Saturday, July 2, 2016

Advanced PWM for Arduino Zero or any Atmel SAMD21 Based Arduino Board


The Arduino PWM library leaves a lot to be desired since it really only scratches the surface on PWM capabilities built into today's MCUs. In this video we look at how to unlock some of the more advanced PWM features on any Arduino board based on the Atmel SAMD21 32 bit ARM MCU.



Click here to access the Atmel programming API from the video

Arduino code from video***********************************************************
//This was used for ForceTronics YouTube tutorial on generating PWM signals with SAMD21 based Arduinos
//This code is public domain and can be used by anyone at their own risk
//Some of this code was leveraged from MartinL on Arduino Forum http://forum.arduino.cc/index.php?topic=346731.5;wap2

//sets the period of the PWM signal, PWM period = wPer / gen clock rate 
volatile unsigned char wPer = 255;
//This variable is to generate the duty cycle of the PWM signal 0.5 --> 50%
volatile float pWMDC = .5;
//selects the gen clock for setting the waveform generator clock or sample rate
const unsigned char gClock = 4;
//sets the divide factor for the gen clk, 48MHz / 3 = 16MHz
const unsigned char dFactor = 3;

void setup() 

  pinMode(3, OUTPUT);
  analogReadResolution(8); //set the ADC resolution to match the PWM max resolution (0 to 255)
  
  REG_GCLK_GENDIV = GCLK_GENDIV_DIV(dFactor) |          // Divide the main clock down by some factor to get generic clock
                    GCLK_GENDIV_ID(gClock);            // Select Generic Clock (GCLK) 4
  while (GCLK->STATUS.bit.SYNCBUSY);              // Wait for synchronization

  REG_GCLK_GENCTRL = GCLK_GENCTRL_IDC |           
                     GCLK_GENCTRL_GENEN |         // Enable GCLK4
                     GCLK_GENCTRL_SRC_DFLL48M |   // Set the 48MHz clock source
                     GCLK_GENCTRL_ID(gClock);          // Select GCLK4
  while (GCLK->STATUS.bit.SYNCBUSY);              // Wait for synchronization

  // Enable the port multiplexer for the digital pin. Note commented out line is pin D7, other is D3
 // PORT->Group[g_APinDescription[7].ulPort].PINCFG[g_APinDescription[7].ulPin].bit.PMUXEN = 1;
  PORT->Group[g_APinDescription[3].ulPort].PINCFG[g_APinDescription[3].ulPin].bit.PMUXEN = 1;
  
   //Connect the TCC0 timer to digital output - port pins are paired odd PMUO and even PMUXE (note D7 is commented out and D3 is not)
  // PORT->Group[g_APinDescription[2].ulPort].PMUX[g_APinDescription[2].ulPin >> 1].reg = PORT_PMUX_PMUXO_F | PORT_PMUX_PMUXE_F; 
  PORT->Group[g_APinDescription[4].ulPort].PMUX[g_APinDescription[4].ulPin >> 1].reg = PORT_PMUX_PMUXO_F | PORT_PMUX_PMUXE_F;

  // Feed GCLK4 to TCC0 and TCC1
  REG_GCLK_CLKCTRL = GCLK_CLKCTRL_CLKEN |         // Enable GCLK4 to TCC0 and TCC1
                     GCLK_CLKCTRL_GEN_GCLK4 |     // Select GCLK4
                     GCLK_CLKCTRL_ID_TCC0_TCC1;   // Feed GCLK4 to TCC0 and TCC1
  while (GCLK->STATUS.bit.SYNCBUSY);              // Wait for synchronization

  //Set for Single slope PWM operation: timers or counters count up to TOP value and then repeat
  REG_TCC1_WAVE |= TCC_WAVE_WAVEGEN_NPWM;       // Reverse the output polarity on all TCC0 outputs
                   //TCC_WAVE_POL(0xF)      //this line inverts the output waveform
                   //TCC_WAVE_WAVEGEN_DSBOTH;    // Setup dual slope PWM on TCC0
  while (TCC1->SYNCBUSY.bit.WAVE);               // Wait for synchronization

  // Each timer counts up to a maximum or TOP value set by the PER register,
  // this determines the frequency of the PWM operation: 
  REG_TCC1_PER = wPer;         // This sets the rate or frequency of PWM signal. 
  while (TCC1->SYNCBUSY.bit.PER);                // Wait for synchronization
  
  // Set the PWM signal to output 50% duty cycle initially (0.5 x 255)
  REG_TCC1_CC1 = pWMDC*wPer;        
  while (TCC1->SYNCBUSY.bit.CC1);                // Wait for synchronization

  //enable interrupts
  REG_TCC1_INTENSET = TCC_INTENSET_OVF; //Set up interrupt at TOP of each PWM cycle
  enable_interrupts(); //enable in NVIC
  
  // Set prescaler and enable the outputs
  REG_TCC1_CTRLA |= TCC_CTRLA_PRESCALER_DIV1 |    // Divide GCLK4 by 1
                    TCC_CTRLA_ENABLE;             // Enable the TCC0 output
  while (TCC1->SYNCBUSY.bit.ENABLE);              // Wait for synchronization
}

void loop() { 

  //Put main code here
 }

//This function sets the interrupts priority to highest and then enables the PWM interrupt
void enable_interrupts() {
  NVIC_SetPriority(TCC1_IRQn, 0);    // Set the Nested Vector Interrupt Controller (NVIC) priority
  NVIC_EnableIRQ(TCC1_IRQn);
}

//This ISR is called at the end or TOP of each PWM cycle
void TCC1_Handler() {
    REG_TCC1_PER = analogRead(A1); //Get period from A1
    while (TCC1->SYNCBUSY.bit.PER);
    REG_TCC1_CC1 = (analogRead(A0)/255.0)*analogRead(A1); //calculate PWM using A0 reading and A1 current state
    while (TCC1->SYNCBUSY.bit.CC1);
    REG_TCC0_INTFLAG = TC_INTFLAG_OVF; //Need to reset interrupt
}