AP_HAL: added voltage_average() interface to AnalogIn

returns voltage in Volts, using averaged reading over samples.

Where possible this should be auto-scaled against a known reference
voltage

libraries/AP_HAL/AnalogIn.h

@@ -9,6 +9,10 @@ public:
     virtual float read_average() = 0;
     virtual float read_latest() = 0;
     virtual void set_pin(uint8_t p) = 0;
+
+    // return a voltage from 0.0 to 5.0V, scaled
+    // against a reference voltage
+    virtual float voltage_average() = 0;
 };
 
 class AP_HAL::AnalogIn {

libraries/AP_HAL_AVR/AnalogIn.h

@@ -18,6 +18,8 @@ public:
     float read_average();
     float read_latest();
     void set_pin(uint8_t p);
+    float voltage_average();
+    
 
     /* implementation specific interface: */
 
@@ -38,6 +40,7 @@ private:
     volatile uint8_t _sum_count;
     volatile uint16_t _sum;
     volatile uint16_t _latest;
+    float _last_average;
 
     /* _pin designates the ADC input mux for the sample */
     uint8_t _pin;

libraries/AP_HAL_AVR/AnalogIn_ADC.cpp

@@ -7,6 +7,8 @@
 #include "AnalogIn.h"
 using namespace AP_HAL_AVR;
 
+extern const AP_HAL::HAL& hal;
+
 ADCSource::ADCSource(uint8_t pin, float prescale) :
     _pin(pin),
     _sum_count(0),
@@ -35,6 +37,16 @@ float ADCSource::read_latest() {
     }
 }
 
+/*
+  return voltage from 0.0 to 5.0V, scaled to Vcc
+ */
+float ADCSource::voltage_average(void)
+{
+	float vcc_mV = hal.analogin->channel(ANALOG_INPUT_BOARD_VCC)->read_average();
+	float v = read_average();
+	return v * vcc_mV * 9.765625e-7; // 9.765625e-7 = 1.0/(1024*1000)
+}
+
 void ADCSource::set_pin(uint8_t pin) {
     _pin = pin;
 }
@@ -44,11 +56,10 @@ float ADCSource::_read_average() {
     uint16_t sum;
     uint8_t sum_count;
 
-    /* Block until there is a new sample. Will only happen if you
-     * call read_average very frequently.
-     * I don't like the idea of blocking like this but we don't
-     * have a way to bubble control upwards at the moment. -pch */
-    while( _sum_count == 0 );
+    if (_sum_count == 0) {
+	    // avoid blocking waiting for new samples
+	    return _last_average;
+    }
 
     /* Read and clear in a critical section */
     uint8_t sreg = SREG;
@@ -62,8 +73,9 @@ float ADCSource::_read_average() {
     SREG = sreg;
 
     float avg = sum / (float) sum_count;
-    return avg;
 
+    _last_average = avg;
+    return avg;
 }
 
 void ADCSource::setup_read() {

libraries/AP_HAL_AVR_SITL/AnalogIn.cpp

@@ -21,6 +21,10 @@ float ADCSource::read_average() {
 	return read_latest();
 }
 
+float ADCSource::voltage_average() {
+	return (5.0/1024.0) * read_average();
+}
+
 float ADCSource::read_latest() {
     switch (_pin) {
     case ANALOG_INPUT_BOARD_VCC:

libraries/AP_HAL_AVR_SITL/AnalogIn.h

@@ -18,6 +18,7 @@ public:
     float read_average();
     float read_latest();
     void set_pin(uint8_t p);
+    float voltage_average();
 
 private:
     /* prescale scales the raw measurments for read()*/

libraries/AP_HAL_Empty/AnalogIn.cpp

@@ -11,6 +11,10 @@ float EmptyAnalogSource::read_average() {
     return _v;
 }
 
+float EmptyAnalogSource::voltage_average() {
+    return 5.0 * _v / 1024.0;
+}
+
 float EmptyAnalogSource::read_latest() {
     return _v;
 }

libraries/AP_HAL_Empty/AnalogIn.h

@@ -10,6 +10,7 @@ public:
     float read_average();
     float read_latest();
     void set_pin(uint8_t p);
+    float voltage_average();
 private:
     float _v;
 };

libraries/AP_HAL_PX4/AnalogIn.cpp

@@ -55,6 +55,14 @@ float PX4AnalogSource::read_latest()
     return _latest_value;
 }
 
+/*
+  return voltage in Volts
+ */
+float PX4AnalogSource::voltage_average()
+{
+    return (5.0f/1024.0f) * read_average();
+}
+
 void PX4AnalogSource::set_pin(uint8_t pin)
 {
     _pin = pin;

libraries/AP_HAL_PX4/AnalogIn.h

@@ -15,6 +15,8 @@ public:
     float read_average();
     float read_latest();
     void set_pin(uint8_t p);
+    float voltage_average();
+
 private:
     // what pin it is attached to
     int16_t _pin;

libraries/AP_HAL_SMACCM/AnalogIn.cpp

@@ -11,6 +11,11 @@ float SMACCMAnalogSource::read_average() {
     return _v;
 }
 
+float SMACCMAnalogSource::voltage_average() {
+    // this assumes 5.0V scaling and 1024 range
+    return (5.0/1024.0) * read_average();
+}
+
 float SMACCMAnalogSource::read_latest() {
     return _v;
 }

libraries/AP_HAL_SMACCM/AnalogIn.h

@@ -10,6 +10,7 @@ public:
     float read_average();
     float read_latest();
     void set_pin(uint8_t p);
+    float voltage_average();
 private:
     float _v;
 };