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External Interrupt (IRQ)
IRQ Pin
MC68HC08GP32A MC68HC08GP16A
Data Sheet
MOTOROLA
External Interrupt (IRQ)
91
Interrupt signals on the IRQ pin are latched into the IRQ latch. The interrupt latch
remains set until one of the following actions occurs:
Vector fetch — A vector fetch automatically generates an interrupt
acknowledge signal that clears the latch that caused the vector fetch.
Software clear — Software can clear the interrupt latch by writing a 1 to the
ACK bit in the interrupt status and control register (INTSCR).
Reset — A reset automatically clears the interrupt latch.
The external interrupt pin is falling-edge-triggered and is software-configurable to
be either falling-edge or falling-edge and low-level-triggered. The MODE bit in the
INTSCR controls the triggering sensitivity of the IRQ pin.
When set, the IMASK bit in the INTSCR masks all external interrupt requests. A
latched interrupt request is not presented to the interrupt priority logic unless the
IMASK bit is clear.
NOTE:
The interrupt mask (I) in the condition code register (CCR) masks all interrupt
requests, including external interrupt requests.
7.4 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 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.
Addr.
Register Name
Bit 7
654321
Bit 0
$001D
IRQ Status and Control
Register (INTSCR)
Read:
0000
IRQF
0
IMASK
MODE
Write:
ACK
Reset:
00000000
= Unimplemented
Figure 7-3. IRQ I/O Register Summary