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CHAPTER 21 INTERRUPT AND TEST FUNCTIONS
Main Processing
EI
INTxx
(PR=1)
INTyy
(PR=0)
IE=0
EI
RETI
INTxx
Servicing
INTzz
(PR=0)
IE=0
EI
RETI
INTyy
Servicing
IE=0
RETI
INTzz
Servicing
Figure 21-16. Multiple Interrupt Example (1/2)
Example 1. A multiple interrupt is generated at twice
While servicing interrupt INTxx, two interrupt requests, INTyy and INTzz, are acknowledged, and a multiple
interrupt is generated. Before each interrupt request acknowledgment, the EI instruction is always issued and
interrupt request acknowledgment is enabled.
PR = 0 : High priority level
PR = 1 : Low priority level
IE = 0 : Interrupt request acknowledgment disabled
Example 2. A multiple interrupt is not generated with priority control
Interrupt request INTyy generated while servicing interrupt INTxx is not acknowledged because it has a lower
priority than INTxx, and a multiple interrupt is not generated. The INTyy request is reserved and acknowledged after
execution of one main processing instruction.
PR = 0 : High priority level
PR = 1 : Low priority level
IE = 0 : Interrupt request acknowledgment disabled
Main Processing INTxx
Servicing
INTyy
Servicing
INTxx
(PR=0)
1 Instruction
Execution
IE=0
INTyy
(PR=1)
EI
IE=0
EI
RETI
RETI