AP_HAL: added voltage_average_ratiometric() call to AnalogIn

this is for ratiometric sensors such as the 3DR airspeed sensor and
the Maxbotix analog sonar

libraries/AP_HAL/AnalogIn.h

@@ -27,6 +27,10 @@ public:
     // return a voltage from 0.0 to 5.0V, scaled
     // against a reference voltage
     virtual float voltage_average() = 0;
+
+    // return a voltage from 0.0 to 5.0V, assuming a ratiometric
+    // sensor
+    virtual float voltage_average_ratiometric() = 0;
 };
 
 class AP_HAL::AnalogIn {

libraries/AP_HAL_AVR/AnalogIn.h

@@ -19,6 +19,7 @@ public:
     float read_latest();
     void set_pin(uint8_t p);
     float voltage_average();
+    float voltage_average_ratiometric();
     void set_stop_pin(uint8_t p);
     void set_settle_time(uint16_t settle_time_ms);    
 

libraries/AP_HAL_AVR/AnalogIn_ADC.cpp

@@ -59,6 +59,17 @@ float ADCSource::voltage_average(void)
     return v * vcc_mV * 9.765625e-7; // 9.765625e-7 = 1.0/(1024*1000)
 }
 
+/*
+  return voltage from 0.0 to 5.0V, assuming a ratiometric sensor. This
+  means the result is really a pseudo-voltage, that assumes the supply
+  voltage is exactly 5.0V.
+ */
+float ADCSource::voltage_average_ratiometric(void)
+{
+    float v = read_average();
+    return v * (5.0f / 1023.0f);
+}
+
 void ADCSource::set_pin(uint8_t pin) {
     _pin = pin;
 }

libraries/AP_HAL_AVR_SITL/AnalogIn.cpp

@@ -22,7 +22,7 @@ float ADCSource::read_average() {
 }
 
 float ADCSource::voltage_average() {
-	return (5.0/1024.0) * read_average();
+	return (5.0f/1023.0f) * read_average();
 }
 
 float ADCSource::read_latest() {

libraries/AP_HAL_AVR_SITL/AnalogIn.h

@@ -19,6 +19,7 @@ public:
     float read_latest();
     void set_pin(uint8_t p);
     float voltage_average();
+    float voltage_average_ratiometric() { return voltage_average(); }
     void set_stop_pin(uint8_t pin) {}
     void set_settle_time(uint16_t settle_time_ms) {}
 

libraries/AP_HAL_AVR_SITL/sitl_ins.cpp

@@ -31,7 +31,7 @@ using namespace AVR_SITL;
 uint16_t SITL_State::_airspeed_sensor(float airspeed)
 {
 	const float airspeed_ratio = 1.9936;
-	const float airspeed_offset = 2820;
+	const float airspeed_offset = 503;
 	float airspeed_pressure, airspeed_raw;
 
 	airspeed_pressure = (airspeed*airspeed) / airspeed_ratio;

libraries/AP_HAL_Empty/AnalogIn.h

@@ -13,6 +13,7 @@ public:
     void set_stop_pin(uint8_t p);
     void set_settle_time(uint16_t settle_time_ms);
     float voltage_average();
+    float voltage_average_ratiometric() { return voltage_average(); }
 private:
     float _v;
 };

libraries/AP_HAL_PX4/AnalogIn.h

@@ -26,6 +26,7 @@ public:
     float read_latest();
     void set_pin(uint8_t p);
     float voltage_average();
+    float voltage_average_ratiometric() { return voltage_average(); }
 
     // stop pins not implemented on PX4 yet
     void set_stop_pin(uint8_t p) {}

libraries/AP_HAL_SMACCM/AnalogIn.h

@@ -11,6 +11,7 @@ public:
     float read_latest();
     void set_pin(uint8_t p);
     float voltage_average();
+    float voltage_average_ratiometric() { return voltage_average(); }
     void set_stop_pin(uint8_t p) {}
     void set_settle_time(uint16_t settle_time_ms) {}