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CHAPTER 15 INTERRUPT FUNCTIONS
Preliminary User’s Manual U16035EJ1V0UM
15.4.4 Nesting interrupt servicing
Nesting occurs when another interrupt request is acknowledged during execution of an interrupt.
Nesting does not occur unless the interrupt request acknowledge enable state is selected (IE = 1) (except non-
maskable interrupts). Also, when an interrupt request is acknowledged, interrupt request acknowledge becomes
disabled (IE = 0). Therefore, to enable nesting, it is necessary to set (1) the IE flag with the EI instruction during interrupt
servicing to enable interrupt acknowledge.
Moreover, even if interrupts are enabled, nesting may not be enabled, this being subject to interrupt priority control.
Two types of priority control are available: default priority control and programmable priority control. Programmable
priority control is used for nesting.
In the interrupt enable state, if an interrupt request with a priority equal to or higher than that of the interrupt currently
being serviced is generated, it is acknowledged for nesting. If an interrupt with a priority lower than that of the interrupt
currently being serviced is generated during interrupt servicing, it is not acknowledged for nesting.
Interrupt requests that are not enabled because of the interrupt disable state or they have a lower priority are held
pending. When servicing of the current interrupt ends, the pended interrupt request is acknowledged following
execution of one main processing instruction execution.
Nesting is not possible during non-maskable interrupt servicing.
Table 15-4 shows interrupt requests enabled for nesting and Figure 15-14 shows nesting examples.
Table 15-4. Interrupt Request Enabled for Nesting During Interrupt Servicing
Nesting Request
Non-Maskable Interrupt Request
Maskable Interrupt Request
PR = 0
PR = 1
Interrupt Being Serviced
IE = 1
IE = 0
IE = 1
IE = 0
Non-maskable interrupt
×
×
×
×
×
Maskable interrupt
ISP = 0
×
×
×
ISP = 1
×
×
Software interrupt
×
×
Remarks 1.
: Nesting enabled
2.
×
: Nesting disabled
3.
ISP and IE are flags contained in PSW.
ISP = 0: An interrupt with higher priority is being serviced.
ISP = 1: No interrupt request has been acknowledged, or an interrupt with a lower priority is being
serviced.
IE = 0:
Interrupt request acknowledge is disabled.
IE = 1:
Interrupt request acknowledge is enabled.
4.
PR is a flag contained in PR0L, PR0H, and PR1L.
PR = 0: Higher priority level
PR = 1: Lower priority level
