Datasheet

PICkit™ 3 Debug Express

When an interrupt occurs and the Master Interrupt signal is asserted, the PIC

microcontroller finishes executing the current instruction, stores the next address on

the Return Address Stack, and then jumps to an interrupt vector. At the interrupt vector

it begins executing a function designated as the Interrupt Service Routine. When this

function exits, program execution returns to the address stored on the Return Address

Stack.

Interrupts allow hardware events to be acted upon very quickly and regardless of the

state of the main program because they cause the immediate execution of dedicated

code.

The PIC18FXXXX architecture supports up to two levels of interrupt priority, each of

which have a logic structure like that in Figure 3-37. Most interrupts have a Priority bit

associated with the interrupt flag and enable that assigns it to one of the two priority

levels. Using priority levels is optional, and the PIC18FXXXX may be configured to use

only one level priority.

When two levels of interrupt priority are used, an interrupt of either priority level may

interrupt the main program. However, only a high priority interrupt may interrupt a low

priority interrupt, and nothing may interrupt a high priority interrupt. As shown in Figure

3-38, when a low priority interrupt event occurs during execution of statement3 in the

main code, the program jumps to begin executing the low priority interrupt function.

During execution of the lo_statement2, a high priority interrupt event occurs,

causing program execution to jump to the High Priority Interrupt function. When the

high priority function completes and exits, execution is returned to where it left off in the

low priority function. Similarly, when the low priority function completes and exits,

program execution returns to where it left off in the main code, at statement4.

FIGURE 3-40: PRIORITY INTERRUPT EXECUTION FLOW

The high priority interrupt vector is at program memory address 0x0008. The low

priority interrupt vector is at program memory address 0x0018. If interrupt priorities are

not used, all interrupts jump to the high priority vector at 0x0008.

3.8.2 Exploring the Lesson 8 Source Code

The first thing to note is that the Directions variable is now global, so it may be

accessed in the Interrupt Service Routine functions.

When using interrupts, the interrupt vectors must be defined and placed at the

appropriate vector addresses using the #pragma code directives. An inline assembly

GOTO statement redirects program execution to the interrupt functions, whose name

serves as the GOTO argument.