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<h1 id="nrf52-power-management">nRF52 Power Management</h1>
<h2 id="overview">Overview</h2>
<p>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.</p>
<h2 id="features">Features</h2>
<h3 id="boot-voltage-protection">Boot Voltage Protection</h3>
<ul>
<li>Checks battery voltage immediately after boot and before mesh operations commence</li>
<li>If voltage is below a configurable threshold (e.g., 3300mV), the device configures voltage wake (LPCOMP + VBUS) and enters protective shutdown (SYSTEMOFF)</li>
<li>Prevents boot loops when battery is critically low</li>
<li>Skipped when external power (USB VBUS) is detected</li>
</ul>
<h3 id="voltage-wake-lpcomp-vbus">Voltage Wake (LPCOMP + VBUS)</h3>
<ul>
<li>Configures the nRF52's Low Power Comparator (LPCOMP) before entering SYSTEMOFF</li>
<li>Enables USB VBUS detection so external power can wake the device</li>
<li>Device automatically wakes when battery voltage rises above recovery threshold or when VBUS is detected</li>
</ul>
<h3 id="early-boot-register-capture">Early Boot Register Capture</h3>
<ul>
<li>Captures RESETREAS (reset reason) and GPREGRET2 (shutdown reason) before SystemInit() clears them</li>
<li>Allows firmware to determine why it booted (cold boot, watchdog, LPCOMP wake, etc.)</li>
<li>Allows firmware to determine why it last shut down (user request, low voltage, boot protection)</li>
</ul>
<h3 id="shutdown-reason-tracking">Shutdown Reason Tracking</h3>
<p>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 |</p>
<h2 id="supported-boards">Supported Boards</h2>
<table>
<thead>
<tr>
<th>Board</th>
<th>Implemented</th>
<th>LPCOMP wake</th>
<th>VBUS wake</th>
</tr>
</thead>
<tbody>
<tr>
<td>Seeed Studio XIAO nRF52840 (<code>xiao_nrf52</code>)</td>
<td>Yes</td>
<td>Yes</td>
<td>Yes</td>
</tr>
<tr>
<td>RAK4631 (<code>rak4631</code>)</td>
<td>Yes</td>
<td>Yes</td>
<td>Yes</td>
</tr>
<tr>
<td>Heltec T114 (<code>heltec_t114</code>)</td>
<td>Yes</td>
<td>Yes</td>
<td>Yes</td>
</tr>
<tr>
<td>Promicro nRF52840</td>
<td>No</td>
<td>No</td>
<td>No</td>
</tr>
<tr>
<td>RAK WisMesh Tag</td>
<td>No</td>
<td>No</td>
<td>No</td>
</tr>
<tr>
<td>Heltec Mesh Solar</td>
<td>No</td>
<td>No</td>
<td>No</td>
</tr>
<tr>
<td>LilyGo T-Echo / T-Echo Lite</td>
<td>No</td>
<td>No</td>
<td>No</td>
</tr>
<tr>
<td>SenseCAP Solar</td>
<td>Yes</td>
<td>Yes</td>
<td>Yes</td>
</tr>
<tr>
<td>WIO Tracker L1 / L1 E-Ink</td>
<td>No</td>
<td>No</td>
<td>No</td>
</tr>
<tr>
<td>WIO WM1110</td>
<td>No</td>
<td>No</td>
<td>No</td>
</tr>
<tr>
<td>Mesh Pocket</td>
<td>No</td>
<td>No</td>
<td>No</td>
</tr>
<tr>
<td>Nano G2 Ultra</td>
<td>No</td>
<td>No</td>
<td>No</td>
</tr>
<tr>
<td>ThinkNode M1/M3/M6</td>
<td>No</td>
<td>No</td>
<td>No</td>
</tr>
<tr>
<td>T1000-E</td>
<td>No</td>
<td>No</td>
<td>No</td>
</tr>
<tr>
<td>Ikoka Nano/Stick/Handheld (nRF)</td>
<td>No</td>
<td>No</td>
<td>No</td>
</tr>
<tr>
<td>Keepteen LT1</td>
<td>No</td>
<td>No</td>
<td>No</td>
</tr>
<tr>
<td>Minewsemi ME25LS01</td>
<td>No</td>
<td>No</td>
<td>No</td>
</tr>
</tbody>
</table>
<p>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.</p>
<h2 id="technical-details">Technical Details</h2>
<h3 id="architecture">Architecture</h3>
<p>The power management functionality is integrated into the <code>NRF52Board</code> base class in <code>src/helpers/NRF52Board.cpp</code>. Board variants provide hardware-specific configuration via a <code>PowerMgtConfig</code> struct and override <code>initiateShutdown(uint8_t reason)</code> to perform board-specific power-down work and conditionally enable voltage wake (LPCOMP + VBUS).</p>
<h3 id="early-boot-capture">Early Boot Capture</h3>
<p>A static constructor with priority 101 in <code>NRF52Board.cpp</code> captures the RESETREAS and GPREGRET2 registers before:
- SystemInit() (priority 102) - which clears RESETREAS
- Static C++ constructors (default priority 65535)</p>
<p>This ensures we capture the true reset reason before any initialisation code runs.</p>
<h3 id="board-implementation">Board Implementation</h3>
<p>To enable power management on a board variant:</p>
<ol>
<li>
<p><strong>Enable in platformio.ini</strong>:
