# Power & Fault Handling Features

**Feature Group ID:** FG-PWR  
**Version:** 1.0  
**Date:** 2025-01-19  
**Scope:** Sensor Hub (Sub-Hub only)  
**Target Platform:** ESP32-S3–based Sensor Hub, ESP-IDF v5.4

## 1 Feature Overview

The **Power & Fault Handling Features** ensure that the Sensor Hub operates reliably under power fluctuations and recovers gracefully from power interruptions. These features protect critical data during brownouts and enable clean recovery after power restoration.

**Technology:**
- **Brownout Detection:** Hardware brownout detector (BOD)
- **Power-Loss Protection:** Supercapacitor (optional, recommended)
- **RTC Backup:** External RTC battery (CR2032, optional)

## 2 Scope and Assumptions

**In Scope**

*   Brownout detection and handling
*   Power-loss data protection
*   Graceful shutdown on power loss
*   Clean recovery after power restoration

**Out of Scope**

*   Battery-powered operation (system assumes continuous power)
*   Power management for low-power modes (not applicable for real-time requirements)

## 3 Sub-Feature Breakdown

### 3.1 F-PWR-01: Brownout Detection and Handling

#### Description

The system monitors input voltage and takes immediate action if it drops below safe threshold.

**Configuration:**
- **Brownout Threshold:** 3.0V (hardware-configurable)
- **Detection:** Hardware brownout detector (BOD) in ESP32-S3
- **ISR Action:** Set "Power Loss" flag and immediately flush critical buffers to NVS/SD
- **Recovery:** Perform clean reboot once power is stable

**Hardware Support:**
- **Supercapacitor (Recommended):** 0.5-1.0F for 1-2s at 3.3V
  - Provides runtime during brownout to complete data flush
  - Enables graceful shutdown
- **External RTC Battery (Optional):** CR2032, 3V, 220mAh
  - Maintains time accuracy during power loss
  - Not required for basic operation

#### Responsibilities

*   Monitor input voltage
*   Detect brownout condition
*   Trigger immediate data flush
*   Enter graceful shutdown mode

#### Constraints

*   Brownout detection must be hardware-based (ESP32-S3 BOD)
*   Data flush must complete within supercapacitor runtime (1-2 seconds)
*   System must reboot cleanly after power restoration

### 3.2 F-PWR-02: Power-Loss Recovery

#### Description

The system recovers gracefully from power interruptions (< 1 second).

**Recovery Behavior:**
- Clean reboot after power stabilization
- Data integrity verification
- State restoration from persistent storage
- Diagnostic event generation (if data loss detected)

**Recovery Sequence:**
1. Power restoration detected
2. Wait for power stabilization (100ms)
3. Perform clean reboot
4. Initialize system from persistent storage
5. Verify data integrity
6. Report recovery status via diagnostics

#### Responsibilities

*   Detect power restoration
*   Perform clean reboot
*   Restore system state from persistent storage
*   Verify data integrity
*   Report recovery status

## 4 System Requirements (Formal SHALL Statements)

### Brownout Detection Requirements

*   **SR-PWR-001**: The system shall monitor input voltage and detect brownout conditions below 3.0V.
*   **SR-PWR-002**: The system shall immediately flush critical data buffers to non-volatile storage upon brownout detection.
*   **SR-PWR-003**: The system shall enter graceful shutdown mode during brownout conditions.
*   **SR-PWR-004**: The system shall perform clean reboot after power stabilization.

### Power-Loss Recovery Requirements

*   **SR-PWR-005**: The system shall recover gracefully from power interruptions.
*   **SR-PWR-006**: The system shall verify data integrity after power restoration.
*   **SR-PWR-007**: The system shall restore system state from persistent storage after power restoration.
*   **SR-PWR-008**: The system shall report power-loss and recovery events via diagnostics.

## 5 Traceability Matrix (Feature → System Requirements)

| Feature ID | Related System Requirements |
|------------|----------------------------|
| F-PWR-01 | SR-PWR-001, SR-PWR-002, SR-PWR-003, SR-PWR-004 |
| F-PWR-02 | SR-PWR-005, SR-PWR-006, SR-PWR-007, SR-PWR-008 |

## 6 Design & Implementation Notes (Non-Normative)

*   **Supercapacitor:** Recommended for production deployment to enable graceful shutdown
*   **RTC Battery:** Optional, improves time accuracy during power loss
*   **Brownout Threshold:** 3.0V is conservative; adjust based on power supply characteristics
*   **Data Flush Priority:** Critical data (calibration, diagnostics) must be flushed first
*   **Recovery Time:** System should recover within 5 seconds after power restoration

## 7 Dependencies

*   **Persistence & Data Management Features** (data flush mechanism)
*   **Diagnostics Features** (power-loss event reporting)
*   **System Management Features** (graceful shutdown, state restoration)

## 8 Hardware Recommendations

| Component | Specification | Purpose |
|-----------|---------------|---------|
| **Supercapacitor** | 0.5-1.0F, 3.3V | Provides runtime during brownout for data flush |
| **RTC Battery** | CR2032, 3V, 220mAh | Maintains time accuracy during power loss |
| **Power Supply** | 3.3V ±5%, minimum 500mA | Stable power for reliable operation |