feat(xiao_s3_wio): add battery read with voltage divider

variants/xiao_s3_wio/XiaoS3WIOBoard.h

@@ -3,11 +3,47 @@
 #include <Arduino.h>
 #include <helpers/ESP32Board.h>
 
+/*
+ * This board has no built-in way to read battery voltage.
+ * Nevertheless it's very easy to make it work, you only require two 1% resistors.
+ * If your using the WIO SX1262 Addon for xaio, make sure you dont connect D1!
+ *
+ *    BAT+ -----+
+ *              |
+ *       VSYS --+ -/\/\/\/\- --+
+ *                   200k      |
+ *                             +-- D1
+ *                             |
+ *        GND --+ -/\/\/\/\- --+
+ *              |    100k
+ *    BAT- -----+
+ */
+#define PIN_VBAT_READ     2 // D1
+#define BATTERY_SAMPLES   8
+#define ADC_MULTIPLIER    (3.0f * 3.3f * 1000)
+
 class XiaoS3WIOBoard : public ESP32Board {
 public:
   XiaoS3WIOBoard() { }
 
   const char* getManufacturerName() const override {
     return "Xiao S3 WIO";
   }
+
+  uint16_t getBattMilliVolts() override {
+#if defined(PIN_VBAT_READ) && defined(ADC_MULTIPLIER)
+    analogReadResolution(12);
+
+    uint32_t raw = 0;
+    for (int i = 0; i < BATTERY_SAMPLES; i++) {
+      raw += analogRead(PIN_VBAT_READ);
+    }
+    raw = raw / BATTERY_SAMPLES;
+
+    return (ADC_MULTIPLIER * raw) / 4096;
+#else
+    return 0;
+#endif
+  }
+
 };