PWM Workbook: Configuring PWM on the ATmega328P

1. Introduction to PWM on ATmega328P

2. Timer0 Registers for PWM

TCCR stands for Timer/Counter Control Register. On the ATmega328P and similar microcontrollers, these registers control the behavior of each timer/counter module.

Each timer has one or more TCCR registers that configure aspects such as:

• PWM mode (e.g., Fast PWM, Phase Correct PWM)
• Compare Output Mode (inverting or non-inverting PWM signal)
• Clock source and prescaler (e.g., no prescaler, prescaler of 8, 64, etc.)

2.1 Types of TCCR Registers on ATmega328P

The ATmega328P has three timers, each with associated TCCR registers:

1. Timer0 (8-bit):

   • TCCR0A and TCCR0B: These control the settings for Timer0, including PWM configuration, clock selection, and output modes.

2. Timer1 (16-bit):

   • TCCR1A and TCCR1B: These control Timer1, which supports 16-bit resolutions. This is useful for applications needing finer control, such as precise PWM or high-resolution counters.

3. Timer2 (8-bit):

   • TCCR2A and TCCR2B: These control Timer2, similar to Timer0 but with different prescaler and clock source options.

2.2 TCCR0A - Timer/Counter Control Register A

• COM0A1: Compare Output Mode for OC0A.

  • Setting COM0A1 to 1 enables non-inverting PWM output on OC0A (Pin PD6).

    Note

    • In non-inverting mode, the output starts high at the beginning of the timer cycle and goes low when the timer reaches the value in OCR0A.
    • The duty cycle is therefore determined by the value in OCR0A: a higher value keeps the signal high for a longer portion of the cycle

• WGM01 and WGM00: Waveform Generation Mode.

  • Setting both WGM01 and WGM00 to 1 puts Timer0 in Fast PWM mode.

2.3TCCR0B - Timer/Counter Control Register B

• CS02:CS00: Clock Select bits.
  • Sets the prescaler to control the PWM frequency.
  • Example: Setting CS01 and CS00 to 1 (CS01 | CS00) applies a prescaler of 64 to the timer.

3. Prescalers

The prescaler in the ATmega328P (and similar microcontrollers) is a divider that reduces the main system clock frequency to a lower frequency for the timer. The prescaler allows you to control the frequency of the timer, which directly affects the speed of operations like PWM or delays that the timer generates.

3.1 Purpose of the Prescaler

The microcontroller’s main clock frequency is often quite high (e.g., 16 MHz on the ATmega328P). Running the timer directly at this speed would result in very high-frequency signals, making it challenging to create longer delays or lower-frequency PWM signals. The prescaler helps by slowing down the timer, making it easier to generate slower or more manageable signals.

3.2 How the Prescaler Works

The prescaler takes the main clock frequency and divides it by a specified factor, which is then used as the input clock for the timer. For example, if the system clock is 16 MHz and the prescaler is set to 64, then the timer operates at:

\[250,000\ Hz\ =\ \frac{16,000,000\ Hz}{64}\]

This effectively slows down the timer and allows more extended timing intervals or lower-frequency PWM output.

3.4 Prescaler Options on ATmega328P

Each timer on the ATmega328P has several prescaler options, typically selectable by configuring specific bits in the TCCR (Timer/Counter Control Register).

3.5 Example Calculation of PWM Frequency with a Prescaler

When using Fast PWM mode on an 8-bit timer, the PWM frequency can be calculated as:

\[PWM\ Frequency\ =\ \frac{CPU\ Clock\ Frequency}{Prescaler\ \cdot 256}\]

For example, with a 16 MHz CPU clock and a prescaler of 64:

\[976\ Hz \approx\ \frac{16,000,000\ Hz}{64\cdot 256}\]

This frequency can then be adjusted by changing the OCR(OCR0A) (Output Compare Register) values to control the duty cycle, with the prescaler setting determining the overall timing speed of the signal.

To set the prescaler for Timer0 on the ATmega328P, you configure the CS00, CS01, and CS02 bits in the TCCR0B register. Each combination of these bits determines a different prescaler value. Here’s how you can configure Timer0 for prescalers of 8, 64, 256, and 1024.

