Eliminate readVcc(), include that functionality in begin().

Add end() function to stop timer-driven ADC readings (call begin() again to restart).
Change example sketches accordingly.
Add derived class to gsCurrentSensor sketch to make LCD optional.

examples/CT2_LCD/CT2_LCD.ino

@@ -32,9 +32,7 @@ void setup()
     lcd.begin(16, 2);
     lcd.clear();
     delay(1000);
-    float vcc = readVcc() / 1000.0;
-    Serial << millis() << F(" Vcc ") << vcc << endl;
-    ct.begin(vcc);
+    ct.begin();
 }
 
 void loop()

examples/CT2_Serial/CT2_Serial.ino

@@ -13,7 +13,6 @@
 
 const float ctRatio(1000);                  // current transformer winding ratio
 const float rBurden(200);                   // current transformer burden resistor value
-const float vcc(5.070);                     // adjust to actual value for best accuracy
 const uint32_t MS_BETWEEN_SAMPLES(5000);    // milliseconds
 const int32_t BAUD_RATE(115200);
 
@@ -25,7 +24,7 @@ void setup()
 {
     delay(1000);
     Serial.begin(BAUD_RATE);
-    ct.begin(vcc);
+    ct.begin();
 }
 
 void loop()

examples/CT_LCD/CT_LCD.ino

@@ -30,9 +30,7 @@ void setup()
     lcd.begin(16, 2);
     lcd.clear();
     delay(1000);
-    float vcc = readVcc() / 1000.0;
-    Serial << millis() << F(" Vcc ") << vcc << endl;
-    ct.begin(vcc);
+    ct.begin();
 }
 
 void loop()

examples/CT_Serial/CT_Serial.ino

@@ -13,7 +13,6 @@
 
 const float ctRatio(1000);                  // current transformer winding ratio
 const float rBurden(200);                   // current transformer burden resistor value
-const float vcc(5.070);                     // adjust to actual value for best accuracy
 const uint32_t MS_BETWEEN_SAMPLES(5000);    // milliseconds
 const int32_t BAUD_RATE(115200);
 
@@ -24,7 +23,7 @@ void setup()
 {
     delay(1000);
     Serial.begin(BAUD_RATE);
-    ct.begin(vcc);
+    ct.begin();
 }
 
 void loop()

examples/gsCurrentSensor/classes.h

@@ -1,14 +1,64 @@
 #include <CurrentTransformer.h>     // https://github.com/JChristensen/CurrentTransformer
 #include <LiquidTWI.h>              // https://forums.adafruit.com/viewtopic.php?t=21586
+#include <Wire.h>
 
 CT_Sensor ct0(A0, 1000, 200);
-LiquidTWI lcd(0);   //i2c address 0 (0x20)
+
+// An I2C LCD class that doesn't hang if an LCD is not connected.
+// LCD presence is detected when begin() is called. Subsequent calls
+// to clear(), setCursor() and write() communicate to the LCD or not
+// based on the results of the detection in begin().
+
+class OptionalLCD : public LiquidTWI
+{
+    public:
+        OptionalLCD(uint8_t i2cAddr) : LiquidTWI(i2cAddr), m_i2cAddr(i2cAddr + 0x20) {}
+        void begin(uint8_t cols, uint8_t rows, uint8_t charsize = LCD_5x8DOTS);
+        void clear();
+        void setCursor(uint8_t col, uint8_t row);
+        virtual size_t write(uint8_t value);
+        bool isPresent() {return m_lcdPresent;}
+
+    private:
+        uint8_t m_i2cAddr;
+        bool m_lcdPresent;
+};
+
+void OptionalLCD::begin(uint8_t cols, uint8_t rows, uint8_t charsize)
+{
+    Wire.begin();
+    Wire.beginTransmission(m_i2cAddr);
+    uint8_t s = Wire.endTransmission();     // status is zero if successful
+    m_lcdPresent = (s == 0);
+    if (m_lcdPresent) LiquidTWI::begin(cols, rows, charsize);
+}
+
+void OptionalLCD::clear()
+{
+    if (m_lcdPresent) LiquidTWI::clear();
+}
+
+void OptionalLCD::setCursor(uint8_t col, uint8_t row)
+{
+    if (m_lcdPresent) LiquidTWI::setCursor(col, row);
+}
+
+size_t OptionalLCD::write(uint8_t value)
+{
+    if (m_lcdPresent)
+        return LiquidTWI::write(value);
+    else
+        return 0;
+}
+
+OptionalLCD lcd(0);   //i2c address 0 (0x20)
 
 class CurrentSensor : public CT_Control
 {
     public:
         CurrentSensor(uint32_t threshold, int8_t led = -1);
         void begin();
+        void restart();
         float sample();
         void clearSampleData();
 
