Simple PWM example for the PIC18F4620

The PIC18F4620 includes a hardware pulse width modulation feature which is really useful for generating periodic pulse sequences with different frequencies and duty cycles. Applications include motor speed control, servo control, and varying the brightness of an LED. This simple example program generates a 1kHz PWM (pulse width modulation) signal on pin 17 (RC2 / CCP1). The program is written in C for Microchip’s XC8 C compiler.

NB The PIC18F4620 provides 10-bit resolution in setting the PWM pulse width. However, to keep things simple, I’m disregarding the 2 least significant bits, which reside in a different register to the 8 most significant bits. Unless you really need the extra resolution, I recommend ignoring the 2 least significant bits and treating both the PWM period and pulse width as 8-bit values. Because I’m making this simplification, the calculations I present below are a bit simpler than those described in the PIC18F4620 datasheet.

Setting the PWM period and duty cycle

In this example, the clock oscillator frequency of the 18F4620 is left at the default value of Fosc = 1MHz. The oscillator period is therefore Tosc = 1us.

The PIC18F4620 performs one machine code instruction every four clock cycles, so the instruction cycle in this example is Tcy = 4us. This is an important figure because Tcy is really the fundamental unit of time when measuring time in PIC programs.

The period of the PWM waveform is determined by the value written to a special function register called PR2 (short for Period Register for Timer 2). The formula for calculating PWM period is:

PWM period = (PR2+1) * Timer 2 prescaler value * Tcy

In this example, PR2 = 249, the Timer 2 prescaler = 1 and Tcy = 4us. Therefore,

PWM period = 250 * 1 * 4us = 1ms

The pulse width of the PWM waveform is determined by the value written to a special function register called CCPR1L (short for Capture/Compare/PWM channel 1 Register, low byte). The formula for calculating PWM pulse width is:

PWM pulse width = CCPR1L * Timer 2 presacler value * Tcy

Initially, CCPR1L = 125, the Timer 2 prescaler = 1 and Tcy = 4us. Initially therefore,

PWM period = 125 * 1 * 4us = 0.5ms

The values of PR2 and CCPR1L can be changed at any time, which will change the period or pulse width of the waveform. In this example, the period is constant at 1ms, giving a frequency of 1kHz. However, the value of CCPR1L changes every half a seconds, causing the duty cycle of the waveform to cycle through the values 50%, 10%, 0% repeatedly.

C code and build script

This is the full C code for the XC8 compiler.

//
// PIC18F4620 1kHz PWM example program
// Written by Ted Burke (http://batchloaf.com)
// Last updated 4-4-2013
//
// To compile with XC8:
//     xc8 --chip=18F4620 main.c
//

#include <xc.h>

#pragma config OSC=INTIO67,MCLRE=OFF,WDT=OFF,LVP=OFF,BOREN=OFF

int main(void)
{
	// Set up PWM (see section 15.4 of the PIC18F4620 datasheet)
	CCP1CON = 0b00001100;   // Enable PWM on CCP1
	TRISC = 0b11111001;     // Make pin 17 (RC1/CCP2) an output
	T2CON = 0b00000100;     // Enable TMR2 with prescaler = 1
	PR2 = 249;   // PWM period = (PR2+1) * prescaler * Tcy = 1ms
	CCPR1L = 25; // pulse width = CCPR1L * prescaler * Tcy = 100us
	
	while(1)
	{
		// 50% duty cycle for 500ms
		CCPR1L = 125;
		_delay(125000);
		
		// 10% duty cycle for 500ms
		CCPR1L = 25;
		_delay(125000);
		
		// 0% duty cycle for 500ms
		CCPR1L = 0;
		_delay(125000);
	}
}

I don’t use MPLAB; I just write my C code in a text editor, compile it with XC8 in a command window, then use the PICkit 2 application to transfer the hex file to the PIC. The build script I use (which contains just one command) is shown below. I save the C code as “main.c” and the build script in the same folder as “build.bat”.

xc8 --chip=18F4620 main.c

Here’s how it looked when I compiled the program on my laptop:

Measuring the waveform period and pulse width

I used the PICkit2 Logic Analyzer to take the following snapshot of the waveform when the duty cycle was 50% (when CCPR1L = 125). The signal was connected to channel 3 of the logic analyzer (pin 6 of the PICkit 2):

Here, the logic analyzer cursors are measuring the period of the waveform:

Here, the logic analyzer cursors are measuring the pulse width of the waveform:

Finally, here’s the waveform when the duty cycle is set to 10% (i.e. when CCPR1L = 25):