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/xiao_nrf52/XiaoNrf52Board.cpp

@@ -5,12 +5,40 @@
 
 #include "XiaoNrf52Board.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 XiaoNrf52Board::initiateShutdown(uint8_t reason) {
+  bool enable_lpcomp = (reason == SHUTDOWN_REASON_LOW_VOLTAGE ||
+                        reason == SHUTDOWN_REASON_BOOT_PROTECT);
+
+  pinMode(VBAT_ENABLE, OUTPUT);
+  digitalWrite(VBAT_ENABLE, enable_lpcomp ? LOW : HIGH);
+
+  if (enable_lpcomp) {
+    configureVoltageWake(power_config.lpcomp_ain_channel, power_config.lpcomp_refsel);
+  }
+
+  enterSystemOff(reason);
+}
+#endif // NRF52_POWER_MANAGEMENT
+
 void XiaoNrf52Board::begin() {
   NRF52BoardDCDC::begin();
 
+  // Configure battery voltage ADC
   pinMode(PIN_VBAT, INPUT);
   pinMode(VBAT_ENABLE, OUTPUT);
-  digitalWrite(VBAT_ENABLE, HIGH);
+  digitalWrite(VBAT_ENABLE, LOW);  // Enable VBAT divider for reading
+  analogReadResolution(12);
+  analogReference(AR_INTERNAL_3_0);
+  delay(50);  // Allow ADC to settle
 
 #ifdef PIN_USER_BTN
   pinMode(PIN_USER_BTN, INPUT_PULLUP);
@@ -27,9 +55,20 @@ void XiaoNrf52Board::begin() {
   digitalWrite(P_LORA_TX_LED, HIGH);
 #endif
 
-  //  pinMode(SX126X_POWER_EN, OUTPUT);
-  //  digitalWrite(SX126X_POWER_EN, HIGH);
-  delay(10); // give sx1262 some time to power up
+#ifdef NRF52_POWER_MANAGEMENT
+  // Boot voltage protection check (may not return if voltage too low)
+  checkBootVoltage(&power_config);
+#endif
+
+  delay(10);  // Give sx1262 some time to power up
+}
+
+uint16_t XiaoNrf52Board::getBattMilliVolts() {
+  // https://wiki.seeedstudio.com/XIAO_BLE#q3-what-are-the-considerations-when-using-xiao-nrf52840-sense-for-battery-charging
+  // VBAT_ENABLE must be LOW to read battery voltage
+  digitalWrite(VBAT_ENABLE, LOW);
+  int adcvalue = analogRead(PIN_VBAT);
+  return (adcvalue * ADC_MULTIPLIER * AREF_VOLTAGE) / 4.096;
 }
 
 #endif