Microchip Technology MCU PIC PIC18F87K22-I/PTRSL TQFP-80 MCP PIC18F87K22-I/PTRSL Data Sheet

 2009-2011 Microchip Technology Inc.

DS39960D-page 141

Members of the PIC18F87K22 family of devices have
multiple interrupt sources and an interrupt priority
feature that allows most interrupt sources to be
assigned a high-priority level or a low-priority level. The
high-priority interrupt vector is at 0008h and the
low-priority interrupt vector is at 0018h. High-priority
interrupt events will interrupt any low-priority interrupts
that may be in progress.
The registers for controlling interrupt operation 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.
In general, interrupt sources have three bits to control
their operation. They are:
• Flag bit – Indicating that an interrupt event 

occurred

• Enable bit – Enabling program execution to 

branch to the interrupt vector address when the 
flag bit is set

• Priority bit – Specifying high priority or low priority
The interrupt priority feature is enabled by setting the
IPEN bit (RCON<7>). When interrupt priority is
enabled, there are two bits that enable interrupts
globally. Setting the GIEH bit (INTCON<7>) enables all
interrupts that have the priority bit set (high priority).
Setting the GIEL bit (INTCON<6>) and GIEH bit
(INTCON<7>) enables all interrupts that have the
priority bit cleared (low priority). When the interrupt flag,
enable bit and appropriate Global Interrupt Enable bit
are set, the interrupt will vector immediately to address,
0008h or 0018h, depending on the priority bit setting.
Individual interrupts can be disabled through their
corresponding enable bits.

When the IPEN bit is cleared (default state), the
interrupt priority feature is disabled and interrupts are
compatible with PIC

®

 mid-range devices. In

Compatibility mode, the interrupt priority bits for each
source have no effect. INTCON<6> is the PEIE bit that
enables/disables all peripheral interrupt sources.
INTCON<7> is the GIE bit that enables/disables all
interrupt sources. All interrupts branch to address,
0008h, 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. Low-priority interrupts are not
processed while high-priority interrupts are in progress.
The return address is pushed onto the stack and the
PC is loaded with the interrupt vector address (0008h
or 0018h). Once in the Interrupt Service Routine (ISR),
the source(s) of the interrupt can be determined by poll-
ing 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) that re-enables interrupts.
For external interrupt events, such as the INTx 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.

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.