PIC18F2450/4450
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.
8.0
INTERRUPTS
The PIC18F2450/4450 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
interrupt any low priority interrupts that may be in
progress.
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 determined by polling
the interrupt flag bits. The interrupt flag bits must be
cleared in software before re-enabling interrupts to avoid
recursive interrupts.
There are ten registers which are used to control
interrupt operation. These registers are:
• RCON
• INTCON
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.
• INTCON2
• INTCON3
• PIR1, PIR2
• PIE1, PIE2
• IPR1, IPR2
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.
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.
Note:
Do not use the MOVFFinstruction to modify
any of the interrupt control registers while
any interrupt is enabled. Doing so may
cause erratic microcontroller behavior.
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
8.1
USB Interrupts
• Enable bit that allows program execution to
branch to the interrupt vector address when the
flag bit is set
Unlike other peripherals, the USB module is capable of
generating a wide range of interrupts for many types of
events. These include several types of normal commu-
nication and status events and several module level
error events.
• 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
globally. Setting the GIEH bit (INTCON<7>) enables all
interrupts that have the priority bit set (high priority).
Setting the GIEL bit (INTCON<6>) 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 000008h or 000018h,
depending on the priority bit setting. Individual inter-
rupts can be disabled through their corresponding
enable bits.
To handle these events, the USB module is equipped
with its own interrupt logic. The logic functions in a
manner similar to the microcontroller level interrupt
funnel, with each interrupt source having separate flag
and enable bits. All events are funneled to a single
device level interrupt, USBIF (PIR2<5>). Unlike the
device level interrupt logic, the individual USB interrupt
events cannot be individually assigned their own prior-
ity. This is determined at the device level interrupt
funnel for all USB events by the USBIP bit.
For additional details on USB interrupt logic, refer to
Section 14.5 “USB Interrupts”.
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. 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.
© 2006 Microchip Technology Inc.
Advance Information
DS39760A-page 85
MICROCHIP [ MICROCHIP ]
