DC Motor Control with PIC16F877A PWM Signals

Pulse Width Modulation (PWM) is a powerful technique used to control motors, lights, and other devices by varying the duty cycle of a digital signal. In this blog post, we’ll explore how to generate PWM signals using the PIC16F877A microcontroller for motor control applications. This post will cover the basic concepts of PWM, configuring the PWM module in the PIC16F877A, and how to apply it for DC motor control.

What is PWM?

PWM is a method of controlling the power delivered to electrical devices by switching the power on and off at high frequencies. The key parameter in PWM is the duty cycle, which determines the proportion of time the signal stays high (on) compared to the total period of the waveform. A higher duty cycle means more power is delivered to the load.

• Duty Cycle (%): $\text{Duty Cycle} = \left( \frac{\text{On Time}}{\text{Period}} \right) \times 100$

For motor control, a higher duty cycle will make the motor spin faster, while a lower duty cycle reduces the speed.

Configuring PWM in PIC16F877A

The PIC16F877A comes with a built-in CCP (Capture/Compare/PWM) module that can generate PWM signals. In this tutorial, we will configure this module to generate a PWM signal to control the speed of a DC motor.

Key Features of the CCP Module:

• PWM Frequency Control: Set by the Timer2 module and the PR2 register.
• Duty Cycle Control: Adjusted using the CCPR1L register and two least significant bits from the CCP1CON register.

Steps to Generate PWM:

1. Set the Clock Frequency: We assume an 8 MHz crystal oscillator for this tutorial, but the same logic applies to other frequencies.
2. Configure Timer2: The frequency and period of the PWM signal are determined by Timer2.
3. Configure the CCP Module: Set the CCP1 module to PWM mode.
4. Control the Duty Cycle: Adjust the duty cycle to vary the motor speed.

Basic PWM Formula

The PWM frequency is determined by the following formula:

• FOSC: The oscillator frequency (in Hz).
• PR2: The Timer2 Period Register.
• Prescaler: Timer2 prescaler, which can be set to 1, 4, or 16.

Circuit Diagram and Operation:

• Transistor: 2N2222A
  • Collector: Connected to the motor's negative terminal
  • Base: Connected to RC2 via a 1kΩ resistor
  • Emitter: Grounded
• Flyback Diode: 1N4007 (connected across the motor terminals with the cathode to the positive terminal)
• DC Motor: Driven by the transistor and controlled by the PWM signal from RC2.
• Smoothing Capacitor
  

This setup enables efficient motor speed control using PWM generated by the PIC16F877A.

Circuit Diagram

The following is the circuit diagram of controlling a DC motor using a PIC16F877A microcontroller.

Circuit Operation

In the circuit setup for controlling the DC motor using PWM, a 2N2222A NPN transistor is used as a switch. The motor is connected to the collector of the transistor, and its emitter is grounded. To control the base of the 2N2222A, the RC2 pin of the PIC16F877A is used, which outputs the PWM signal. A 1kΩ resistor is placed between the RC2 pin and the base of the transistor to limit the base current and protect the microcontroller. This arrangement allows the transistor to switch the motor on and off rapidly, depending on the PWM signal, effectively controlling the speed of the motor.

A flyback diode is placed across the motor terminals to protect the transistor from voltage spikes caused by the inductive nature of the motor when it is turned off. In this case, a 1N4148 or 1N4007 diode can be used. However, the 1N4007 is a better option for this application as it is designed for higher current handling and is more suitable for inductive loads like motors, while the 1N4148 is a fast switching diode typically used in signal processing.

Setting Up the Code

Below is an example of how to set up the PIC16F877A to generate a PWM signal for motor control.

Code Explanation:

We configure Timer2 and the CCP1 module for PWM generation, then use the PWM signal to control the speed of a DC motor connected to the CCP1 output pin (RC2).