3.6 Setting the Prescaler for Timer0

The following code snippets show how to set each prescaler in TCCR0B:

TCCR0B = (1 << CS01); // Prescaler = 8

TCCR0B = (1 << CS01) | (1 << CS00); // Prescaler = 64

TCCR0B = (1 << CS02); // Prescaler = 256

TCCR0B = (1 << CS02) | (1 << CS00); // Prescaler = 1024

4. Building the pwm.c source code

4.1 Basic pwm.ino

Now we have an understanding of the what is going on under the hood we can convert this pwm.ino into and embedded version.

int pwmPin = 6;      // LED connected to digital pin 9

void setup() {
    pinMode(pwmPin, OUTPUT);  // sets the pin as output
    analogWrite(ledPin, 127); // analogRead values go from 0 to 1023, analogWrite values from 0 to 255
}

void loop(){}

4.2 Create pwm.c

1. Create a new directory inside embeddedC called pwm

   Terminal

   $ mkdir -p ~/embeddedC/pwm && cd embeddedC/pwm
   

2. Create a new file inside the the pwm directory called pwm.c

   Code

   $ touch pwm.c
   

3. Now it's time start wrighting out the program:

   Code

   #include <avr/io.h>
   
   void PWM_Init()
   {
       // Set PD6 (OC0A) as output
       DDRD |= (1 << PD6); // PWM pin 6
   
       // Set Timer0 to Fast PWM mode with non-inverting output on OC0A
       TCCR0A = (1 << COM0A1) | (1 << WGM01) | (1 << WGM00);
   
       // Set the prescaler to 64 for Timer0
       TCCR0B = (1 << CS01) | (1 << CS00);
   
       // Initialize with 0% duty cycle
       OCR0A = 0;
   }
   
   int main()
   {
       PWM_Init(); // Initialize PWM on PD6
   
       while (1) {
           // Adjust OCR0A as needed for duty cycle from 0 to 255
           OCR0A = 127; // Example: 50% duty cycle
       }
   }
   

   Explanation of code

   1. Setting PD6 as Output:
      • DDRD |= (1 << PD6); configures PD6 as an output pin to output the PWM signal.
   2. Configuring Timer0 for Fast PWM Mode:
      • TCCR0A:
        • (1 << COM0A1): Sets non-inverting mode for PWM on OC0A.
        • (1 << WGM01) | (1 << WGM00): Sets Fast PWM mode.
      • TCCR0B:
        • (1 << CS01) | (1 << CS00): Sets a prescaler of 64, controlling the PWM frequency. . Setting the Duty Cycle:
      • OCR0A: Controls the duty cycle. A value of 127 provides approximately a 50% duty cycle.
      • Range: OCR0A can range from 0 to 255, where:
        • 0: 0% duty cycle (always low)
        • 127: 50% duty cycle
        • 255: 100% duty cycle (always high)

4. We are now going to copy the Makefile from the blink/ created last time, and replace all instances of blink with pwm using the regex feature:

   Terminal

   $ cp ../blink/Makefile .
   $ vim Makefile
   

5. Inside vim type the following an press enter to find all instances of blink with pwm

   Terminal

   :%s/\<blink\>/pwm/g
   

   

6. Remember like with blink we need to compile and upload the code to the board, ensure it is plugged in, to find the com port on windows:

   

7. If the COM port has changed remember to change it in the Makefile

   Code

   PORT = COM#
   

8. Use make:

   • make clean
   • make
   • make upload

9. Ensure you have wired up an led circuit in to pin 6 with a 220\(\Omega\) resistor

5. Exercises

5.1 Exercise 1: Changing the Duty Cycle

Modify the OCR0A register in the main loop to create different duty cycles. For example, try setting it to 64, 127, and 192. Observe how the brightness of an LED connected to PD6 changes.

Task:

• Set OCR0A = 64; // ~25% duty cycle
• Set OCR0A = 192; // ~75% duty cycle
• Record the observed LED brightness.

5.2 Exercise 2: Creating a Smooth Fade Effect

Modify the code to gradually increase the duty cycle from 0 to 255 and then decrease it back to 0 to create a smooth "breathing" LED effect.

Hint:

• Use a for loop to increment OCR0A from 0 to 255, adding a delay between each increment.

• Then, use another for loop to decrement it from 255 back to 0.

  Suppress code here... [31 lines]

  #include <avr/io.h>
  
  void PWM_Init()
  {
      // Set PD6 (OC0A) as output
      DDRD |= (1 << PD6); // PWM pin 6
  
      // Set Timer0 to Fast PWM mode with non-inverting output on OC0A
      TCCR0A = (1 << COM0A1) | (1 << WGM01) | (1 << WGM00);
  
      // Set the prescaler to 64 for Timer0
      TCCR0B = (1 << CS01) | (1 << CS00);
  
      // Initialize with 0% duty cycle
      OCR0A = 0;
  }
  
  int main()
  {
      PWM_Init(); // Initialize PWM on PD6
  
      while (1) {
          for (uint8_t i = 0; i <= 255; i++) {
              OCR0A = i;
              _delay_ms(10); // Adjust delay as needed
          }
          for (uint8_t i = 255; i > 0; i--) {
              OCR0A = i;
              _delay_ms(10); // Adjust delay as needed
          }
      }
  }
  

5.3 Exercise 3: Changing the PWM Frequency

Experiment with different prescaler values to change the PWM frequency. Change the CS02:CS00 bits in TCCR0B to see the effects.

Task:

• Try prescalers of 8, 64, and 256, and observe how the frequency and brightness change.