APPENDIX
Appendix 8. 7906 Group Q & A
Interrupts
Q
If an interrupt request (b) occurs while an interrupt routine (a) is executed, is it true that the main
routine is not executed at all after the execution of the interrupt routine (a) is completed until the
execution of the INTACK sequence for the next interrupt (b) starts?
Sequence of
execution
?
RTI instruction
Interrupt routine (a)
INTACK sequence
for interrupt (b)
Main routine
Conditions:
➀➀ Flag I is cleared to “0” by executing the RTI instruction.
➀➀ The interrupt priority level of interrupt (b) is higher than IPL of the main routine.
➀➀ The interrupt priority detection time = 2 cycles of fsys
.
A
An interrupt request is sampled by a sampling pulse generated synchronously with the CPU’s op-code
fetch cycle.
(1) If the next interrupt request (b) occurs before sampling pulse ➀ for the RTI instruction is gener-
ated, the microcomputer executes the INTACK sequence for (b) without executing the main routine.
(No instruction of the main routine is executed.) It is because that sampling is completed while
executing the RTI instruction.
Interrupt request (b)
➀
Sampling pulse
RTI instruction
Interrupt routine (a)
INTACK sequence for interrupt (b)
(2) If the next interrupt request (b) occurs immediately after sampling pulse ➀ 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 ➀.
Interrupt request (b)
➀
➀
Sampling pulse
RTI instruction One instruction executed
INTACK sequence
for interrupt (b)
Interrupt routine (a)
Main routine
7906 Group User’s Manual Rev.2.0
20-103
RENESAS [ RENESAS TECHNOLOGY CORP ]
