Added SAMD specific library for ADC result correction.

libraries/SAMD_AnalogCorrection/examples/CorrectADCResponse/CorrectADCResponse.ino

+/*
+  This sketch easily and quickly finds the right ADC correction values for a particular Arduino ZERO board.
+  The correction values that are found are only valid for the board where the sketch is executed.
+
+  This example code is in the public domain.
+
+  Written 6 May 2015 by Claudio Indellicati
+*/
+
+/*
+  How to use this sketch
+  
+  1) Remove any connection cable, shield or jumper from your Arduino ZERO
+  2) Connect pin A1 to the nearest GND pin using the shortest jumper possible
+  3) Connect pin A2 to the 3.3V pin using the shortest jumper possible
+  4) Connect the Arduino ZERO to your PC using a USB cable plugged in the USB programming port of the board
+  5) Upload this sketch and leave the board powered on for at least one minute
+  6) Open the Serial Monitor and press the reset button on the Arduino ZERO
+  7) At the and of the procedure you can find logged
+       - the offset and gain values for the board where the sketch has been just executed
+       - the instruction line to copy/paste in the final sketch
+*/
+
+#include "SAMD_AnalogCorrection.h"
+
+#define ADC_GND_PIN          A1
+#define ADC_3V3_PIN          A2
+
+#define ADC_READS_SHIFT      8
+#define ADC_READS_COUNT      (1 << ADC_READS_SHIFT)
+
+#define ADC_MIN_GAIN         0x0400
+#define ADC_UNITY_GAIN       0x0800
+#define ADC_MAX_GAIN         (0x1000 - 1)
+#define ADC_RESOLUTION_BITS  12
+#define ADC_RANGE            (1 << ADC_RESOLUTION_BITS)
+#define ADC_TOP_VALUE        (ADC_RANGE - 1)
+
+#define MAX_TOP_VALUE_READS  10
+
+void setup()
+{
+  Serial.begin(9600);
+
+  Serial.println("\r\nCalibrating ADC with factory values");
+
+  analogReadResolution(ADC_RESOLUTION_BITS);
+
+  Serial.println("\r\nReading GND and 3.3V ADC levels");
+  Serial.print("   ");
+  readGndLevel();
+  Serial.print("   ");
+  read3V3Level();
+
+  int offsetCorrectionValue = 0;
+  uint16_t gainCorrectionValue = ADC_UNITY_GAIN;
+  
+  Serial.print("\r\nOffset correction (@gain = ");
+  Serial.print(gainCorrectionValue);
+  Serial.println(" (unity gain))");
+
+  // Set default correction values and enable correction
+  analogReadCorrection(offsetCorrectionValue, gainCorrectionValue);
+
+  for (int offset = 0; offset < (int)(ADC_OFFSETCORR_MASK >> 1); ++offset)
+  {
+    analogReadCorrection(offset, gainCorrectionValue);
+    
+    Serial.print("   Offset = ");
+    Serial.print(offset);
+    Serial.print(", ");
+
+    if (readGndLevel() == 0)
+    {
+      offsetCorrectionValue = offset;
+      break;
+    }
+  }
+
+  Serial.println("\r\nGain correction");
+
+  uint8_t topValueReadsCount = 0U;
+  
+  uint16_t minGain = 0U,
+           maxGain = 0U;
+
+  analogReadCorrection(offsetCorrectionValue, gainCorrectionValue);
+  Serial.print("   Gain = ");
+  Serial.print(gainCorrectionValue);
+  Serial.print(", ");
+  uint16_t highLevelRead = read3V3Level();
+  
+  if (highLevelRead < ADC_TOP_VALUE)
+  {
+    for (uint16_t gain = ADC_UNITY_GAIN + 1; gain <= ADC_MAX_GAIN; ++gain)
+    {
+      analogReadCorrection(offsetCorrectionValue, gain);
+
+      Serial.print("   Gain = ");
+      Serial.print(gain);
+      Serial.print(", ");
+      highLevelRead = read3V3Level();
+      
+      if (highLevelRead == ADC_TOP_VALUE)
+      {
+        if (minGain == 0U)
+          minGain = gain;
+
+        if (++topValueReadsCount >= MAX_TOP_VALUE_READS)
+        {
+          maxGain = minGain;
+          break;
+        }
+        
+        maxGain = gain;
+      }
+
+      if (highLevelRead > ADC_TOP_VALUE)
+        break;
+    }
+  }
+  else if (highLevelRead >= ADC_TOP_VALUE)
+  {
+    if (highLevelRead == ADC_TOP_VALUE)
+      maxGain = ADC_UNITY_GAIN;
+
+    for (uint16_t gain = ADC_UNITY_GAIN - 1; gain >= ADC_MIN_GAIN; --gain)
+    {
+      analogReadCorrection(offsetCorrectionValue, gain);
+
+      Serial.print("   Gain = ");
+      Serial.print(gain);
+      Serial.print(", ");
+      highLevelRead = read3V3Level();
+      
+      if (highLevelRead == ADC_TOP_VALUE)
+      {
+        if (maxGain == 0U)
+          maxGain = gain;
+        
+        minGain = gain;
+      }
+
+      if (highLevelRead < ADC_TOP_VALUE)
+        break;
+    }
+  }
+
+  gainCorrectionValue = (minGain + maxGain) >> 1;
+
+  analogReadCorrection(offsetCorrectionValue, gainCorrectionValue);
+
+  Serial.println("\r\nReadings after corrections");
+  Serial.print("   ");
+  readGndLevel();
+  Serial.print("   ");
+  read3V3Level();
+
+  Serial.println("\r\n==================");
+  Serial.println("\r\nCorrection values:");
+  Serial.print("   Offset = ");
+  Serial.println(offsetCorrectionValue);
+  Serial.print("   Gain = ");
+  Serial.println(gainCorrectionValue);
+  Serial.println("\r\nAdd the next line to your sketch:");
+  Serial.print("   analogReadCorrection(");
+  Serial.print(offsetCorrectionValue);
+  Serial.print(", ");
+  Serial.print(gainCorrectionValue);
+  Serial.println(");");
+  Serial.println("\r\n==================");
+}
+
+void loop()
+{
+}
+
+uint16_t readGndLevel()
+{
+  uint32_t readAccumulator = 0;
+
+  for (int i = 0; i < ADC_READS_COUNT; ++i)
+    readAccumulator += analogRead(ADC_GND_PIN);
+
+  uint16_t readValue = readAccumulator >> ADC_READS_SHIFT;
+  
+  Serial.print("ADC(GND) = ");
+  Serial.println(readValue);
+
+  return readValue;
+}
+
+uint16_t read3V3Level()
+{
+  uint32_t readAccumulator = 0;
+
+  for (int i = 0; i < ADC_READS_COUNT; ++i)
+    readAccumulator += analogRead(ADC_3V3_PIN);
+
+  uint16_t readValue = readAccumulator >> ADC_READS_SHIFT;
+  
+  if (readValue < (ADC_RANGE >> 1))
+    readValue += ADC_RANGE;
+
+  Serial.print("ADC(3.3V) = ");
+  Serial.println(readValue);
+
+  return readValue;
+}
+

