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CHAPTER 19 INTERRUPT FUNCTIONS
Figure 19-14. Multiple Interrupt Examples (1/2)
Example 1. Multiple interrupts occur twice
Main processing
INTxx servicing
INTyy servicing
INTzz servicing
EI
EI
EI
RETI
RETI
RETI
INTxx
(PR = 1)
INTyy
(PR = 0)
INTzz
(PR = 0)
IE = 0
IE = 0
IE = 0
During servicing of interrupt INTxx, two interrupt requests, INTyy and INTzz, are acknowledged, and multiple
interrupt servicing takes place. Before each interrupt request is acknowledged, the EI instruction must always be
issued to enable interrupt request acknowledge.
Example 2. Multiple interrupt servicing does not occur due to priority control
Main processing
INTxx servicing
INTyy servicing
INTxx
(PR = 0)
INTyy
(PR = 1)
EI
RETI
IE = 0
IE = 0
EI
1 instruction execution
RETI
Interrupt request INTyy issued during servicing of interrupt INTxx is not acknowledged because its priority is lower
than that of INTxx, and multiple interrupt servicing does not take place. The INTyy interrupt request is held pending,
and is acknowledged following execution of one main processing instruction.
PR = 0 : Higher priority level
PR = 1 : Lower priority level
IE = 0
: Interrupt request acknowledge disable
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