AP_HAL_AVR: support voltage_latest()

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_latest();
     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

@@ -60,6 +60,26 @@ 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, scaled to Vcc
+ */
+float ADCSource::voltage_latest(void)
+{
+    if (_pin == ANALOG_INPUT_BOARD_VCC) {
+        return read_latest() * 0.001f;
+    }
+    float vcc_mV = hal.analogin->channel(ANALOG_INPUT_BOARD_VCC)->read_average();
+    float v = read_latest();
+    // constrain Vcc reading so that a bad Vcc doesn't throw off
+    // the reading of other sources too badly
+    if (vcc_mV < 4000) {
+        vcc_mV = 4000;
+    } else if (vcc_mV > 6000) {
+        vcc_mV = 6000;
+    }
+    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