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

#include <xc.h>

// CONFIGURATION BITS
#pragma config FOSC = HS     // High-Speed Oscillator
#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
#pragma config CPD = OFF     // Data EEPROM Code Protection Disabled
#pragma config WRT = OFF     // Flash Program Write Protection Disabled
#pragma config CP = OFF      // Flash Program Code Protection Disabled

void PWM_Init(unsigned int freq) {
    // Set the PWM frequency using the PR2 register
    PR2 = (_XTAL_FREQ / (4 * freq * 16)) - 1;  // Assuming prescaler of 16

    // Set CCP1 in PWM mode
    CCP1M3 = 1;
    CCP1M2 = 1;

    // Configure Timer2
    T2CKPS0 = 1;  // Prescaler = 16
    T2CKPS1 = 1;
    TMR2ON = 1;   // Turn on Timer2

    // Set RC2 as output (CCP1 pin)
    TRISC2 = 0;

    // Wait for Timer2 to stabilize
    while (!TMR2IF);
    TMR2IF = 0;
}

void PWM_Set_Duty(unsigned int duty) {
    if (duty < 1024) {
        duty = (duty * 4) / 1023; // Scale duty cycle to 10-bit range

        CCPR1L = duty >> 2; // Set the 8 MSB
        CCP1CONbits.DC1B = duty & 0x03; // Set the 2 LSB
    }
}

void main() {
    // Initialize PWM with a frequency of 5kHz
    PWM_Init(5000); 

    while(1) {
        // Set duty cycle to 50% for motor control
        PWM_Set_Duty(512); 
        
        // Infinite loop
        while (1) {
            // The motor speed can be changed dynamically by adjusting the duty cycle
            PWM_Set_Duty(768);  // Change duty cycle to 75% for higher speed
            __delay_ms(1000);
            PWM_Set_Duty(256);  // Reduce to 25% for lower speed
            __delay_ms(1000);
        }
    }
}

Key Parts of the Code:

1. PWM Initialization (PWM_Init):

   • Configures Timer2 with a prescaler and sets the PR2 value based on the desired frequency.
   • Enables PWM mode in CCP1.

2. Setting Duty Cycle (PWM_Set_Duty):

   • Adjusts the duty cycle of the PWM signal using the CCPR1L and DC1B bits in CCP1CON.
   • The duty cycle value is scaled to the 10-bit range.

3. Main Function:

   • Initializes the PWM module to generate a signal with a frequency of 5 kHz.
   • Sets the motor speed by adjusting the duty cycle.

Understanding the PR2 Calculation

To calculate PR2 based on your desired frequency, use the formula:

For example, to generate a 5 kHz PWM signal with an 8 MHz oscillator and a Timer2 prescaler of 16:

Applications of PWM in Motor Control

Using PWM to control a motor provides several advantages:

• Speed Control: The motor's speed can be adjusted by changing the PWM duty cycle.
• Power Efficiency: Unlike analog voltage control, PWM maintains high power efficiency since the output is fully on or off, reducing heat loss.
• Smooth Control: PWM provides smooth and precise motor control, which is useful for robotics and automation.

Video Demonstration of DC Motor Control with PIC16F877A PWM Signals

 The following video demonstrates how the motor control circuit works. It shows how the motor spins faster or slower when the duty cycle of the PWM signal is increased or decreased. It also shows the effecct on the motor when the frequency of the PWM signal is varied. Thus the video illustrates effect on loads like dc motors with varying frequency and duty cycle of the PWM signal.

Conclusion

Generating PWM signals with the PIC16F877A is straightforward and can be effectively used for motor control applications. By adjusting the duty cycle of the PWM signal, you can easily control the speed of a DC motor with precision. This tutorial covered the configuration of the CCP module, Timer2 setup, and how to dynamically adjust the motor speed in code.

If you're building projects that involve DC motors or need precise control over devices, understanding PWM is essential. Experiment with different frequencies and duty cycles to optimize performance for your application.

Happy coding! Let us know in the comments how you're using PWM in your projects, or if you have any questions!

Further Reading:

• PIC16f877A LED Blink Code
• How to Control an LED Using a Switch with PIC16F877A
• ADC Example Code for PIC16F877A
• How to use UART with PIC16F877A
• How to Calculate the PR2 register value in PIC16F877A