ATmega328p 20kHz PWM for control of Delta THB1748BG

main.c

void setup() {
  // Core clock runs at 16MHz, three different timers can divide this freq
  // by using a prescalar factor. Different PWM modes are available for each
  // timer. We are using Fast PWM, which has the counter count from 0 to TOP. 
  // Thus, timer frequency would be 16MHz / prescalar / TOP count. The output 
  // turns on when the timer is at 0, and turns off when reaches the compare 
  // register value. As the compare register approaches TOP, the duty cycle 
  // approaches 100%. Since 0 is included in the high state, the effective
  // counts in the high state will be the register value + 1. 
  //
  // Timer to pin pairs:
  // OC0A   6
  // OC0B   5   * use
  // OC1A   9
  // OC1B   10  * use
  // OC2A   11
  // OC2B   3   * use
  // Note: Timer 0 will affect functions using user time e.g. millis() and
  // delay(). We will be using A to set TOP, so we can only use B in each 
  // pair.

  // Set data direction to output for relevant pins
  pinMode(3, OUTPUT);
  pinMode(5, OUTPUT);
  pinMode(10, OUTPUT);
 
  // Reset existing configuration and counters
  TCCR0A = 0;
  TCCR0B = 0;
  TCCR1A = 0;
  TCCR1B = 0;
  TCCR2A = 0;
  TCCR2B = 0;
  TCNT0 = 0;
  TCNT1 = 0;
  TCNT2 = 0;

  // Set waveform generation mode bits WGM to FAST PWM using OCRnA for TOP.
  // Timer 0 & 2:
  // Mode WGMn2 WGMn1 WGMn0 TOP   Description
  // 3    0     1     1     0xFF  Fast PWM
  // 7    1     1     1     OCRnA Fast PWM
  // Timer 1:
  // Mode WGM13 WGM12 WGM11 WGM10 TOP   Description
  // 5    0     1     0     1     0xFF  Fast PWM, 8bit
  // 15   1     1     1     1     OCRnA Fast PWM
  //  
  // Set COMnA/B to 10 provides non-inverted PWM to outputs A and B
  // COMnA1 COMnA0  Description
  // 0      0       OCnA disabled
  // 0      1       WMGn2 = 0: Normal, WGMn2 = 1: Toggle OCnA on Match
  // 1      0       Non-inverted mode
  // 1      1       Inverted mode
  // COMnB1 COMnB0  Description
  // 0      0       OCnB disabled
  // 0      1       Reserved
  // 1      0       Non-inverted mode
  // 1      1       Inverted mode
  //
  // Prescalar is a 3 bit value stored in last three LSB of the register
  // as CSn0, CSn1, CSn2. Timer 0/1 and 2 have different value mappings for these
  // three bits:
  // Timer 0 - 001:1, 010:8, 011:64, 100:256, 101:1024
  // Timer 1 - 001:1, 010:8, 011:64, 100:256, 101:1024
  // Timer 2 - 001:1, 010:8, 011:32, 100:64, 101:128, 110:256, 111:1024
  //
  // Note: _BV() is a macro function defined in avr-libc which performs
  // _BV(x) (1 << x). 
  //
  // To achieve 20kHz, we solve for a reasonable prescalar that will give
  // us a high enough count to fit the level of precision desired, e.g. if
  // we wanted a 100 point control scale:
  //    16000000 / n / 100 = 20000
  //    n = 16000000 / 100 / 20000
  //    n = 8
  // Prescalar 8 is the same for all Timers: 010 or CSn1
  TCCR0A = _BV(COM0A1) | _BV(COM0B1)  | _BV(WGM01) | _BV(WGM00);
  TCCR0B = _BV(WGM02) | _BV(CS01);
  TCCR1A = _BV(COM1A1) | _BV(COM1B1) | _BV(WGM11) | _BV(WGM10);
  TCCR1B = _BV(WGM13) | _BV(WGM12) | _BV(CS11);
  TCCR2A = _BV(COM2A1) | _BV(COM2B1) | _BV(WGM21) | _BV(WGM20);
  TCCR2B = _BV(WGM22) | _BV(CS21);

  // Use OCRnA to set TOP limit for all timers (99 has a count of 100)
  OCR0A = 99;
  OCR1A = 99;
  OCR2A = 99;

  // Set starting compare registers to ~5% duty cycle (you may use OCRnB directly, also). 
  analogWrite(3, 5);
  analogWrite(5, 5);
  analogWrite(10, 5);
  
  // References:
  // * pg. 94, 95, 99, 100, 104
  //   http://ww1.microchip.com/downloads/en/DeviceDoc/Atmel-8271-8-bit-AVR-Microcontroller-ATmega48A-48PA-88A-88PA-168A-168PA-328-328P_datasheet_Complete.pdf
  // * http://www.righto.com/2009/07/secrets-of-arduino-pwm.html
  // * https://sites.google.com/site/qeewiki/books/avr-guide/pwm-on-the-atmega328
}

void loop() {
  // Control goes here
}