Datasheet

PICkit™ 3 Debug Express Lessons

See Section 19.0 “10-Bit Analog-to-Digital Converter (A/D) Module” in the

PIC18F45K20 Data Sheet (DS41303) for more information on the ADC peripheral.

3.8 LESSON 8: INTERRUPTS

This lesson changes the Lesson 7 code to use interrupts to act on the switch press and

Timer0 events instead of polling them. The switch uses the RB0/INT0 external interrupt

capability.

3.8.1 PIC18FXXXX Interrupt Architecture

When a peripheral requires attention or an event occurs, it sets an interrupt flag. Each

flag has an interrupt enable bit that determines whether it will generate an interrupt to

the microcontroller or not. In the previous lessons, interrupt flags such as TMR0IF were

polled, but did not create an interrupt as the enable bit was not set. The enable bits

allow only selected events to cause in interrupt. All interrupts are ORed together, and

then ANDed with a global interrupt enable.

FIGURE 3-39: SIMPLIFIED INTERRUPT LOGIC

Note: If TMR0L is added to the Watch window, it will cause incorrect operation

when stepping through the following 2 lines of code:

TMR0H = ADC_Convert();

TMR0L = 0;

This is caused by the buffered nature of TMR0H. When “Stepping Over” the

TMR0H assignment statement, the MPLAB IDE will read the TMR0L

upper byte of TMR0 is loaded into the TMR0H buffer, wiping out the value

written in the previous TMR0H assignment statement.

One workaround to be able to add TMR0L to the Watch window is to make

sure not to step from the TMR0H to the TMR0L statement. Set a breakpoint

on the TMR0L assignment statement, and Run from the TMR0H

assignment statement.

Key Concepts

- An interrupt is a hardware-based event that “interrupts” the program code to

execute a special function. When the interrupt function exits, program exe-

cution returns to where it left off.

- The PIC18FXXXX supports a single interrupt priority or two levels of inter-

rupt priority.

- A Low Priority interrupt can interrupt the main program. A High Priority inter-

rupt can interrupt the main program or a low priority interrupt.

- The directives #pragma interruptlow and #pragma interrupt are

used to define the interrupt functions.