HAL_PX4: support voltage monitoring on multi-connector pin 5

libraries/AP_HAL_PX4/AnalogIn.cpp

@@ -19,6 +19,10 @@
 // for a range of known inputs from 1.3V to 5.0V
 #define PX4_AIRSPEED_VOLTAGE_SCALING (6.76f/4096.0f)
 
+// the FMU battery mon port has some additional scaling to make it
+// safe for up to 18.68V.
+#define PX4_FMU_BATTERY_VOLTAGE_SCALING (18.68f/4096.0f)
+
 // pin4 in the SPI port is analog input 13, marked as analog3 on the
 // PX4IO schematic v1.3, and is scaled quite strangely
 #define PX4_ANALOG3_VOLTAGE_SCALING (16.88f/4096.0f)
@@ -73,6 +77,9 @@ float PX4AnalogSource::read_latest()
  */
 float PX4AnalogSource::voltage_average()
 {
+    if (_pin == PX4_ANALOG_FMU_BATTERY) {
+        return PX4_FMU_BATTERY_VOLTAGE_SCALING * read_average();
+    }
     if (_pin == PX4_ANALOG_AIRSPEED_PIN) {
         return PX4_AIRSPEED_VOLTAGE_SCALING * read_average();
     }

libraries/AP_HAL_PX4/AnalogIn.h

@@ -14,6 +14,7 @@
 #define PX4_ANALOG_BATTERY_VOLTAGE_PIN 100
 #define PX4_ANALOG_BATTERY_CURRENT_PIN 101
 
+#define PX4_ANALOG_FMU_BATTERY          10 // on multi-connector pin 5
 #define PX4_ANALOG_AIRSPEED_PIN         11
 #define PX4_ANALOG_ANALOG2_PIN          12 // on SPI port pin 3
 #define PX4_ANALOG_ANALOG3_PIN          13 // on SPI port pin 4