
External Interrupt (IRQ)
IRQ Pin
MC68HC908GR60A MC68HC908GR48A MC68HC908GR32A
Data Sheet
MOTOROLA
External Interrupt (IRQ)
123
8.4 IRQ Pin
A falling edge 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 MODE bit is set, the IRQ 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 1 to the ACK bit in the interrupt status and
control register (INTSCR). The ACK bit is useful in applications that poll the
IRQ 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 IRQ pin. A falling edge that occurs after writing to the ACK bit latches
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 IRQ pin to a high level — As long as the IRQ pin is low, IRQ
remains active.
The vector fetch or software clear and the return of the IRQ pin to a high level may
occur in any order. The interrupt request remains pending as long as the IRQ pin
is low. 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 IRQ pin is falling-edge-sensitive only. With MODE
clear, a vector fetch or software clear immediately clears the IRQ latch.
The IRQF bit in the INTSCR register can be used to check for pending interrupts.
The IRQF bit is not affected by the IMASK bit, which makes it useful in applications
where polling is preferred.
Use the BIH or BIL instruction to read the logic level on the IRQ pin.
NOTE:
When using the level-sensitive interrupt trigger, avoid false interrupts by masking
interrupt requests in the interrupt routine.
8.5 IRQ Module During Break Interrupts
The BCFE bit in the SIM break flag control register (SBFCR) enables software to
To allow software to clear the IRQ latch during a break interrupt, write a 1 to the
BCFE bit. If a latch is cleared during the break state, it remains cleared when the
MCU exits the break state.