2024-04-26

Intro and important terms

• The ATMega328p has 3 timers, all of them have PWM support, and they are named by a number: timer 0, timer 1, timer 2.
• Both timer 0 and 2 are 8 bit timers, only timer 1 is 16 bits.
• The counter value can be accessed via register TCNTn (n=0,1,2).

Timer/Counter 0

The following are details about operation mode of timer/counter 0. The are few differences between the timers, but most of the content below is generalisable.

Normal mode

In this mode, the counter always go up (incremental instead of decremental), and no counter clear is performed. The counter value is simply wrapped when it reach the maximum (255) back to zero.

This mode is set when the waveform generation mode value is WGM02:0 = 0.

Fast PWM mode

In this mode, a high frequency PWM waveform generation is available. The counter counts from BOTTOM to TOP then restarts from BOTTOM. TOP is defined as 0xFF when WGM2:0 = 3, and OCR0A when WGM2:0 = 7.

The PWM frequency for the output can be calculated by the following equation:

f_pwm = f_clk / (N*256)

For the 16bit timer this becomes:

`f_pwm = f_clk / (N*(1+TOP))

N is a pre-scale value that can be configured (1, 8, 64, 256, or 1024).

This mode is set when the waveform generation mode value is WGM02:0 = 3 or 7.

Examples

void timer0_init() {
  /*
   * Initialise timer 0 with PWM.
   * Frequency: 62.5khz
   *
   * Output:
   *  OC0A: PD6 (duty cycle: 50%) | Arduino D6
   *  OC0B: PD5 (duty cycle: 30%) | Arduino D5
   *
   * A Mode: Fast PWM mode (non-inverting).
   *         COM0A1 = 1
   *         COM0A0 = 0
   *
   * B Mode: Fast PWM mode (non-inverting).
   *         COM0B1 = 1
   *         COM0B0 = 0
   *
   * Wave form: Fast PWM with TOP at 0xFF
   *            WGM01 = WGM00 = 1
   *
   * Pre-scaler: No pre-scaler
   *             CS01 = 0
   *             CS00 = 1
   *
   * Output compare registers:
   *  OCR0A = (256*0.5) = 128.
   *  OCR0B = (256*0.3) = 77.
   */
  DDRD |= (1 << PD5) | (1 << PD6);
  TCCR0A |= (1 << COM0A1) | (1 << COM0B1) | (1 << WGM01) | (1 << WGM00);
  TCCR0B |= (1 << CS00);
  OCR0A = (256 * 0.5);
  OCR0B = (256 * 0.3);
}

void timer1_init() {
  /*
   * Initialise timer 1 with PWM.
   * Frequency: 1,953khz
   *
   * Output:
   *  OC1A: PB1 (duty cycle: 50%) | Arduino D9
   *  OC1B: PB2 (duty cycle: 25%) | Arduino D10
   *
   * A Mode: Fast PWM mode (non-inverting).
   *         COM1A1 = 1
   *         COM1A0 = 0
   *
   * B Mode: Fast PWM mode (non-inverting).
   *         COM1B1 = 1
   *         COM1B0 = 0
   *
   * Wave form: Fast PWM with TOP at 0x03FF (1023)
   *            WGM10 = WGM11 = WGM12 = 1
   *
   * Pre-scaler: 1/8 (an eighth).
   *             CS11 = 1
   *
   * Output compare registers:
   *  OCR1A = (65536*0.5) = 32768.
   *  OCR1B = (65536*0.25) = 16384.
   */
  DDRB |= (1 << PB1) | (1 << PB2);
  TCCR1A |= (1 << COM1A1) | (1 << COM1B1) | (1 << WGM12) | (1 << WGM11) |
            (1 << WGM10);
  TCCR1B |= (1 << CS11);
  OCR1A = (uint16_t)(65535 * 0.5);
  OCR1B = (uint16_t)(65535 * 0.25);
}

void timer2_init() {
  /*
   * Initialise timer 0 with PWM.
   * Frequency: 488.28hz
   *
   * Output:
   *  OC2A: PB3 (duty cycle: 50%) | Arduino D11
   *  OC2B: PD3 (duty cycle: 30%) | Arduino D3
   *
   * A Mode: Fast PWM mode (non-inverting).
   *         COM2A1 = 1
   *         COM2A0 = 0
   *
   * B Mode: Fast PWM mode (non-inverting).
   *         COM2B1 = 1
   *         COM2B0 = 0
   *
   * Wave form: Fast PWM with TOP at 0xFF
   *            WGM21 = WGM20 = 1
   *
   * Pre-scaler: 1/128
   *             CS22 = 1
   *             CS21 = 0
   *             CS20 = 1
   *
   * Output compare registers:
   *  OCR2A = (256*0.5) = 128.
   *  OCR2B = (256*0.3) = 77.
   */
  DDRD |= (1 << PD3);
  DDRB |= (1 << PB3);

  TCCR2A |= (1 << COM2A1) | (1 << COM2B1) | (1 << WGM21) | (1 << WGM20);
  TCCR2B |= (1 << CS22) | (1 << CS20);
  OCR2A = (256 * 0.5);
  OCR2B = (256 * 0.3);
}

• TCCR1A: Timer/Counter1 Control Register A.
• TCCR1B: Timer/Counter1 Control Register B.
• CS12: Clock select. A value of 1 << CS12 | 1 << CS10, gives clk/1024 (from prescaler) which is 16Mhz/1024 == 15,625hz.

Further reading

source