platform independent VCC measurement

.gitignore

@@ -0,0 +1 @@
+.DS_Store

opticalDensity.ino

@@ -71,7 +71,7 @@ float readSensor(int sensorPin)
 
 // taken from
 // http://hacking.majenko.co.uk/making-accurate-adc-readings-on-arduino
-long readVcc() {
+long readVcc_uno() {
   long result;
   // Read 1.1V reference against AVcc
   ADMUX = _BV(REFS0) | _BV(MUX3) | _BV(MUX2) | _BV(MUX1);
@@ -85,11 +85,32 @@ long readVcc() {
 }
 
 
+// taken from https://code.google.com/p/tinkerit/wiki/SecretVoltmeter
+//
+//const long scaleConst = 1156.300 * 1000 ; // internalRef * 1023 * 1000;
+const long scaleConst = 1125300L;
+long readVcc() {
+  // Read 1.1V reference against AVcc
+  // set the reference to Vcc and the measurement to the internal 1.1V reference
+#if defined(__AVR_ATmega32U4__) || defined(__AVR_ATmega1280__) || defined(__AVR_ATmega2560__)
+  ADMUX = _BV(REFS0) | _BV(MUX4) | _BV(MUX3) | _BV(MUX2) | _BV(MUX1);
+#elif defined (__AVR_ATtiny24__) || defined(__AVR_ATtiny44__) || defined(__AVR_ATtiny84__)
+  ADMUX = _BV(MUX5) | _BV(MUX0);
+#elif defined (__AVR_ATtiny25__) || defined(__AVR_ATtiny45__) || defined(__AVR_ATtiny85__)
+  ADMUX = _BV(MUX3) | _BV(MUX2);
+#else
+  ADMUX = _BV(REFS0) | _BV(MUX3) | _BV(MUX2) | _BV(MUX1);
+#endif 
 
+  delay(2); // Wait for Vref to settle
+  ADCSRA |= _BV(ADSC); // Start conversion
+  while (bit_is_set(ADCSRA,ADSC)); // measuring
 
+  uint8_t low  = ADCL; // must read ADCL first - it then locks ADCH 
+  uint8_t high = ADCH; // unlocks both
 
+  long result = (high<<8) | low;
 
-
-
-
-
+  result = scaleConst / result; // Calculate Vcc (in mV); 1125300 = 1.1*1023*1000
+  return (long)result; // Vcc in millivolts
+}