APM: added board_voltage() function

ArduPlane/system.pde

@@ -565,3 +565,36 @@ void flash_leds(bool on)
     digitalWrite(A_LED_PIN, on?LED_OFF:LED_ON);
     digitalWrite(C_LED_PIN, on?LED_ON:LED_OFF);
 }
+
+#ifndef DESKTOP_BUILD
+/*
+ * Read Vcc vs 1.1v internal reference
+ *
+ * This call takes about 150us total. ADC conversion is 13 cycles of
+ * 125khz default changes the mux if it isn't set, and return last
+ * reading (allows necessary settle time) otherwise trigger the
+ * conversion
+ */
+uint16_t board_voltage(void)
+{
+	static uint16_t vcc = 5000;
+	const uint8_t mux = (_BV(REFS0)|_BV(MUX4)|_BV(MUX3)|_BV(MUX2)|_BV(MUX1));
+
+	if (ADMUX == mux) {
+		ADCSRA |= _BV(ADSC);                // Convert
+		uint16_t counter=4000; // normally takes about 1700 loops
+		while (bit_is_set(ADCSRA, ADSC) && counter)  // Wait
+			counter--;
+		if (counter == 0) {
+			// we don't actually expect this timeout to happen,
+			// but we don't want any more code that could hang
+			return vcc;
+		}
+		uint32_t result = ADCL | ADCH<<8;
+		vcc = 1126400L / result;       // Read and back-calculate Vcc in mV
+	} else {
+		ADMUX = mux; // switch mux, settle time is needed
+	}
+	return vcc;  // in mV
+}
+#endif