From 17fdc7ca1415442922cfef5fa2051d1877eb50f1 Mon Sep 17 00:00:00 2001
From: Laurentiu Badea <laurb9@gmail.com>
Date: Sun, 1 Mar 2015 23:16:50 -0800
Subject: [PATCH 1/2] Teensy: AREF calibration

---
 adc_avr.cpp     |  2 +-
 adc_teensy3.cpp | 24 ++++++++++++++++++++++++
 adc_teensy3.h   |  1 +
 3 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/adc_avr.cpp b/adc_avr.cpp
index c5d896f..592e15a 100644
--- a/adc_avr.cpp
+++ b/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
diff --git a/adc_teensy3.cpp b/adc_teensy3.cpp
index e5bbc5c..1f7dd36 100644
--- a/adc_teensy3.cpp
+++ b/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.
  */
diff --git a/adc_teensy3.h b/adc_teensy3.h
index 3f691f7..1debc72 100644
--- a/adc_teensy3.h
+++ b/adc_teensy3.h
@@ -35,6 +35,7 @@ class ADCInput : public ADCBase {
             *buffer++ = analogRead(input);
         }
     }
+    uint16_t calibrateAREF();
 };
 
 #endif /* ADC_TEENSY3_ */

From c862224632e73928828927b69ab88423424b7931 Mon Sep 17 00:00:00 2001
From: Laurentiu Badea <laurb9@gmail.com>
Date: Sun, 1 Mar 2015 23:18:44 -0800
Subject: [PATCH 2/2] Add AREF calibration feature

---
 README.md | 1 +
 1 file changed, 1 insertion(+)

diff --git a/README.md b/README.md
index 46e8933..328c04b 100644
--- a/README.md
+++ b/README.md
@@ -5,6 +5,7 @@ Features
 ========
 - Configurable ADC speed for sampling rates from 10KHz to 300KHz (on UNO)
 - Auto grid on time base from 2ms to 50us (on UNO)
+- Vmax calibration and 3.3-5V detection using internal reference
 - 0V auto trigger
 - Display minimum-maximum voltage of captured signal
 - Square waveforms are displayed properly