
External Interrupt Module (IRQ)
Functional Description
MC68HC(7)08LN56 — Rev. 2.0
General Release Specication
MOTOROLA
External Interrupt Module (IRQ)
123
NONDISCLOSURE
AGREEMENT
REQUIRED
8.4.1 IRQ1/V
PP Pin
A logic zero on the IRQ1/V
PP pin can latch an interrupt request into the
IRQ1 latch. A vector fetch, software clear, or reset clears the IRQ1 latch.
If the MODE1 bit is set, the IRQ1/V
PP pin is both falling-edge-sensitive
and low-level-sensitive. With MODE1 set, both of the following actions
must occur to clear IRQ1:
Vector fetch or software clear — A vector fetch generates an
interrupt acknowledge signal to clear the latch. Software can
generate the interrupt acknowledge signal by writing a logic one to
the ACK1 bit in the interrupt status and control register (ISCR).
The ACK1 bit is useful in applications that poll the IRQ1/V
PP pin
and require software to clear the IRQ1 latch. Writing to the ACK1
bit prior to leaving an interrupt service routine can also prevent
spurious interrupts due to noise. Setting ACK1 does not affect
subsequent transitions on the IRQ1/V
PP pin. A falling edge that
occurs after writing to the ACK1 bit latches another interrupt
request. If the IRQ1 mask bit, IMASK1, is clear, the CPU loads the
program counter with the vector address at locations $FFFA and
$FFFB.
Return of the IRQ1/V
PP pin to logic one — As long as the IRQ1/VPP
pin is at logic zero, IRQ1 remains active.
The vector fetch or software clear and the return of the IRQ1/V
PP pin to
logic one may occur in any order. The interrupt request remains pending
as long as the IRQ1/V
PP pin is at logic zero. A reset will clear the latch
and the MODEx control bit, thereby clearing the interrupt even if the pin
stays low.
If the MODE1 bit is clear, the IRQ1/V
PP pin is falling-edge-sensitive only.
With MODE1 clear, a vector fetch or software clear immediately clears
the IRQ1 latch.
The IRQF1 bit in the ISCR register can be used to check for pending
interrupts. The IRQF1 bit is not affected by the IMASK1 bit, which makes
it useful in applications where polling is preferred.