ASF_01_sys_sw_arch/system_arch_final/[PWR] Power & Fault Handling Features.md

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