MeshCore/docs/nrf52_power_management.md

# nRF52 Power Management

## Overview

The nRF52 Power Management module provides battery protection features to prevent over-discharge, minimise likelihood of brownout and flash corruption conditions existing, and enable safe voltage-based recovery.

## Features

### Boot Voltage Protection
- Checks battery voltage immediately after boot and before mesh operations commence
- If voltage is below a configurable threshold (e.g., 3300mV), the device configures voltage wake (LPCOMP + VBUS) and enters protective shutdown (SYSTEMOFF)
- Prevents boot loops when battery is critically low
- Skipped when external power (USB VBUS) is detected

### Voltage Wake (LPCOMP + VBUS)
- Configures the nRF52's Low Power Comparator (LPCOMP) before entering SYSTEMOFF
- Enables USB VBUS detection so external power can wake the device
- Device automatically wakes when battery voltage rises above recovery threshold or when VBUS is detected

### Early Boot Register Capture
- Captures RESETREAS (reset reason) and GPREGRET2 (shutdown reason) before SystemInit() clears them
- Allows firmware to determine why it booted (cold boot, watchdog, LPCOMP wake, etc.)
- Allows firmware to determine why it last shut down (user request, low voltage, boot protection)

### Shutdown Reason Tracking
Shutdown reason codes (stored in GPREGRET2):
| Code | Name | Description |
|------|------|-------------|
| 0x00 | NONE | Normal boot / no previous shutdown |
| 0x4C | LOW_VOLTAGE | Runtime low voltage threshold reached |
| 0x55 | USER | User requested powerOff() |
| 0x42 | BOOT_PROTECT | Boot voltage protection triggered |

## Supported Boards


| Board | Implemented | LPCOMP wake | VBUS wake |
|-------|-------------|-------------|-----------|
| Seeed Studio XIAO nRF52840 (`xiao_nrf52`) | Yes | Yes | Yes |
| RAK4631 (`rak4631`) | Yes | Yes | Yes |
| Heltec T114 (`heltec_t114`) | Yes | Yes | Yes |
| GAT562 Mesh Watch13 | Yes | Yes | Yes |
| Promicro nRF52840 | No | No | No |
| RAK WisMesh Tag | No | No | No |
| Heltec Mesh Solar | No | No | No |
| LilyGo T-Echo / T-Echo Lite | No | No | No |
| SenseCAP Solar | Yes | Yes | Yes |
| WIO Tracker L1 / L1 E-Ink | No | No | No |
| WIO WM1110 | No | No | No |
| Mesh Pocket | No | No | No |
| Nano G2 Ultra | No | No | No |
| ThinkNode M1/M3/M6 | No | No | No |
| T1000-E | No | No | No |
| Ikoka Nano/Stick/Handheld (nRF) | No | No | No |
| Keepteen LT1 | No | No | No |
| Minewsemi ME25LS01 | No | No | No |

Notes:
- "Implemented" reflects Phase 1 (boot lockout + shutdown reason capture).
- User power-off on Heltec T114 does not enable LPCOMP wake.
- VBUS detection is used to skip boot lockout on external power, and VBUS wake is configured alongside LPCOMP when supported hardware exposes VBUS to the nRF52.

## Technical Details

### Architecture

The power management functionality is integrated into the `NRF52Board` base class in `src/helpers/NRF52Board.cpp`. Board variants provide hardware-specific configuration via a `PowerMgtConfig` struct and override `initiateShutdown(uint8_t reason)` to perform board-specific power-down work and conditionally enable voltage wake (LPCOMP + VBUS).

### Early Boot Capture

A static constructor with priority 101 in `NRF52Board.cpp` captures the RESETREAS and GPREGRET2 registers before:
- SystemInit() (priority 102) - which clears RESETREAS
- Static C++ constructors (default priority 65535)

This ensures we capture the true reset reason before any initialisation code runs.

### Board Implementation

To enable power management on a board variant:

1. **Enable in platformio.ini**:
   ```ini
   -D NRF52_POWER_MANAGEMENT
   ```

2. **Define configuration in variant.h**:
   ```c
   #define PWRMGT_VOLTAGE_BOOTLOCK    3300   // Won't boot below this voltage (mV)
   #define PWRMGT_LPCOMP_AIN          7      // AIN channel for voltage sensing
   #define PWRMGT_LPCOMP_REFSEL       2      // REFSEL (0-6=1/8..7/8, 7=ARef, 8-15=1/16..15/16)
   ```

3. **Implement in board .cpp file**:
   ```cpp
   #ifdef NRF52_POWER_MANAGEMENT
   const PowerMgtConfig power_config = {
     .lpcomp_ain_channel = PWRMGT_LPCOMP_AIN,
     .lpcomp_refsel = PWRMGT_LPCOMP_REFSEL,
     .voltage_bootlock = PWRMGT_VOLTAGE_BOOTLOCK
   };

   void MyBoard::initiateShutdown(uint8_t reason) {
     // Board-specific shutdown preparation (e.g., disable peripherals)
     bool enable_lpcomp = (reason == SHUTDOWN_REASON_LOW_VOLTAGE ||
                           reason == SHUTDOWN_REASON_BOOT_PROTECT);

     if (enable_lpcomp) {
       configureVoltageWake(power_config.lpcomp_ain_channel, power_config.lpcomp_refsel);
     }

     enterSystemOff(reason);
   }
   #endif

   void MyBoard::begin() {
     NRF52Board::begin();  // or NRF52BoardDCDC::begin()
     // ... board setup ...

   #ifdef NRF52_POWER_MANAGEMENT
     checkBootVoltage(&power_config);
   #endif
   }
   ```

   For user-initiated shutdowns, `powerOff()` remains board-specific. Power management only arms LPCOMP for automated shutdown reasons (boot protection/low voltage).

