Generating PWM Signals with PIC CCP Module Tutorial

Pulse Width Modulation (PWM) is a technique used to control analog devices using digital signals.

It is widely used for motor speed control, LED dimming, and communication applications.

Most PIC microcontrollers include CCP (Capture/Compare/PWM) modules that allow precise PWM signal generation with minimal CPU intervention.

Understanding PWM in PIC Microcontrollers

A PWM signal consists of:

• Frequency: How often the signal repeats per second.
• Duty Cycle: The percentage of time the signal is HIGH within one period.

Formula:

Duty Cycle(%)=(Total Period/ON time​)×100

Example:

• If a 1 kHz PWM signal has a duty cycle of 50%, it means the signal is HIGH for 0.5ms and LOW for 0.5ms.

CCP Module Overview

The CCP (Capture/Compare/PWM) module in PIC microcontrollers supports PWM mode, where it generates a periodic pulse with adjustable duty cycle.

CCP Registers for PWM Mode

PWM Frequency Calculation

The PWM frequency is determined by the PR2 register and the Timer 2 prescaler.

PWM Period=(PR2+1)×4×Tosc​×Prescaler

PWM Frequency = 1/PWM Period​

Where:

• Tosc=1FoscT_{osc} = \frac{1}{F_{osc}} (Clock cycle time)
• FoscF_{osc} is the oscillator frequency
• PR2 is an 8-bit register (0 to 255)
• Prescaler can be 1, 4, or 16

Example Calculation

If:

• Fosc=8MHzF_{osc} = 8MHz
• PR2 = 124
• Timer2 Prescaler = 16

PWM Period = (124+1)×4 × 1/8MHz ×16

PWM Frequency = 1/(125 × 4 × 1/8MHz​ × 16)

PWM Frequency = 1kHz​

Setting PWM Duty Cycle

The PWM duty cycle is controlled using CCPR1L and CCP1CON<5:4> bits.

Duty Cycle (10-bit)=(CCPR1L:CCP1CON<5:4>)×Tosc×4\text{Duty Cycle (10-bit)} = \left( CCPR1L:CCP1CON<5:4> \right) \times T_{osc} \times 4

• CCPR1L holds the upper 8 bits.
• CCP1CON<5:4> holds the lower 2 bits.

Example

For 50% duty cycle on a 1 kHz PWM:

Duty Cycle (10-bit)=125×42=250\text{Duty Cycle (10-bit)} = \frac{125 \times 4}{2} = 250

• CCPR1L = 0x3E (62 in decimal)
• CCP1CON<5:4> = 0b10

Implementing PWM on PIC Microcontroller

We will configure PWM using CCP1 module on a PIC16F877A to control LED brightness.

Hardware Requirements

• We used a PIC16F877A microcontroller
• 8 MHz external crystal
• LED with resistor (330Ω)
• Power supply (5V)

Circuit

• PWM output is on RC2 (CCP1 pin).
• The LED is connected to RC2 with a 330Ω resistor.

PIC PWM Code Example

We used MPLAB X with the XC8 compiler for this example

#include <xc.h>

// Define Configuration Bits (For PIC16F877A)
#pragma config FOSC = XT // Oscillator Selection
#pragma config WDTE = OFF // Watchdog Timer Disabled
#pragma config PWRTE = OFF // Power-up Timer Disabled
#pragma config BOREN = ON // Brown-out Reset Enabled
#pragma config LVP = OFF // Low Voltage Programming Disabled

#define _XTAL_FREQ 8000000  // Define clock frequency (8MHz)

void PWM_Init(unsigned int frequency);
void PWM_DutyCycle(unsigned int duty);

void main() {
    TRISC2 = 0;  // Set CCP1 (RC2) as output
    PWM_Init(1000);  // Initialize PWM with 1 kHz frequency

    while(1) {
        PWM_DutyCycle(512);  // Set 50% duty cycle (512 out of 1023)
        __delay_ms(1000);
        
        PWM_DutyCycle(256);  // Set 25% duty cycle (256 out of 1023)
        __delay_ms(1000);
        
        PWM_DutyCycle(768);  // Set 75% duty cycle (768 out of 1023)
        __delay_ms(1000);
    }
}

// Function to Initialize PWM
void PWM_Init(unsigned int frequency) {
    unsigned int PR2_value;
    
    PR2_value = (_XTAL_FREQ / (frequency * 4 * 16)) - 1;
    PR2 = PR2_value;  // Set PR2 for the required PWM frequency
    
    CCP1CON = 0x0C;  // Configure CCP1 as PWM mode
    T2CON = 0x04;  // Enable Timer2 with Prescaler = 1
    
    // Start Timer2
    TMR2 = 0;
    TMR2ON = 1;
}

// Function to Set PWM Duty Cycle (0 to 1023)
void PWM_DutyCycle(unsigned int duty) {
    CCPR1L = duty >> 2;  // Upper 8 bits
    CCP1CONbits.DC1B = duty & 0x03;  // Lower 2 bits
}

Explanation

1. Initialize PWM (PWM_Init())
   • Configures CCP1 module in PWM mode.
   • Calculates PR2 value for the required PWM frequency.
   • Enables Timer2 with an appropriate prescaler.
2. Set Duty Cycle (PWM_DutyCycle())
   • Converts 10-bit duty cycle value into 8-bit (CCPR1L) and 2-bit (CCP1CON<5:4>) values.
3. Main Loop
   • Changes LED brightness by setting different duty cycles (25%, 50%, 75%).

Summary

Applications of PWM

• Motor Speed Control
• LED Brightness Control
• Analog Signal Generation
• Audio Signal Processing

This tutorial covers everything needed to generate PWM signals using PIC CCP module