Add readVcc() function, update README.

README.md

@@ -40,17 +40,28 @@ Operating frequency for the current transformer.
 ## Constructors
 
 ### CT_Sensor(uint8_t channel, float ratio, float burden)
-##### Description
-Defines a CT_Sensor object. One or more CT_Sensor objects can be defined as needed.
+### CT_Sensor(uint8_t channel, float amps, float volts)
+##### Description 
+Defines a CT_Sensor object. One or more CT_Sensor objects can be defined as needed.  
+The first form is for current transformers with a user-supplied burden resistor. In this case, the turns ratio and the burden resistor value are given.  
+The second form is for current transformers with a built-in burden resistor. These are often specified as the output voltage corresponding to the maximum input current, e.g. 20A/1V.
 ##### Syntax
-`CT_Sensor myCT(channel, ratio, burden);`
+```c++
+CT_Sensor myCT(channel, ratio, burden);
+/* or */
+CT_Sensor myCT(channel, amps, volts);
+```
 ##### Parameters
 **channel:** ADC channel number that the current transformer is connected to. (Arduino pin numbers can also be used, e.g. A0-A5). *(uint8_t)*  
 **ratio:** Secondary-to-primary turns ratio for the current transformer. *(float)*  
-**burden:** Current transformer burden resistor value in ohms. *(float)*
+**burden:** Current transformer burden resistor value in ohms. *(float)*  
+**amps:** Maximum rated current for the current transformer. *(float)*  
+**volts:** Voltage output corresponding to the maximum current input. *(float)*
 ##### Example
 ```c++
-CT_Sensor mySensor(0, 1000, 200);
+CT_Sensor mySensor(0, 1000, 200);   // 1000:1 turns ratio, 200Ω burden resistor
+/* or */
+CT_Sensor mySensor(0, 20, 1);       // 1 volt output at 20 amps (built-in burden resistor)
 ```
 
 ### CT_Control(ctFreq_t freq)
@@ -115,3 +126,18 @@ myCtrl.read(&mySensor);
 float rmsCurrent = myCT.read();
 
 ```
+### float CT_Control::readVcc()
+##### Description
+Reads the value of the microcontroller's supply voltage and returns it in volts. Call this function only before calling `begin()`, since `begin()` configures the ADC for continuous timer-driven readings. This function is useful for accurately determining the Vcc value to be passed to the `begin()` function.
+##### Syntax
+`myCtrl.readVcc();`
+##### Parameters
+None.
+##### Returns
+Supply voltage in volts *(float)*
+##### Example
+```c++
+CT_Control myCtrl(CT_FREQ_50HZ);
+float vcc = myCtrl.readVcc();
+myCtrl.begin(vcc);
+```

examples/gsCurrentSensor/classes.h

@@ -1,5 +1,5 @@
-#include <CurrentTransformer.h>             // https://github.com/JChristensen/CurrentTransformer
-#include <LiquidTWI.h>                 //http://forums.adafruit.com/viewtopic.php?t=21586
+#include <CurrentTransformer.h>     // https://github.com/JChristensen/CurrentTransformer
+#include <LiquidTWI.h>              // https://forums.adafruit.com/viewtopic.php?t=21586
 
 CT_Sensor ct0(A0, 1000, 200);
 LiquidTWI lcd(0);   //i2c address 0 (0x20)
@@ -11,7 +11,6 @@ class CurrentSensor : public CT_Control
         void begin();
         float sample();
         void clearSampleData();
-        int readVcc();
 
         int nSample;            // number of times CT was read
         int nRunning;           // number of times current was >= threshold value
@@ -29,9 +28,14 @@ CurrentSensor::CurrentSensor(uint32_t threshold) : maThreshold(threshold)
 
 void CurrentSensor::begin()
 {
-    CT_Control::begin(static_cast<float>(readVcc()) / 1000.0);
+    float vcc = readVcc();
+    CT_Control::begin(vcc);
     lcd.begin(16, 2);
     lcd.clear();
+    lcd.setCursor(0, 0);
+    lcd << F("Vcc = ") << _FLOAT(vcc, 3);
+    delay(1000);
+    lcd.clear();
 }
 
 // read the ct and collect sample data, display on lcd
@@ -61,18 +65,3 @@ void CurrentSensor::clearSampleData()
     maMax = 0;
     maMin = 999999;
 }
-
-// read 1.1V reference against AVcc
-// from http://code.google.com/p/tinkerit/wiki/SecretVoltmeter
-int CurrentSensor::readVcc()
-{
-    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
-    delay(5);
-    ADCSRA |= _BV(ADSC);                    // start conversion
-    loop_until_bit_is_clear(ADCSRA, ADSC);  // wait for it to complete
-    return 1125300L / ADC;                  // calculate AVcc in mV (1.1 * 1000 * 1023)
-}
-

examples/gsCurrentSensor/gsCurrentSensor.ino

@@ -29,7 +29,7 @@ const time_t SYNC_RETRY_INTERVAL(5*60); // time sync retry interval, sec
 
 //object instantiations
 gsXBee xb;                              // the XBee
-CurrentSensor cs(100);                  // current transformer
+CurrentSensor cs(100);                  // current transformer, 100mA threshold
 
 //time, time zone, etc.
 uint32_t ms;                            // current time from millis()
@@ -186,7 +186,8 @@ void xmit(xbeeReadStatus_t xbStatus)
             payloadInteger(n, cs.nSample);
             payloadInteger(r, cs.nRunning);
             payloadInteger(t, cs.maSum);
-            payloadInteger(m, cs.maMin);
+            // if no current observed, send 0 for the min instead of 999999
+            payloadInteger(m, (cs.maMax == 0 ? 0 : cs.maMin));
             payloadInteger(x, cs.maMax);
             xb.destAddr = coordinator;
             xb.sendData(payload);
@@ -219,7 +220,7 @@ void xmit(xbeeReadStatus_t xbStatus)
                 // suspect that the time sync request arrives too soon and the
                 // base station reads it as the response to the DB command
                 // used to get last hop RSS. was seeing RSS UNEXP RESP messages.
-                delay(50);
+                delay(10);
                 timeSyncRetry += SYNC_RETRY_INTERVAL;
                 xb.destAddr = coordinator;
                 xb.requestTimeSync(utc);

library.properties

@@ -1,5 +1,5 @@
 name=CurrentTransformer
-version=2.0.0
+version=2.1.0
 author=Jack Christensen <[email protected]>
 maintainer=Jack Christensen <[email protected]>
 sentence=Arduino Library for measuring current in 50/60Hz circuits using current transformers.

src/CurrentTransformer.cpp

@@ -87,6 +87,22 @@ 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

@@ -33,15 +33,16 @@ class CT_Control
         // 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
+        float m_vcc;                        // mcu supply voltage
+        uint16_t m_tcOCR1;                  // compare value for timer
 };
 
 #endif