PIC18CXX2
When the IPEN bit is cleared (default state), the inter-
rupt priority feature is disabled and interrupts are com-
7.0
INTERRUPTS
The PIC18CXX2 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 over-
ride any low priority interrupts that may be in progress.
patible with PICmicro mid-range devices.
In
compatibility mode, the interrupt priority bits for each
source have no effect. INTCON<6> is the PEIE bit,
which enables/disables all peripheral interrupt sources.
INTCON<7> is the GIE bit, which enables/disables all
interrupt sources. All interrupts branch to address
000008h in compatibility mode.
There are ten registers which are used to control inter-
rupt operation. These registers are:
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 prior-
ity levels are used, this will be either the GIEH or GIEL
bit. High priority interrupt sources can interrupt a low
priority interrupt.
• RCON
• INTCON
• INTCON2
• INTCON3
• PIR1, PIR2
• PIE1, PIE2
• IPR1, IPR2
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.
It is recommended that the Microchip header files sup-
plied with MPLAB 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.
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.
Each interrupt source has three bits to control its oper-
ation. The functions of these bits are:
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.
• 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<7>). When interrupt priority is
enabled, there are two bits which enable interrupts glo-
bally. Setting the GIEH bit (INTCON<7>) enables all
interrupts that have the priority bit set. Setting the GIEL
bit (INTCON<6>) enables all interrupts that have the
priority bit cleared. When the interrupt flag, enable bit
and appropriate global interrupt enable it are set, the
interrupt will vector immediately to address 000008h or
000018h depending on the priority level. Individual
interrupts can be disabled through their corresponding
enable bits.
7/99 Microchip Technology Inc.
Preliminary
DS39026B-page 65
MICROCHIP [ MICROCHIP ]
