M68ICS08GR datasheet(170/408 Pages) FREESCALE

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External Interrupt (IRQ)

Technical Data

MC68HC908GR8 — Rev 4.0

170

External Interrupt (IRQ)

MOTOROLA

12.5 IRQ1 Pin

A logic 0 on the IRQ1 pin can latch an interrupt request into the IRQ

latch. A vector fetch, software clear, or reset clears the IRQ latch.

If the MODE bit is set, the IRQ1 pin is both falling-edge-sensitive and

low-level-sensitive. With MODE set, both of the following actions must

occur to clear IRQ:

•

Vector fetch or software clear — A vector fetch generates an

interrupt acknowledge signal to clear the latch. Software may

generate the interrupt acknowledge signal by writing a logic 1 to

the ACK bit in the interrupt status and control register (INTSCR).

The ACK bit is useful in applications that poll the IRQ1 pin and

require software to clear the IRQ latch. Writing to the ACK bit prior

to leaving an interrupt service routine can also prevent spurious

interrupts due to noise. Setting ACK does not affect subsequent

transitions on the IRQ1 pin. A falling edge that occurs after writing

to the ACK bit another interrupt request. If the IRQ mask bit,

IMASK, is clear, the CPU loads the program counter with the

vector address at locations $FFFA and $FFFB.

•

Return of the IRQ1 pin to logic 1 — As long as the IRQ1 pin is at

logic 0, IRQ remains active.

The vector fetch or software clear and the return of the IRQ1 pin to logic

1 may occur in any order. The interrupt request remains pending as long

as the IRQ1 pin is at logic 0. A reset will clear the latch and the MODE

control bit, thereby clearing the interrupt even if the pin stays low.

If the MODE bit is clear, the IRQ1 pin is falling-edge-sensitive only. With

MODE clear, a vector fetch or software clear immediately clears the IRQ

latch.

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