Interrupts are disabled upon any device Reset. They are enabled by setting the
following bits:
- GIE bit
- PEIE bit (if the Interrupt Enable bit of the interrupt
event is contained in the PIE registers)
- Interrupt Enable bit(s) for the
specific interrupt event(s)
The PIR registers record individual interrupts via interrupt flag bits.
Interrupt flag bits will be set, regardless of the status of the GIE, PEIE and
individual interrupt enable bits.
The following events happen when an interrupt event occurs while the GIE bit
is set:
- Current prefetched instruction is flushed
- GIE bit is cleared
- Current Program Counter (PC) is pushed onto the stack
- Critical registers are
automatically saved to the shadow registers (see Automatic Context Saving)
- PC is loaded with the interrupt
vector 0004h
The firmware within the Interrupt Service Routine (ISR) may determine the source of the
interrupt by polling the interrupt flag bits. The interrupt flag bits must be cleared
before exiting the ISR to avoid repeated interrupts. Because the GIE bit is cleared, any
interrupt that occurs while executing the ISR will be recorded through its interrupt
flag, but will not cause the processor to redirect to the interrupt vector.
The RETFIE
instruction exits the ISR by popping the previous address
from the stack, restoring the saved context from the shadow registers and setting the
GIE bit.
For additional information on a specific interrupts operation, refer to its peripheral
section.
Important:
- 1.Individual interrupt flag
bits are set, regardless of the state of any other enable bits.
- 2.All interrupts will be
ignored while the GIE bit is cleared. Any interrupt occurring while the GIE
bit is clear will be serviced when the GIE bit is set again.