@@ -29,18 +79,26 @@ CurrentSensor::CurrentSensor(uint32_t threshold, int8_t led) : maThreshold(thres
     clearSampleData();
 }
 
+// configure led and lcd hardware, initialize CT_Control
 void CurrentSensor::begin()
 {
     if (m_led >= 0) pinMode(m_led, OUTPUT);
-    float vcc = readVcc();
-    CT_Control::begin(vcc);
     lcd.begin(16, 2);
+    if (lcd.isPresent())
+        Serial << millis() << F(" LCD detected\n");
+    else
+        Serial << millis() << F(" LCD not present\n");
     lcd.clear();
-    lcd.setCursor(0, 0);
-    lcd << F("Vcc = ") << _FLOAT(vcc, 3);
+    restart();
+}
+
+// initialize CT_Control, display vcc value
+void CurrentSensor::restart()
+{
+    float vcc = CT_Control::begin();
+    lcd.setCursor(0, 1);
+    lcd << F("VCC  " ) << _FLOAT(vcc, 3) << F(" V ");
     Serial << millis() << F(" Vcc = ") << _FLOAT(vcc, 3) << endl;
-    delay(1000);
-    lcd.clear();
 }
 
 // read the ct and collect sample data, display on lcd
@@ -63,7 +121,7 @@ float CurrentSensor::sample()
         if (m_led >= 0) digitalWrite(m_led, LOW);
     }
     lcd.setCursor(0, 0);
-    lcd << F("CT-0 " ) << _FLOAT(a, 3) << F(" AMP ");
+    lcd << F("CT-0 " ) << _FLOAT(a, 3) << F(" A ");
     return a;
 }
 

examples/gsCurrentSensor/gsCurrentSensor.ino

@@ -17,8 +17,6 @@
 #include <XBee.h>               // https://github.com/andrewrapp/xbee-arduino
 #include "classes.h"
 
-const char *sketchVersion = "1.1.0";
-
 // pin definitions and other constants
 const uint8_t
     xbeeReset(4),
@@ -159,6 +157,8 @@ void loop()
                 nextTimePrint += 60;
                 Serial << millis() << F(" Local: ");
                 printDateTime(local);
+                cs.end();               // stop & restart the current sensor to re-read Vcc
+                cs.restart();
             }
         }
         break;

src/CurrentTransformer.cpp

@@ -25,26 +25,50 @@ CT_Control::CT_Control(ctFreq_t freq)
     m_tcOCR1 = (freq == CT_FREQ_50HZ) ? CT_Control::OCR50 : CT_Control::OCR60;
 }
 
-void CT_Control::begin(float vcc)
+// configure the timer and adc. reads and returns Vcc value in volts.
+float CT_Control::begin()
 {
-    m_vcc = vcc;
+    // read Vcc
+    ADCSRA = _BV(ADEN) | _BV(ADPS2) | _BV(ADPS1) | _BV(ADPS0);  // default adc configuration
+    ADCSRB = 0;
+    // set AVcc as reference, 1.1V bandgap reference voltage as input
+    ADMUX = _BV(REFS0) | _BV(MUX3) | _BV(MUX2) | _BV(MUX1);
+    delay(10);                              // Vref settling time
+    ADCSRA |= _BV(ADSC);                    // start conversion
+    loop_until_bit_is_clear(ADCSRA, ADSC);  // wait for it to complete
+    int mv = 1125300L / ADC;                // calculate AVcc in mV (1.1 * 1000 * 1023)
+    m_vcc = static_cast<float>(mv) / 1000.0;
 
     // set up the timer
-    TCCR1B = 0;                 // stop the timer
+    TCCR1B = 0;                             // stop the timer
     TCCR1A = 0;
-    TIFR1 = 0xFF;               // ensure all interrupt flags are cleared
-    OCR1A = m_tcOCR1;           // set timer output compare value
+    TIFR1 = 0xFF;                           // ensure all interrupt flags are cleared
+    OCR1A = m_tcOCR1;                       // set timer output compare value
     OCR1B = m_tcOCR1;
     cli();
-    TCNT1 = 0;                  // clear the timer
-    TIMSK1 = _BV(OCIE1B);       // enable timer interrupts
+    TCNT1 = 0;                              // clear the timer
+    TIMSK1 = _BV(OCIE1B);                   // enable timer interrupts
     sei();
     TCCR1B = _BV(WGM12) | _BV(CS10);    // start the timer, ctc mode, prescaler divide by 1
 
