* refactor: new helpers/nrf52 dir

* RAK, startOTAUpdate()

src/helpers/nrf52/RAK4631Board.h

@@ -0,0 +1,64 @@
+#pragma once
+
+#include <MeshCore.h>
+#include <Arduino.h>
+
+// LoRa radio module pins for RAK4631
+#define  P_LORA_DIO_1   47
+#define  P_LORA_NSS     42
+#define  P_LORA_RESET  RADIOLIB_NC   // 38
+#define  P_LORA_BUSY    46
+#define  P_LORA_SCLK    43
+#define  P_LORA_MISO    45
+#define  P_LORA_MOSI    44
+#define  SX126X_POWER_EN  37
+ 
+#define SX126X_DIO2_AS_RF_SWITCH  true
+#define SX126X_DIO3_TCXO_VOLTAGE   1.8
+
+// built-ins
+#define  PIN_VBAT_READ    5
+#define  ADC_MULTIPLIER   (3 * 1.73)
+
+class RAK4631Board : public mesh::MainBoard {
+protected:
+  uint8_t startup_reason;
+
+public:
+  void begin() {
+    // for future use, sub-classes SHOULD call this from their begin()
+    startup_reason = BD_STARTUP_NORMAL;
+
+    pinMode(PIN_VBAT_READ, INPUT);
+
+    pinMode(SX126X_POWER_EN, OUTPUT);
+    digitalWrite(SX126X_POWER_EN, HIGH);
+    delay(10);   // give sx1262 some time to power up
+  }
+
+  uint8_t getStartupReason() const override { return startup_reason; }
+
+  #define BATTERY_SAMPLES 8
+
+  uint16_t getBattMilliVolts() override {
+    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;
+  }
+
+  const char* getManufacturerName() const override {
+    return "RAK 4631";
+  }
+
+  void reboot() override {
+    NVIC_SystemReset();
+  }
+
+  bool startOTAUpdate() override;
+};