nRF52840 Power Management - Phase 1 - Boot Low VBAT Voltage Lockout

Added NRF52840 power management core functionality:
- Boot‑voltage lockout
- Initial support for shutdown/reset reason storage and capture (via RESETREAS/GPREGRET2)
- LPCOMP wake (for voltage-driven shutdowns)
- VBUS wake (for voltage-driven shutdowns)
- Per-board shutdown handler for board-specific tasks
- Exposed CLI queries for power‑management status in CommonCLI.cpp
- Added documentation in docs/nrf52_power_management.md.
- Enabled power management support in Xiao nRF52840, RAK4631, Heltec T114 boards

variants/rak4631/RAK4631Board.cpp

@@ -3,6 +3,28 @@
 
 #include "RAK4631Board.h"
 
+#ifdef NRF52_POWER_MANAGEMENT
+// Static configuration for power management
+// Values set in variant.h defines
+const PowerMgtConfig power_config = {
+  .lpcomp_ain_channel = PWRMGT_LPCOMP_AIN,
+  .lpcomp_refsel = PWRMGT_LPCOMP_REFSEL,
+  .voltage_bootlock = PWRMGT_VOLTAGE_BOOTLOCK
+};
+
+void RAK4631Board::initiateShutdown(uint8_t reason) {
+  // Disable LoRa module power before shutdown
+  digitalWrite(SX126X_POWER_EN, LOW);
+
+  if (reason == SHUTDOWN_REASON_LOW_VOLTAGE ||
+      reason == SHUTDOWN_REASON_BOOT_PROTECT) {
+    configureVoltageWake(power_config.lpcomp_ain_channel, power_config.lpcomp_refsel);
+  }
+
+  enterSystemOff(reason);
+}
+#endif // NRF52_POWER_MANAGEMENT
+
 void RAK4631Board::begin() {
   NRF52BoardDCDC::begin();
   pinMode(PIN_VBAT_READ, INPUT);
@@ -21,6 +43,11 @@ void RAK4631Board::begin() {
   Wire.begin();
 
   pinMode(SX126X_POWER_EN, OUTPUT);
+#ifdef NRF52_POWER_MANAGEMENT
+  // Boot voltage protection check (may not return if voltage too low)
+  // We need to call this after we configure SX126X_POWER_EN as output but before we pull high
+  checkBootVoltage(&power_config);
+#endif
   digitalWrite(SX126X_POWER_EN, HIGH);
   delay(10);   // give sx1262 some time to power up
 }