AP_HAL: added board_voltage AnalogIn method

this makes it easier to get the board voltage from any library,
without having to allocate another analog channel object

libraries/AP_HAL/AnalogIn.h

@@ -41,6 +41,9 @@ class AP_HAL::AnalogIn {
 public:
     virtual void init(void* implspecific) = 0;
     virtual AP_HAL::AnalogSource* channel(int16_t n) = 0;
+
+    // board 5V rail voltage in volts
+    virtual float board_voltage(void) = 0;
 };
 
 #define ANALOG_INPUT_BOARD_VCC 254

libraries/AP_HAL_AVR/AnalogIn.h

@@ -68,6 +68,7 @@ public:
     AVRAnalogIn();
     void init(void* ap_hal_scheduler);
     AP_HAL::AnalogSource* channel(int16_t n);
+    float board_voltage(void);
 
 protected: 
     ADCSource* _create_channel(int16_t num);

libraries/AP_HAL_AVR/AnalogIn_Common.cpp

@@ -114,4 +114,12 @@ AP_HAL::AnalogSource* AVRAnalogIn::channel(int16_t ch)
     }
 }
 
+/*
+  return board voltage in volts
+ */
+float AVRAnalogIn::board_voltage(void)
+{
+    return _vcc.voltage_latest();
+}
+
 #endif

libraries/AP_HAL_AVR_SITL/AnalogIn.h

@@ -38,7 +38,7 @@ public:
     }
     void init(void* ap_hal_scheduler);
     AP_HAL::AnalogSource* channel(int16_t n);
-
+    float board_voltage(void) { return 5.0f; }
 private:
     static ADCSource* _channels[SITL_INPUT_MAX_CHANNELS];
     SITL_State *_sitlState;

libraries/AP_HAL_Empty/AnalogIn.cpp

@@ -41,4 +41,7 @@ AP_HAL::AnalogSource* EmptyAnalogIn::channel(int16_t n) {
     return new EmptyAnalogSource(1.11);
 }
 
-
+float EmptyAnalogIn::board_voltage(void)
+{
+    return 0;
+}

libraries/AP_HAL_Empty/AnalogIn.h

@@ -24,5 +24,6 @@ public:
     EmptyAnalogIn();
     void init(void* implspecific);
     AP_HAL::AnalogSource* channel(int16_t n);
+    float board_voltage(void);
 };
 #endif // __AP_HAL_EMPTY_ANALOGIN_H__

libraries/AP_HAL_FLYMAPLE/AnalogIn.cpp

@@ -126,4 +126,9 @@ AP_HAL::AnalogSource* FLYMAPLEAnalogIn::channel(int16_t ch)
     }
 }
 
+float FLYMAPLEAnalogIn::board_voltage(void)
+{
+    return _vcc.voltage_latest();
+}
+
 #endif

libraries/AP_HAL_FLYMAPLE/AnalogIn.h

@@ -84,6 +84,7 @@ public:
     FLYMAPLEAnalogIn();
     void init(void* implspecific);
     AP_HAL::AnalogSource* channel(int16_t n);
+    float board_voltage(void);
 
 protected: 
     FLYMAPLEAnalogSource* _create_channel(int16_t num);
@@ -93,7 +94,7 @@ protected:
     int16_t _num_channels;
     int16_t _active_channel;
     uint16_t _channel_repeat_count;
-
+    
 private:
     // On Flymaple, VCC measurement is at pin 20. VCC (=VIN) of 5V is only present
     // if external voltage (not USB) is connected. Also there is a voltage

libraries/AP_HAL_Linux/AnalogIn.h

@@ -24,5 +24,9 @@ public:
     LinuxAnalogIn();
     void init(void* implspecific);
     AP_HAL::AnalogSource* channel(int16_t n);
+
+    // we don't yet know how to get the board voltage
+    float board_voltage(void) { return 0.0f; }
+
 };
 #endif // __AP_HAL_LINUX_ANALOGIN_H__

libraries/AP_HAL_PX4/AnalogIn.cpp

@@ -160,16 +160,16 @@ void PX4AnalogSource::set_pin(uint8_t pin)
 /*
   apply a reading in ADC counts
  */
-void PX4AnalogSource::_add_value(float v, uint16_t vcc5V_mV)
+void PX4AnalogSource::_add_value(float v, float vcc5V)
 {
     _latest_value = v;
     _sum_value += v;
-    if (vcc5V_mV == 0) {
+    if (vcc5V < 3.0f) {
         _sum_ratiometric += v;
     } else {
         // this compensates for changes in the 5V rail relative to the
         // 3.3V reference used by the ADC.
-        _sum_ratiometric += v * 5000 / vcc5V_mV;
+        _sum_ratiometric += v * 5.0f / vcc5V;
     }
     _sum_count++;
     if (_sum_count == 254) {
@@ -180,7 +180,8 @@ void PX4AnalogSource::_add_value(float v, uint16_t vcc5V_mV)
 }
 
 
-PX4AnalogIn::PX4AnalogIn()
+PX4AnalogIn::PX4AnalogIn() :
+    _board_voltage(0)
 {}
 
 void PX4AnalogIn::init(void* machtnichts)
@@ -211,14 +212,13 @@ void PX4AnalogIn::_timer_tick(void)
     /* read all channels available */
     int ret = read(_adc_fd, &buf_adc, sizeof(buf_adc));
     if (ret > 0) {
-        uint16_t vcc5V_mV = 0;
         // match the incoming channels to the currently active pins
         for (uint8_t i=0; i<ret/sizeof(buf_adc[0]); i++) {
 #ifdef CONFIG_ARCH_BOARD_PX4FMU_V2
             if (buf_adc[i].am_channel == 4) {
                 // record the Vcc value for later use in
                 // voltage_average_ratiometric()
-                vcc5V_mV = buf_adc[i].am_data * 6600 / 4096;
+                _board_voltage = buf_adc[i].am_data * 6.6f / 4096;
             }
 #endif
         }
@@ -229,7 +229,7 @@ void PX4AnalogIn::_timer_tick(void)
             for (uint8_t j=0; j<PX4_ANALOG_MAX_CHANNELS; j++) {
                 PX4::PX4AnalogSource *c = _channels[j];
                 if (c != NULL && buf_adc[i].am_channel == c->_pin) {
-                    c->_add_value(buf_adc[i].am_data, vcc5V_mV);
+                    c->_add_value(buf_adc[i].am_data, _board_voltage);
                 }
             }
         }

libraries/AP_HAL_PX4/AnalogIn.h

@@ -46,7 +46,7 @@ private:
     uint8_t _sum_count;
     float _sum_value;
     float _sum_ratiometric;
-    void _add_value(float v, uint16_t vcc5V_mV);
+    void _add_value(float v, float vcc5V);
     float _pin_scaler();
 };
 
@@ -56,6 +56,7 @@ public:
     void init(void* implspecific);
     AP_HAL::AnalogSource* channel(int16_t pin);
     void _timer_tick(void);
+    float board_voltage(void) { return _board_voltage; }
 
 private:
     int _adc_fd;
@@ -65,5 +66,6 @@ private:
     uint64_t _servorail_timestamp;
     PX4::PX4AnalogSource* _channels[PX4_ANALOG_MAX_CHANNELS];
     uint32_t _last_run;
+    float _board_voltage;
 };
 #endif // __AP_HAL_PX4_ANALOGIN_H__