Datasheet

PICkit™ 3 Debug Express Lessons

3.9.3 Exploring the Lesson 9 Source Code

The Lesson 9 program code has a simple background loop in the main() function that

displays a binary count on the demo board LEDs, as shown in Figure 3-46. Each count

increment is delayed by 64,000 instruction cycles. As the clock frequency is changed,

the instruction rate changes and so the total time in seconds of the delay gets shorter

as the clock frequency increases. The effect is that the LED display will count faster as

the clock speed is increased.

At the start of the program, the internal oscillator is running at 250 kHz. Each press of

the demo board switch creates an interrupt that increases the clock frequency by a

factor of 2 up through 64 MHz, after which it returns to 250 kHz.

FIGURE 3-48: SOURCE CODE BACKGROUND LOOP

A few other things of interest in the Lesson 9 source code are:

• The interrupts are configured for only a single level of priority, where interrupt pri-

orities are disabled. This differs from the Lesson 8 source code where interrupt

priorities were enabled.

• Instead of using ADCON1 to configure the switch input RB0 as a digital input as

was done in previous lessons, the Lesson 9 source sets the Configuration bit

PBADEN = OFF. This causes all PORTB pins to default to digital, instead of ana-

log, inputs on a Reset.

• The Lesson 9 interrupt service function void InterruptService(void) dem-

onstrates calling another function

void SetIntOSC(IntOSCFreq *ClockSet)

from within the interrupt service code.

3.9.4 Build and Run the Lesson 9 Code with PICkit 3 Debug Express

Build and program the Lesson 9 project, then Run the application in the debugger.

Pressing the demo board switch causes the program to change the oscillator frequency

during execution. As the oscillator frequency increases, the rate at which the LEDs

count increases.

while (1)

{// delayand counton LEDs here.Interrupt handles switch and freqchanges

LATD =LED_Count++; //output countto PORTD LEDs

Delay1KTCYx(32); //delay 32,000cycles or about 1 secat 125kHz

}