How do I configure an interrupt for the hardware PWM on the Arduino Due?

Question

I'm working on a project on the Arduino Due to make a digital synthesizer. I will use PWM to generate an output wave at the desired notefrequency, and sample the wave at 192kHz.

I am able to get PWM out at the desired frequency and adjust the pulse width (tested with oscilloscope), but I need to configure the interrupt so that I can calculate the next needed PWM value for different types of waves at different audible frequencies (sin, sawtooth, etc.). Each time the interrupt triggers (at 192kHz), a function will calculate the next pulse width value based on the note value (audible frequency).

Here is my code so far. I am able to get the PWM to work, but my dummy code in the interrupt (which turns a pin on and off) does not toggle the pin. I have tested this with an oscilloscope. I know that the problem with the code is the interrupt not triggering/executing properly, because the output pin is set high in the setup code, and that also happens on the oscilloscope. However, the interrupt test pin does not toggle. What am I missing?

uint32_t pwmPin = 8; // PWM output pin
uint32_t channel = g_APinDescription[pwmPin].ulPWMChannel;
uint32_t sampFreq = 192000ul; // sample at 192kHz
uint32_t clkAFreq = 42000000ul;
uint32_t pwmFreq = (clkAFreq * 2)/sampFreq;
uint16_t dutyPercent = 50;
uint16_t dutyAct = pwmFreq * (100-dutyPercent) / 100;

void setup() {
  // put your setup code here, to run once:


 pinMode(5, OUTPUT); // interrupt test pin

  pmc_enable_periph_clk(PWM_INTERFACE_ID);
  PWMC_ConfigureClocks(clkAFreq, 0, VARIANT_MCK);

  PIO_Configure(
     g_APinDescription[pwmPin].pPort,
     g_APinDescription[pwmPin].ulPinType,
     g_APinDescription[pwmPin].ulPin,
     g_APinDescription[pwmPin].ulPinConfiguration);

  //uint32_t
  channel = g_APinDescription[pwmPin].ulPWMChannel;
  PWMC_ConfigureChannel(PWM_INTERFACE, channel, clkAFreq, 0, 0);
  PWMC_SetPeriod(PWM_INTERFACE, channel, pwmFreq);
  PWMC_EnableChannel(PWM_INTERFACE, channel);
  PWMC_SetDutyCycle(PWM_INTERFACE, channel, dutyAct);
  PWMC_EnableChannelIt(PWM_INTERFACE, channel);
  dutyPercent = 50; //square wave

}

void loop() {
  // put your main code here, to run repeatedly:

}


void PWM_Handler()          // this is what I looked up in startup_sam4s.c
{
  int i;                    // 
  digitalWrite(5, HIGH);    // toggle pin
  i++;                      // delay a little
  digitalWrite(5, LOW);     // toggle pin
}

Can you clarify "sample the wave at 192kHz"? Is an external device is sampling the Arduino output? Or the Arduino is sampling an external input? — mhopeng
I normally think of PWM signals as square waves with a modulated duty cycle. What do you expect the "PWM value for different types of waves (sin, sawtooth, etc.)" to look like? — mhopeng
No value for i is defined in the PWM_Handler(), so the time that pin 5 is high is very short. Try using delay(500) instead. — mhopeng
The PWM outputs at a constant 192kHz. So the frequency of the PWM output always rises at 192kHz. The program then calculates the next pulse width for the desired waveform and frequency. — snazziosity

snazziosity snazziosity · Accepted Answer · 2016-04-18T22:06:46

For anyone who stumbles across this, here is how I got the interrupt to work:

uint32_t totalTime = 0; // total elapsed time

uint32_t pwmPin = 8; // PWM output pin
uint32_t irqPin = 5; // interrupt test pin
uint32_t channel = g_APinDescription[pwmPin].ulPWMChannel; // set channel for PWM out
uint32_t sampFreq = 192000ul; // sampling frequency (Hz)
uint16_t maxDutyCount = 255;
uint32_t clkAFreq = 42000000ul; // clock frequency (Hz)
uint32_t pwmFreq = (clkAFreq * 2)/sampFreq; // calculate PWM frequency
uint16_t dutyPercent = 128; // starting duty percent
uint16_t dutyAct = pwmFreq * (maxDutyCount-dutyPercent) / maxDutyCount; // 

void setup() {
  // put your setup code here, to run once:
  pinMode(irqPin, OUTPUT);

  pmc_enable_periph_clk(PWM_INTERFACE_ID);
  PWMC_ConfigureClocks(clkAFreq, 0, VARIANT_MCK);

  PIO_Configure(
   g_APinDescription[pwmPin].pPort,
   g_APinDescription[pwmPin].ulPinType,
   g_APinDescription[pwmPin].ulPin,
   g_APinDescription[pwmPin].ulPinConfiguration);

   channel = g_APinDescription[pwmPin].ulPWMChannel; // channel 5
   PWMC_ConfigureChannel(PWM_INTERFACE, channel, clkAFreq, 0, 0);
   PWMC_SetPeriod(PWM_INTERFACE, channel, pwmFreq);
   PWMC_EnableChannel(PWM_INTERFACE, channel);
   PWMC_SetDutyCycle(PWM_INTERFACE, channel, dutyAct);
   PWM_INTERFACE->PWM_IER1 = 0x20; //enable interrupt on channel 5
   PWM_INTERFACE->PWM_IDR1 = 0xFFFFFFDF; //enable interrupt on channel 5
   PWM_INTERFACE->PWM_IER2 = 0x00002001; //enable interrupt on channel 5
   PWM_INTERFACE->PWM_IDR2 = 0xFFFFDFFE; //enable interrupt on channel 5

   NVIC_DisableIRQ(PWM_IRQn); // set up interrupt
   NVIC_ClearPendingIRQ(PWM_IRQn);
   NVIC_SetPriority(PWM_IRQn, 0);
   NVIC_EnableIRQ((IRQn_Type)36); //NVIC_EnableIRQ(PWM_IRQn);
   PWMC_EnableChannel(PWM_INTERFACE, channel);
   //Enable of the Interrupts (writing CHIDx and FCHIDx
   //in PWM_IER1 register, and writing WRDYE, ENDTXE,
   //TXBUFE, UNRE, CMPMx and CMPUx in PWM_IER2 register)
}

void loop() {
}


void PWM_Handler(void) // PWM interrupt handler
{
   volatile long dummy = PWM_INTERFACE->PWM_ISR1; // clear interrupt flag
   dummy = PWM_INTERFACE->PWM_ISR2; // clear interrupt flag
//your code here!
}

How do I configure an interrupt for the hardware PWM on the Arduino Due?

1 Answers