libraries/SAMD_AnalogCorrection/library.properties

+name=SAMD_AnalogCorrection
+version=1.0
+author=Arduino
+maintainer=Arduino <info@arduino.cc>
+sentence=Sets and enables the digital correction logic of the SAMD ADC.
+paragraph=Useful to correct the values returned by the ADC.
+url=
+architectures=samd

libraries/SAMD_AnalogCorrection/src/SAMD_AnalogCorrection.cpp

+/*
+  Copyright (c) 2015 Arduino LLC.  All right reserved.
+
+  This library is free software; you can redistribute it and/or
+  modify it under the terms of the GNU Lesser General Public
+  License as published by the Free Software Foundation; either
+  version 2.1 of the License, or (at your option) any later version.
+
+  This library is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+  See the GNU Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public
+  License along with this library; if not, write to the Free Software
+  Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+*/
+
+#include "SAMD_AnalogCorrection.h"
+
+void analogReadCorrection (int offset, uint16_t gain)
+{
+  // Set correction values
+  ADC->OFFSETCORR.reg = ADC_OFFSETCORR_OFFSETCORR(offset);
+  ADC->GAINCORR.reg = ADC_GAINCORR_GAINCORR(gain);
+
+  // Enable digital correction logic
+  ADC->CTRLB.bit.CORREN = 1;
+  while(ADC->STATUS.bit.SYNCBUSY);
+}
+

libraries/SAMD_AnalogCorrection/src/SAMD_AnalogCorrection.h

+/*
+  Copyright (c) 2015 Arduino LLC.  All right reserved.
+
+  This library is free software; you can redistribute it and/or
+  modify it under the terms of the GNU Lesser General Public
+  License as published by the Free Software Foundation; either
+  version 2.1 of the License, or (at your option) any later version.
+
+  This library is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+  See the GNU Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public
+  License along with this library; if not, write to the Free Software
+  Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+*/
+
+#pragma once
+
+#include <Arduino.h>
+
+void analogReadCorrection (int offset, uint16_t gain);
+