for interrupt (b)

Sequence of

execution

RTI instruction

INTACK sequence for interrupt (b)

Interrupt request (b)

Interrupt routine (a)

Sampling pulse

RTI instruction

Main routine

Interrupt request (b)

Sampling pulse

INTACK sequence

for interrupt (b)

One instruction executed

Interrupt routine (a)

RTI instruction

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