     // set up the adc
     ADCSRA  = _BV(ADEN)  | _BV(ADATE) | _BV(ADIE);      // enable ADC, auto trigger, interrupt when conversion complete
     ADCSRA |= _BV(ADPS2) | _BV(ADPS1) | _BV(ADPS0);     // ADC clock prescaler: divide by 128 (for 125kHz)
     ADCSRB = _BV(ADTS2) | _BV(ADTS0);                   // trigger ADC on Timer/Counter1 Compare Match B    
+
+    return m_vcc;
+}
+
+void CT_Control::end()
+{
+    // reset adc to default configuration (enabled, clock prescaler 128)
+    ADCSRA = _BV(ADEN) | _BV(ADPS2) | _BV(ADPS1) | _BV(ADPS0);
+    ADCSRB = 0;
+
+    // stop the timer
+    TCCR1B = 0;
+    TCCR1A = 0;
+    TIFR1 = 0xFF;               // ensure all interrupt flags are cleared
 }
 
 void CT_Control::read(CT_Sensor *ct0, CT_Sensor *ct1)
@@ -61,17 +85,13 @@ void CT_Control::read(CT_Sensor *ct0, CT_Sensor *ct1)
         ADMUX = _BV(REFS0) | ct0->m_channel;    // set channel, and AVcc as reference
         while (!adcBusy);                       // wait for next conversion to start
         while (adcBusy);                        // wait for conversion to complete
-        //cli();
         int32_t v0 = adcVal;                    // get the reading, promote to 32 bit
-        //sei();
 
         // read ct1
         ADMUX = _BV(REFS0) | ct1->m_channel;    // set channel, and AVcc as reference
         while (!adcBusy);                       // wait for next conversion to start
         while (adcBusy);                        // wait for conversion to complete
-        //cli();
         int32_t v1 = adcVal;                    // get the reading, promote to 32 bit
-        //sei();
 
         // accumulate the sum of squares,
         // subtract 512 (half the adc range) to remove dc component
@@ -87,22 +107,6 @@ void CT_Control::read(CT_Sensor *ct0, CT_Sensor *ct1)
     return;
 }
 
-// read 1.1V reference against AVcc
-// call this function only before calling begin() as the ADC is
-// then automatically triggered by the timer.
-// returns the value of Vcc in volts.
-// from http://code.google.com/p/tinkerit/wiki/SecretVoltmeter
-float CT_Control::readVcc()
-{
-    // set AVcc as reference, 1.1V bandgap reference voltage as input
-    ADMUX = _BV(REFS0) | _BV(MUX3) | _BV(MUX2) | _BV(MUX1);
-    delay(5);                               // Vref settling time
-    ADCSRA |= _BV(ADSC);                    // start conversion
-    loop_until_bit_is_clear(ADCSRA, ADSC);  // wait for it to complete
-    int mv = 1125300L / ADC;                // calculate AVcc in mV (1.1 * 1000 * 1023)
-    return static_cast<float>(mv) / 1000.0;
-}
-
 // adc conversion complete, pass the value back to the main code
 ISR(ADC_vect)
 {

src/CurrentTransformer.h

@@ -17,29 +17,30 @@ class CT_Sensor
         float amps() {return m_amps;}
 
     protected:
-        uint8_t m_channel;  // adc channel number
-        float m_ratio;      // ct turns ratio
-        float m_burden;     // ct burden resistor, ohms
-        float m_amps;       // rms amperes measured
-        
+        uint8_t m_channel;                  // adc channel number
+        float m_ratio;                      // ct turns ratio
+        float m_burden;                     // ct burden resistor, ohms
+        float m_amps;                       // rms amperes measured
+
     friend class CT_Control;
 };
 
 class CT_Control
 {
     public:
         CT_Control(ctFreq_t freq=CT_FREQ_60HZ);
-        void begin(float vcc=5.0);          // initializations
+        float begin();                      // initializations; returns Vcc
+        void end();                         // reset adc and timer to defaults
         // read the rms value of one cycle for one or two CTs
         void read(CT_Sensor *ct0, CT_Sensor *ct1);
         void read(CT_Sensor *ct0) {read(ct0, ct0);}
-        float readVcc();                    // read Vcc value in millivolts
         static volatile bool adcBusy;       // adc busy flag
         static volatile int adcVal;         // value returned from adc
         static const uint16_t sampleSize;   // number of samples to cover one cycle
         static const uint16_t ADC_MAX;      // maximum adc reading
         static const uint16_t OCR50;        // timer output compare register value for 50Hz
         static const uint16_t OCR60;        // timer output compare register value for 60Hz
+
     private:
         float m_vcc;                        // mcu supply voltage
         uint16_t m_tcOCR1;                  // compare value for timer