<code>ini
-D NRF52_POWER_MANAGEMENT</code></p>
</li>
<li>
<p><strong>Define configuration in variant.h</strong>:
<code>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)</code></p>
</li>
<li>
<p><strong>Implement in board .cpp file</strong>:
```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
};</p>
</li>
</ol>
<p>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);</p>
<pre><code> if (enable_lpcomp) {
configureVoltageWake(power_config.lpcomp_ain_channel, power_config.lpcomp_refsel);
}
enterSystemOff(reason);
</code></pre>
<p>}
#endif</p>
<p>void MyBoard::begin() {
NRF52Board::begin(); // or NRF52BoardDCDC::begin()
// ... board setup ...</p>
<p>#ifdef NRF52_POWER_MANAGEMENT
checkBootVoltage(&amp;power_config);
#endif
}
```</p>
<p>For user-initiated shutdowns, <code>powerOff()</code> remains board-specific. Power management only arms LPCOMP for automated shutdown reasons (boot protection/low voltage).</p>
<ol>
<li><strong>Declare override in board .h file</strong>:
<code>cpp
#ifdef NRF52_POWER_MANAGEMENT
void initiateShutdown(uint8_t reason) override;
#endif</code></li>
</ol>
<h3 id="voltage-wake-configuration">Voltage Wake Configuration</h3>
<p>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</p>
<p>VBUS wake is enabled via the POWER peripheral USBDETECTED event whenever <code>configureVoltageWake()</code> is used. This requires USB VBUS to be routed to the nRF52 (typical on nRF52840 boards with native USB).</p>
<p><strong>LPCOMP Reference Selection (PWRMGT_LPCOMP_REFSEL)</strong>:
| 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 |</p>
<p><strong>Important</strong>: For boards with a voltage divider on the battery sense pin, LPCOMP measures the divided voltage. Use:
<code>VBAT_threshold ≈ (VDD * fraction) * divider_scale</code>, where <code>divider_scale = (Rtop + Rbottom) / Rbottom</code> (e.g., 2.0 for 1M/1M, 2.5 for 1.5M/1M, 3.0 for XIAO).</p>
<h3 id="softdevice-compatibility">SoftDevice Compatibility</h3>
<p>The power management code checks whether SoftDevice is enabled and uses the appropriate API:
- When SD enabled: <code>sd_power_*</code> functions
- When SD disabled: Direct register access (NRF_POWER-&gt;*)</p>
<p>This ensures compatibility regardless of BLE stack state.</p>
<h2 id="cli-commands">CLI Commands</h2>
<p>Power management status can be queried via the CLI:</p>
<table>
<thead>
<tr>
<th>Command</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td><code>get pwrmgt.support</code></td>
<td>Returns "supported" or "unsupported"</td>
</tr>
<tr>
<td><code>get pwrmgt.source</code></td>
<td>Returns current power source - "battery" or "external" (5V/USB power)</td>
</tr>
<tr>
<td><code>get pwrmgt.bootreason</code></td>
<td>Returns reset and shutdown reason strings</td>
</tr>
<tr>
<td><code>get pwrmgt.bootmv</code></td>
<td>Returns boot voltage in millivolts</td>
</tr>
</tbody>
</table>
<p>On boards without power management enabled, all commands except <code>get pwrmgt.support</code> return:</p>
<pre><code>ERROR: Power management not supported
</code></pre>
<h2 id="debug-output">Debug Output</h2>
<p>When <code>MESH_DEBUG=1</code> is enabled, the power management module outputs:</p>
<pre><code>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)
</code></pre>
<h2 id="phase-2-planned">Phase 2 (Planned)</h2>
<ul>
<li>Runtime voltage monitoring</li>
<li>Voltage state machine (Normal -&gt; Warning -&gt; Critical -&gt; Shutdown)</li>
<li>Configurable thresholds</li>
<li>Load shedding callbacks for power reduction</li>
<li>Deep sleep integration</li>
<li>Scheduled wake-up</li>
<li>Extended sleep with periodic monitoring</li>
</ul>
<h2 id="references">References</h2>
<ul>
<li><a href="https://infocenter.nordicsemi.com/topic/ps_nrf52840/power.html">nRF52840 Product Specification - POWER</a></li>
<li><a href="https://infocenter.nordicsemi.com/topic/ps_nrf52840/lpcomp.html">nRF52840 Product Specification - LPCOMP</a></li>
<li><a href="https://infocenter.nordicsemi.com/topic/sdk_nrf5_v17.1.0/group__nrf__sdm__api.html">SoftDevice S140 API - Power Management</a></li>
</ul>
</article>
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