4. **Declare override in board .h file**:
   ```cpp
   #ifdef NRF52_POWER_MANAGEMENT
     void initiateShutdown(uint8_t reason) override;
   #endif
   ```

### Voltage Wake Configuration

The LPCOMP (Low Power Comparator) is configured to:
- Monitor the specified AIN channel (0-7 corresponding to P0.02-P0.05, P0.28-P0.31)
- Compare against VDD fraction reference (REFSEL: 0-6=1/8..7/8, 7=ARef, 8-15=1/16..15/16)
- Detect UP events (voltage rising above threshold)
- Use 50mV hysteresis for noise immunity
- Wake the device from SYSTEMOFF when triggered

VBUS wake is enabled via the POWER peripheral USBDETECTED event whenever `configureVoltageWake()` is used. This requires USB VBUS to be routed to the nRF52 (typical on nRF52840 boards with native USB).

**LPCOMP Reference Selection (PWRMGT_LPCOMP_REFSEL)**:
| REFSEL | Fraction | VBAT @ 1M/1M divider (VDD=3.0-3.3) | VBAT @ 1.5M/1M divider (VDD=3.0-3.3) |
|--------|----------|------------------------------------|--------------------------------------|
| 0 | 1/8 | 0.75-0.82 V | 0.94-1.03 V |
| 1 | 2/8 | 1.50-1.65 V | 1.88-2.06 V |
| 2 | 3/8 | 2.25-2.47 V | 2.81-3.09 V |
| 3 | 4/8 | 3.00-3.30 V | 3.75-4.12 V |
| 4 | 5/8 | 3.75-4.12 V | 4.69-5.16 V |
| 5 | 6/8 | 4.50-4.95 V | 5.62-6.19 V |
| 6 | 7/8 | 5.25-5.77 V | 6.56-7.22 V |
| 7 | ARef | - | - |
| 8 | 1/16 | 0.38-0.41 V | 0.47-0.52 V |
| 9 | 3/16 | 1.12-1.24 V | 1.41-1.55 V |
| 10 | 5/16 | 1.88-2.06 V | 2.34-2.58 V |
| 11 | 7/16 | 2.62-2.89 V | 3.28-3.61 V |
| 12 | 9/16 | 3.38-3.71 V | 4.22-4.64 V |
| 13 | 11/16 | 4.12-4.54 V | 5.16-5.67 V |
| 14 | 13/16 | 4.88-5.36 V | 6.09-6.70 V |
| 15 | 15/16 | 5.62-6.19 V | 7.03-7.73 V |

**Important**: For boards with a voltage divider on the battery sense pin, LPCOMP measures the divided voltage. Use:
`VBAT_threshold ≈ (VDD * fraction) * divider_scale`, where `divider_scale = (Rtop + Rbottom) / Rbottom` (e.g., 2.0 for 1M/1M, 2.5 for 1.5M/1M, 3.0 for XIAO).

### SoftDevice Compatibility

The power management code checks whether SoftDevice is enabled and uses the appropriate API:
- When SD enabled: `sd_power_*` functions
- When SD disabled: Direct register access (NRF_POWER->*)

This ensures compatibility regardless of BLE stack state.

## CLI Commands

Power management status can be queried via the CLI:

| Command | Description |
|---------|-------------|
| `get pwrmgt.support` | Returns "supported" or "unsupported" |
| `get pwrmgt.source` | Returns current power source - "battery" or "external" (5V/USB power) |
| `get pwrmgt.bootreason` | Returns reset and shutdown reason strings |
| `get pwrmgt.bootmv` | Returns boot voltage in millivolts |

On boards without power management enabled, all commands except `get pwrmgt.support` return:
```
ERROR: Power management not supported
```

## Debug Output

When `MESH_DEBUG=1` is enabled, the power management module outputs:
```
DEBUG: PWRMGT: Reset = Wake from LPCOMP (0x20000); Shutdown = Low Voltage (0x4C)
DEBUG: PWRMGT: Boot voltage = 3450 mV (threshold = 3300 mV)
DEBUG: PWRMGT: LPCOMP wake configured (AIN7, ref=3/8 VDD)
```

## Phase 2 (Planned)

- Runtime voltage monitoring
- Voltage state machine (Normal -> Warning -> Critical -> Shutdown)
- Configurable thresholds
- Load shedding callbacks for power reduction
- Deep sleep integration
- Scheduled wake-up
- Extended sleep with periodic monitoring

## References

- [nRF52840 Product Specification - POWER](https://infocenter.nordicsemi.com/topic/ps_nrf52840/power.html)
- [nRF52840 Product Specification - LPCOMP](https://infocenter.nordicsemi.com/topic/ps_nrf52840/lpcomp.html)
- [SoftDevice S140 API - Power Management](https://infocenter.nordicsemi.com/topic/sdk_nrf5_v17.1.0/group__nrf__sdm__api.html)