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INTERRUPT STRUCTURE S3C8275X/F8275X/C8278X/F8278X/C8274X/F8274X
5-14
INTERRUPT PENDING FUNCTION TYPES
Overview
There are two types of interrupt pending bits: one type that is automatically cleared by hardware after the interrupt
service routine is acknowledged and executed; the other that must be cleared in the interrupt service routine.
Pending Bits Cleared Automatically by Hardware
For interrupt pending bits that are cleared automatically by hardware, interrupt logic sets the corresponding
pending bit to "1" when a request occurs. It then issues an IRQ pulse to inform the CPU that an interrupt is waiting
to be serviced. The CPU acknowledges the interrupt source by sending an IACK, executes the service routine,
and clears the pending bit to "0". This type of pending bit is not mapped and cannot, therefore, be read or written
by application software.
Pending Bits Cleared by the Service Routine
The second type of pending bit is the one that should be cleared by program software. The service routine must
clear the appropriate pending bit before a return-from-interrupt subroutine (IRET) occurs. To do this, a "0" must be
written to the corresponding pending bit location in the source’s mode or control register.
Programming Tip ⎯ How to clear an interrupt pending bit
As the following examples are shown, a load instruction should be used to clear an interrupt pending bit.
Examples:
1. SB0
LD EXTIPND, #11111011B ; Clear P0.2's interrupt pending bit
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IRET