Teensy: AREF calibration

adc_avr.cpp

@@ -27,7 +27,7 @@
 #if defined(ADCSRA) && defined(ADCL)
 
 // Actual voltage of Vbg (internal 1.1V ref) in mV. This is different from chip to chip.
-// = 5000*1100/VREFmeasured (with accurate multimeter). 3300*1100/VREFmeasured for 3.3V systems.
+// = VrefMeasured*1100/5.000 (with accurate multimeter). VrefMeasured*1100/3.300 for 3.3V systems.
 #define INTERNAL_REF_MV 1090
 
 const uint8_t ADCInput::prescalers[] = {7,6,5,4,3,2}; // 1:8MHz clock is out of ADC spec for 16MHz AVR

adc_teensy3.cpp

@@ -11,6 +11,11 @@
 #ifdef __MK20DX256__
 #include "adc_teensy3.h"
 
+// Actual voltage of internal 1.2V ref in mV. This is different from chip to chip.
+// = VRefMeasured*1200/3.3 for 3.3V systems.
+#define INTERNAL_REF_MV (3280*1200L/3300)
+
+#define INTERNAL_REF_PORT 39
 #define ADC_CLOCK_TO_SAMPLING 15
 
 static unsigned int averagingTable[] = {32, 16,  8,  4,  0, 0};
@@ -45,6 +50,25 @@ bool ADCInput::setMode(uint8_t mode=0){
     }
 }
 
+/*
+ * Read internal reference voltage.
+ */
+uint16_t ADCInput::calibrateAREF(){
+    // reset ADC to default mode for highest precision.
+    uint8_t oldMode = curMode;
+    setMode(0);
+    analogReadRes(16);
+    analogRead(INTERNAL_REF_PORT);
+    delay(200);
+    uint16_t rangemV = INTERNAL_REF_MV * ((1L<<16)-1) / analogRead(INTERNAL_REF_PORT);
+
+    // set ADC to previous mode
+    setMode(oldMode);
+    read();
+
+    return rangemV;
+}
+
 /*
  * Return ADC clock in Hz. This is only useful to estimate sampling rate.
  */

adc_teensy3.h

@@ -35,6 +35,7 @@ class ADCInput : public ADCBase {
             *buffer++ = analogRead(input);
         }
     }
+    uint16_t calibrateAREF();
 };
 
 #endif /* ADC_TEENSY3_ */