PIC18FXX8 Datasheet, PDF (79/402 Pages) Microchip Technology – 28/40-Pin High-Performance, Enhanced Flash Microcontrollers with CAN Module

8.0 INTERRUPTS
The PIC18FXX8 devices have multiple interrupt
sources and an interrupt priority feature that allows
each interrupt source to be assigned a high priority
level or a low priority level. The high priority interrupt
vector is at 000008h and the low priority interrupt vector
is at 000018h. High priority interrupt events will
override any low priority interrupts that may be in
progress.
There are 13 registers that are used to control interrupt
operation. These registers are:
• RCON
• INTCON
• INTCON2
• INTCON3
• PIR1, PIR2, PIR3
• PIE1, PIE2, PIE3
• IPR1, IPR2, IPR3
It is recommended that the Microchip header files,
supplied with MPLAB^® IDE, be used for the symbolic bit
names in these registers. This allows the assembler/
compiler to automatically take care of the placement of
these bits within the specified register.
Each interrupt source has three bits to control its
operation. The functions of these bits are:
• Flag bit to indicate that an interrupt event
occurred
• Enable bit that allows program execution to
branch to the interrupt vector address when the
flag bit is set
• Priority bit to select high priority or low priority
The interrupt priority feature is enabled by setting the
IPEN bit (RCON register). When interrupt priority is
enabled, there are two bits that enable interrupts
globally. Setting the GIEH bit (INTCON<7>) enables all
interrupts. Setting the GIEL bit (INTCON register)
enables all interrupts that have the priority bit cleared.
When the interrupt flag, enable bit and appropriate
global interrupt enable bit are set, the interrupt will vec-
tor immediately to address 000008h or 000018h,
depending on the priority level. Individual interrupts can
be disabled through their corresponding enable bits.
PIC18FXX8
When the IPEN bit is cleared (default state), the
interrupt priority feature is disabled and interrupts are
compatible with PICmicro^® mid-range devices. In
Compatibility mode, the interrupt priority bits for each
source have no effect. The PEIE bit (INTCON register)
enables/disables all peripheral interrupt sources. The
GIE bit (INTCON register) enables/disables all interrupt
sources. All interrupts branch to address 000008h in
Compatibility mode.
When an interrupt is responded to, the global interrupt
enable bit is cleared to disable further interrupts. If the
IPEN bit is cleared, this is the GIE bit. If interrupt priority
levels are used, this will be either the GIEH or GIEL bit.
High priority interrupt sources can interrupt a low
priority interrupt.
The return address is pushed onto the stack and the
PC is loaded with the interrupt vector address
(000008h or 000018h). Once in the Interrupt Service
Routine, the source(s) of the interrupt can be deter-
mined by polling the interrupt flag bits. The interrupt
flag bits must be cleared in software before re-enabling
interrupts to avoid recursive interrupts.
The “return from interrupt” instruction, RETFIE, exits
the interrupt routine and sets the GIE bit (GIEH or GIEL
if priority levels are used), which re-enables interrupts.
For external interrupt events, such as the INT pins or
the PORTB input change interrupt, the interrupt latency
will be three to four instruction cycles. The exact
latency is the same for one or two-cycle instructions.
Individual interrupt flag bits are set regardless of the
status of their corresponding enable bit or the GIE bit.
Note:
Do not use the MOVFF instruction to modify
any of the interrupt control registers while
any interrupt is enabled. Doing so may
cause erratic microcontroller behavior.
 2004 Microchip Technology Inc.
DS41159D-page 77