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CHAPTER 19 INTERRUPT FUNCTIONS
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User’s Manual U14260EJ3V1UD
19.4.4 Multiple interrupt servicing
Multiple interrupt servicing occurs when an interrupt request is acknowledged during execution of another interrupt.
Multiple interrupt servicing does not occur unless the interrupt request acknowledgment enable state is selected
(IE = 1) (except non-maskable interrupts). When an interrupt request is received, interrupt request acknowledgment
becomes disabled (IE = 0). Therefore, to enable multiple interrupt servicing, it is necessary to set (1) the IE flag with
the EI instruction during interrupt servicing to enable interrupt acknowledgment.
Moreover, even if interrupts are enabled, multiple interrupt servicing 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 multiple interrupt servicing.
In the interrupt enabled 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 multiple interrupt servicing. 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 multiple interrupt servicing.
Interrupt requests that are not enabled because of the interrupt disabled state or they have a lower priority are
held pending. When servicing of the current interrupt ends, the pending interrupt request is acknowledged following
execution of at least one main processing instruction.
Multiple interrupt servicing is not possible during non-maskable interrupt servicing.
Table 19-5 shows relationship between interrupt requests enabled for multiple interrupt servicing and Figure 19-
13 shows multiple interrupt servicing examples.
Table 19-5. Relationship Between Interrupt Requests Enabled for Multiple Interrupt Servicing
Multiple Interrupt
Request
Non-Maskable
Interrupt Request
Maskable Interrupt Request
Software
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.
: Multiple interrupt servicing enabled
2.
×
: Multiple interrupt servicing disabled
3.
ISP and IE are flags contained in the 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 acknowledgment is disabled.
IE = 1:
Interrupt request acknowledgment is enabled.
4.
PR is a flag contained in PR0L, PR0H, and PR1L.
PR = 0: Higher priority level
PR = 1: Lower priority level