MC908MR32CFUE Datasheet, PDF (94/282 Pages) Freescale Semiconductor, Inc – Microcontrollers

External Interrupt (IRQ)
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 these actions must occur to clear the IRQ1 latch:
• Vector fetch, software clear, or reset — A vector fetch generates an interrupt acknowledge signal
to clear the latch. Software can 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 IRQ1 latch. Writing to the ACK1
bit 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 IRQ1 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, the IRQ1 latch remains set.
The vector fetch or software clear and the return of the IRQ pin to logic 1 can occur in any order. The
interrupt request remains pending as long as the IRQ pin is at logic 0.
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 IRQ1 latch.
Use the branch if IRQ pin high (BIH) or branch if IRQ pin low (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 Status and Control Register
The IRQ status and control register (ISCR) has these functions:
• Clears the IRQ interrupt latch
• Masks IRQ interrupt requests
• Controls triggering sensitivity of the IRQ interrupt pin
Address:
Read:
Write:
Reset:
$003F
Bit 7
6
5
4
3
2
1
0
0
0
0
0
IRQF
IMASK1
R
R
R
R
ACK1
0
0
0
0
0
0
0
R
= Reserved
Figure 8-4. IRQ Status and Control Register (ISCR)
Bit 0
MODE1
0
ACK1 — IRQ Interrupt Request Acknowledge Bit
Writing a logic 1 to this write-only bit clears the IRQ latch. ACK1 always reads as logic 0. Reset clears
ACK1.
MC68HC908MR32 • MC68HC908MR16 Data Sheet, Rev. 6.1
94
Freescale Semiconductor