Datasheet

MC68HC908QY4•MC68HC908QT4•MC68HC908QY2•MC68HC908QT2•MC68HC908QY1•MC68HC908QT1
174 External Interrupt (IRQ) MOTOROLA
External Interrupt (IRQ)
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13.5 IRQ Pin
A logic 0 on the IRQ pin can latch an interrupt request into the IRQ latch.
A vector fetch, software clear, or reset clears the IRQ latch.
If the MODE1 bit is set, the IRQ pin is both falling-edge sensitive and
low-level sensitive. With MODE1 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 ACK1 bit in the interrupt status and control register (ISCR).
The ACK1 bit is useful in applications that poll the IRQ
pin and
require software to clear the IRQ 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 IRQ
pin. A falling edge that occurs
after writing to the ACK1 bit latches another interrupt request. If
the IRQ mask bit, IMASK1, is clear, the CPU loads the program
counter with the vector address at locations $FFFA and $FFFB.
• Return of the IRQ
pin to logic 1 — As long as the IRQ pin is at logic
0, IRQ remains active.
The vector fetch or software clear and the return of the IRQ pin to logic 1
may occur in any order. The interrupt request remains pending as long
as the IRQ
pin is at logic 0. A reset will clear the latch and the MODE1
control bit, thereby clearing the interrupt even if the pin stays low.
If the MODE1 bit is clear, the IRQ
pin is falling-edge sensitive only. With
MODE1 clear, a vector fetch or software clear immediately clears the
IRQ 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.
NOTE: When the IRQ function is enabled in the CONFIG2 register, the BIH and
BIL instructions can be used to read the logic level on the IRQ pin. If the
IRQ function is disabled, these instructions will behave as if the IRQ pin