Microchip Technology MA240017 Data Sheet

DS39927C-page 90

 2008-2011 Microchip Technology Inc.

8.4.1

INITIALIZATION

To configure an interrupt source:
1.

Set the NSTDIS Control bit (INTCON1<15>) if
nested interrupts are not desired.

2.

Select the user-assigned priority level for the
interrupt source by writing the control bits in the
appropriate IPCx register. The priority level will
depend on the specific application and type of
interrupt source. If multiple priority levels are not
desired, the IPCx register control bits, for all
enabled interrupt sources, may be programmed
to the same non-zero value.

3.

Clear the interrupt flag status bit associated with
the peripheral in the associated IFSx register.

4.

Enable the interrupt source by setting the
interrupt enable control bit associated with the
source in the appropriate IECx register.

8.4.2

INTERRUPT SERVICE ROUTINE

The method that is used to declare an ISR and initialize
the IVT with the correct vector address depends on the
programming language (i.e., C or assembler) and the
language development toolsuite that is used to develop
the application. In general, the user must clear the
interrupt flag in the appropriate IFSx register for the
source of the interrupt that the ISR handles. Otherwise,
the ISR will be re-entered immediately after exiting the
routine. If the ISR is coded in assembly language, it
must be terminated using a RETFIE instruction to
unstack the saved PC value, SRL value and old CPU
priority level.

8.4.3

TRAP SERVICE ROUTINE (TSR)

A Trap Service Routine (TSR) is coded like an ISR,
except that the appropriate trap status flag in the
INTCON1 register must be cleared to avoid re-entry
into the TSR.

8.4.4

INTERRUPT DISABLE

All user interrupts can be disabled using the following
procedure:
1.

Push the current SR value onto the software
stack using the PUSH instruction.

2.

Force the CPU to Priority Level 7 by inclusive
ORing the value OEh with SRL.

To enable user interrupts, the POP instruction may be
used to restore the previous SR value.
Only user interrupts with a priority level of 7 or less can
be disabled. Trap sources (Levels 8-15) cannot be
disabled.
The  DISI instruction provides a convenient way to
disable interrupts of Priority Levels 1-6 for a fixed
period. Level 7 interrupt sources are not disabled by
the DISI instruction.

At a device Reset, the IPCx registers are
initialized, such that all user interrupt
sources are assigned to Priority Level 4.