Things Plus SAMD51, need analogReference() functionality

Development Boards
1

I want to give an external analog reference for thermistor wiring.

In Arduino Uno, there is a dedicated pin “AREF” for this purpose but how can I achive this functionality in Things Plus SAMD51?

I have below queries:

  • Which pin on Things Plus SAMD51 should I use for external analog reference.
  • Inside **void setup()**, I am using a function **analogReference(AR_DEFAULT)** , will this configure the designated analog reference pin to 3.3 volt

    • Code given below gets compiled successfully

    #include <math.h>

#define therm_pin A0
float T_approx;

float V_0 = 3.3; // voltage reference

// first resistance value for voltage divider
float R_1 = 220000.0;
// fit coefficients
float a = 283786.2;
float b = 0.06593;
float c = 49886.0;

int avg_size = 10; // averaging size

void setup() {
  // initialize serial communication at 9600 bits per second:
  Serial.begin(9600);
  pinMode(therm_pin,INPUT);

  // set analog reference to read AREF pin
  analogReference(AR_DEFAULT);
}

void loop() {
  
  // loop over several values to lower noise
  float T_sum = 0.0;
  for (int ii;ii<avg_size;ii++){
    // read the input on analog pin 0:
    int sensorValue = analogRead(therm_pin);
    // Convert the analog reading (which goes from 0 - 1023) to voltage reference (3.3V or 5V or other):
    float voltage = (sensorValue/1023.0)*V_0;

    // this is where the thermistor conversion happens based on parameters from fit
    T_sum+=(-1.0/b)*(log(((R_1*voltage)/(a*(V_0-voltage)))-(c/a)));
  }
  
  // averaging values from loop
  T_approx = T_sum/float(avg_size);

  // readout for Celsius and Fahrenheit
  Serial.print("Temperature: ");
  Serial.print(T_approx);
  Serial.print(" (");
  Serial.print((T_approx*(9.0/5.0))+32.0);
  Serial.println(" F)");
  
  delay(500);
}
    2

    It looks like there’s one @ PA03 for VREFA tied to VDDA, and another VREFB @ A3

    https://cdn.sparkfun.com/assets/8/2/1/d … us_v10.pdf

    (and a third if you want more!)

    3

    Thanks for a quick reply! :slight_smile:

    I do not know, how to call any of these specific pins in Arduino IDE?

    Will using “analogReference(AR_DEFAULT)” as below, enable VREFA?

    void setup() {
  // initialize serial communication at 9600 bits per second:
  Serial.begin(9600);
  pinMode(therm_pin,INPUT);

  // set analog reference to read AREF pin
  analogReference(AR_DEFAULT);
}

    I also do not see PA03 available here https://cdn.sparkfun.com/assets/c/c/4/3 … v01-01.pdf

    These pins are multiplexed and for a TQFP/QFN 64 package, “Table 6-1.” at page 32 in https://cdn.sparkfun.com/assets/0/0/5/9/2/60001507C.pdf provides some information but to be honest, I do not know how to achieve this in Arduino IDE.

    Could you please help here little bit please!