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CHAPTER 10 EXTERNAL INTERRUPT CONTROL BLOCK
10.4 External Interrupt Request Level
When the request level is the edge request, a minimum of three machine cycles
(peripheral clock machine cycles) are required for edge detection.
When the request input level is in accordance with the level setting, even if the request
input previously issued externally is canceled, the request to the interrupt controller
remains active because the internal source-holding circuit retains it.
To cancel the request to the interrupt controller, clear the external interrupt request
register.
I External interrupt request level
Figure 10.4-1 "Clearing the source-holding circuit during level setting" shows how the source
holding circuit is cleared when the level is set. Figure 10.4-2 "Interrupt-source input with
interrupts enabled and interrupt request to the interrupt controller" shows how interrupt sources
are input when interrupt sources are enabled and how an interrupt request is issued to the
interrupt controller.
Figure 10.4-1 Clearing the source-holding circuit during level setting
Interrupt input
Level detection
Figure 10.4-2 Interrupt-source Input with interrupts enabled and interrupt request to the interrupt
Interrupt input
Interrupt request to
interrupt controller
254
Source F/F
(Source-holding circuit)
Sources are retained until holding circuit is cleared.
controller
High level
Enable gate
To interrupt controller
Inactivated by clearing source F/F
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