nRF52 Power Management

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
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:
Enable in platformio.ini:
```ini
-D NRF52_POWER_MANAGEMENT
```
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)
```
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).
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
nRF52840 Product Specification - LPCOMP
SoftDevice S140 API - Power Management