Utilizing Advanced ADC Capabilities on Arduino’s with the SAMD21 (Zero, MKR1000, etc) Part 1

We are all familiar with the Arduino "analogRead()" function, but there is a lot more to the SAMD21 ADC then just taking simple readings. In this video series we look at some of the more advanced ADC capabilities of the SAMD21 and how to use them. In part 1 we look at how to use the window monitoring capability of the ADC.

//*******************Arduino code from the video*********************
//This sketch is from a tutorial on the ForceTronics YouTube Channel called 
//Utilizing Advanced ADC Capabilities on Arduino’s with the SAMD21 (Zero, MKR1000, etc)
//This code is public domain and free to anyone to use or modify at your own risk

//declare const for window mode settings
const byte DISABLE = 0;
const byte MODE1 = 1;
const byte MODE2 = 2;
const byte MODE3 = 3;
const byte MODE4 = 4;

void setup() {
  //call this function to start the ADC in window and define the window parameters
  ADCWindowBegin(MODE1, 512, 750); //Do not use the Arduino analog functions until you call ADCWindowEnd()
  Serial.begin(57600);
}

void loop() {
  delay(1500);
  Serial.println(readADC()); //the "readADC()" function can be used to get ADC readings while in Window mode
  Serial.println();
}

//This is the interrupt service routine (ISR) that is called 
//if an ADC measurement falls out of the range of the window 
void ADC_Handler() {
    digitalWrite(LED_BUILTIN, HIGH); //turn LED off
    ADC->INTFLAG.reg = ADC_INTFLAG_WINMON; //Need to reset interrupt
}

//this function sets up the ADC window mode with interrupt
void ADCWindowBegin(byte mode, int upper, int lower) {
  setMeasPin(); //function sets up ADC pin A0 as input
  setGenClock(); //setup ADC clock, using internal 8MHz clock
  setUPADC(); //configure ADC
  setADCWindow(mode, upper, lower); //setup ADC window mode 
  setUpInterrupt(0); //setup window mode interrupt with highest priority
  enableADC(1); //enable ADC 
}

void ADCWindowEnd() {
  NVIC_DisableIRQ(ADC_IRQn); //turn off interrupt
  enableADC(0); //disable ADC 
}

//setup measurement pin, using Arduino ADC pin A3
void setMeasPin() {
  // Input pin for ADC Arduino A3/PA04
  REG_PORT_DIRCLR1 = PORT_PA04;

  // Enable multiplexing on PA04
  PORT->Group[0].PINCFG[4].bit.PMUXEN = 1;
  PORT->Group[0].PMUX[1].reg = PORT_PMUX_PMUXE_B | PORT_PMUX_PMUXO_B;
}

//Function sets up generic clock for ADC
//Uses built-in 8MHz clock
void setGenClock() {
   // Enable the APBC clock for the ADC
  REG_PM_APBCMASK |= PM_APBCMASK_ADC;

  configOSC8M(); //this function sets up the internal 8MHz clock that we will use for the ADC
  
  // Setup clock GCLK3 for no div factor
   GCLK->GENDIV.reg |= GCLK_GENDIV_ID(3)| GCLK_GENDIV_DIV(1);
   while (GCLK->STATUS.reg & GCLK_STATUS_SYNCBUSY);  

  //configure the generator of the generic clock, which is 8MHz clock
  GCLK->GENCTRL.reg |= GCLK_GENCTRL_GENEN | GCLK_GENCTRL_SRC_OSC8M | GCLK_GENCTRL_ID(3) | GCLK_GENCTRL_DIVSEL;
  while (GCLK->STATUS.reg & GCLK_STATUS_SYNCBUSY);
  
  //enable clock, set gen clock number, and ID to where the clock goes (30 is ADC)
  GCLK->CLKCTRL.reg |= GCLK_CLKCTRL_CLKEN | GCLK_CLKCTRL_GEN(3) | GCLK_CLKCTRL_ID(30);
  while (GCLK->STATUS.bit.SYNCBUSY);
}

//Function that does general settings for ADC
//sets it for a single sample
//Uses internal voltage reference
//sets gain factor to 1/2
void setUPADC() {
  // Select reference, internal VCC/2
  ADC->REFCTRL.reg |= ADC_REFCTRL_REFSEL_INTVCC1; // VDDANA/2, combine with gain DIV2 for full VCC range

  // Average control 1 sample, no right-shift
  ADC->AVGCTRL.reg |= ADC_AVGCTRL_ADJRES(0) | ADC_AVGCTRL_SAMPLENUM_1;

  // Sampling time, no extra sampling half clock-cycles
  REG_ADC_SAMPCTRL |= ADC_SAMPCTRL_SAMPLEN(0);

  // Input control: set gain to div by two so ADC has measurement range of VCC, no diff measurement so set neg to gnd, pos input set to pin 0 or A0
  ADC->INPUTCTRL.reg |= ADC_INPUTCTRL_GAIN_DIV2 | ADC_INPUTCTRL_MUXNEG_GND | ADC_INPUTCTRL_MUXPOS_PIN4;
  while (REG_ADC_STATUS & ADC_STATUS_SYNCBUSY);

  // PS16, 8 MHz, ADC_CLK = 500 kHz, ADC sampling rate, single eded, 12 bit, free running, DIV2 gain, 7 ADC_CLKs, 14 usec
  ADC->CTRLB.reg |= ADC_CTRLB_PRESCALER_DIV16 | ADC_CTRLB_RESSEL_10BIT | ADC_CTRLB_FREERUN; // Run ADC continously, 7 ADC_CLKs, 14 usec
  while (REG_ADC_STATUS & ADC_STATUS_SYNCBUSY);
}

//This function is used to setup the ADC windowing mode
//inputs are the mode, upper window value, and lower window value
//
void setADCWindow(byte mode, int upper, int lower) {
  ADC->WINCTRL.reg = mode; //set window mode
  while (ADC->STATUS.bit.SYNCBUSY);

   ADC->WINUT.reg = upper; //set upper threshold
   while (ADC->STATUS.bit.SYNCBUSY);

   ADC->WINLT.reg = lower; //set lower threshold
   while (ADC->STATUS.bit.SYNCBUSY);
}

//This function sets up an ADC interrupt that is triggered 
//when an ADC value is out of range of the window
//input argument is priority of interrupt (0 is highest priority)
void setUpInterrupt(byte priority) {
  
  ADC->INTENSET.reg |= ADC_INTENSET_WINMON; // enable ADC window monitor interrupt
   while (ADC->STATUS.bit.SYNCBUSY);

   NVIC_EnableIRQ(ADC_IRQn); // enable ADC interrupts
   NVIC_SetPriority(ADC_IRQn, priority); //set priority of the interrupt
}

//function allows you to enable or disable ADC
void enableADC(bool en) {
  if(en) ADC->CTRLA.reg = 2; //2 is binary 010 which is register bit to enable ADC
  else ADC->CTRLA.reg = 0; //0 disables ADC
}

//This function will return the latest ADC reading made during free run window mode
//must first start the ADC before calling this function
unsigned int readADC() {
  // Free running, wait for conversion to complete
  while (!(REG_ADC_INTFLAG & ADC_INTFLAG_RESRDY));
  // Wait for synchronization before reading RESULT
  while (REG_ADC_STATUS & ADC_STATUS_SYNCBUSY);
  
  return REG_ADC_RESULT;
}

//function enables the 8MHz clock used for the ADC
void configOSC8M() 
{
  SYSCTRL->OSC8M.reg |= SYSCTRL_OSC8M_ENABLE;
}