USER`S MANUAL

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.

In the S3F80N8 interrupt structure, the timer 0 overflow interrupt (IRQ2) belongs to this category of interrupts in which pending condition is cleared automatically by hardware.

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.

5-13