Microchip Technology MA330024 Data Sheet

DS70000591F-page 178

 2009-2014 Microchip Technology Inc.

7.4.1

INITIALIZATION

Complete the following steps to configure an interrupt
source at initialization:

1.

Set the NSTDIS 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 can 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.

7.4.2

INTERRUPT SERVICE ROUTINE

The method used to declare an ISR and initialize the
IVT with the correct vector address depends on the
programming language (C or assembler) and the
language development toolsuite used to develop the
application. 

In general, the user application must clear the interrupt
flag in the appropriate IFSx register for the source of
interrupt that the ISR handles. Otherwise, program will
re-enter the ISR 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.

7.4.3

TRAP SERVICE ROUTINE

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.

7.4.4

INTERRUPT DISABLE

The following steps outline the procedure to disable all
user interrupts:

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, EOh, with SRL.

To enable user interrupts, the POP instruction can be
used to restore the previous SR value.

The  DISI instruction provides a convenient way to
disable interrupts of Priority Levels 1-6 for a fixed
period of time. 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.

Only user interrupts with a priority level of
7 or lower can be disabled. Trap sources
(Level 8-Level 15) cannot be disabled.