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System Requirements Specification (SRS)

ASF Sensor Hub (Sub-Hub) Embedded System

Document Type: System Requirements Specification
Version: 1.0
Date: 2025-01-19
Platform: ESP32-S3, ESP-IDF v5.4, C/C++
Domain: Industrial/Agricultural Automation (Smart Poultry Farm)
Standard: ISO/IEC/IEEE 29148:2018

1. Introduction

1.1 Purpose

This System Requirements Specification (SRS) defines the complete set of system requirements for the ASF Sensor Hub (Sub-Hub) embedded system. The document serves as the authoritative source for all functional and non-functional requirements that the system must satisfy.

1.2 Scope

The ASF Sensor Hub is a distributed sensing node deployed inside a poultry house for environmental monitoring and data collection. The system is responsible for:

  • Acquisition of multiple environmental sensor data
  • Local preprocessing and validation of sensor data
  • Persistent storage of data and configuration
  • Secure communication with a Main Hub
  • Support for diagnostics, maintenance, and OTA updates
  • Safe operation under fault conditions

Explicitly out of scope:

  • Main Hub processing and control logic
  • Cloud analytics and services
  • Farm control algorithms
  • Actuator management

1.3 Definitions and Acronyms

Term Definition
Sub-Hub The ASF Sensor Hub embedded system (this system)
Main Hub Central processing unit that coordinates multiple Sub-Hubs
MC Machine Constants - persistent configuration data
DP Data Persistence component
STM State Manager component
SAL Sensor Abstraction Layer
HMI Human-Machine Interface (OLED display + buttons)
OTA Over-The-Air firmware update
mTLS Mutual Transport Layer Security
CBOR Concise Binary Object Representation

1.4 References

  • ISO/IEC/IEEE 29148:2018 - Systems and software engineering — Life cycle processes — Requirements engineering
  • IEC 61508 - Functional safety of electrical/electronic/programmable electronic safety-related systems
  • ESP-IDF v5.4 Programming Guide
  • System_Architecture_Documentation.md
  • Cross-Feature Constraints.md

2. Overall Description

2.1 Product Perspective

The ASF Sensor Hub operates as an autonomous embedded system within a distributed poultry farm automation network. It interfaces with:

  • Environmental sensors via I2C, SPI, UART, and analog interfaces
  • Main Hub via encrypted Wi-Fi communication
  • Peer Sub-Hubs via ESP-NOW for limited coordination
  • Local operators via OLED display and button interface
  • SD Card for persistent data storage

2.2 Product Functions

The system provides the following major functions:

  1. Multi-sensor data acquisition with high-frequency sampling and filtering
  2. Data quality assurance through calibration and failure detection
  3. Secure communication with Main Hub using encrypted protocols
  4. Persistent data storage with wear-aware management
  5. Over-the-air firmware updates with rollback capability
  6. Comprehensive diagnostics and health monitoring
  7. Local human-machine interface for status and diagnostics
  8. System state management with controlled transitions
  9. Security enforcement through hardware-based protection
  10. Power and fault handling with graceful degradation

2.3 User Classes and Characteristics

User Class Characteristics System Interaction
Farm Operators Basic technical knowledge, daily monitoring Local HMI for status checking
Maintenance Engineers Advanced technical knowledge, periodic maintenance Debug sessions, diagnostic access
System Integrators Expert technical knowledge, system configuration Machine constants management, OTA updates

2.4 Operating Environment

  • Hardware Platform: ESP32-S3 microcontroller
  • Operating System: FreeRTOS (via ESP-IDF)
  • Development Framework: ESP-IDF v5.4
  • Programming Language: C/C++
  • Environmental Conditions: Industrial poultry house (temperature, humidity, dust)
  • Power Supply: 12V DC with brownout protection
  • Communication: Wi-Fi 802.11n (2.4 GHz), ESP-NOW

2.5 Design and Implementation Constraints

  • Memory: 512KB SRAM, 8MB Flash (ESP32-S3)
  • Real-time: Deterministic sensor sampling within 1-second cycles
  • Security: Hardware-enforced Secure Boot V2 and Flash Encryption
  • Reliability: 99.9% uptime requirement in normal operating conditions
  • Safety: IEC 61508 SIL-1 compliance for sensor data integrity
  • Environmental: IP65 enclosure rating, -10°C to +60°C operating range

3. System Requirements

3.1 Functional Requirements

3.1.1 Sensor Data Acquisition Requirements

SR-DAQ-001: Multi-Sensor Support
The system SHALL support simultaneous data acquisition from the following sensor types:

  • Temperature sensors
  • Humidity sensors
  • Carbon Dioxide (CO₂) sensors
  • Ammonia (NH₃) sensors
  • Volatile Organic Compounds (VOC) sensors
  • Particulate Matter (PM) sensors
  • Light Intensity sensors

SR-DAQ-002: High-Frequency Sampling
The system SHALL sample each enabled sensor a minimum of 10 times per acquisition cycle to enable local filtering.

SR-DAQ-003: Local Data Filtering
The system SHALL apply configurable filtering algorithms (median filter, moving average) to raw sensor samples to produce representative values.

SR-DAQ-004: Timestamped Data Generation
The system SHALL associate each processed sensor value with a timestamp accurate to ±1 second of system time.

SR-DAQ-005: Sensor State Management
The system SHALL maintain operational state for each sensor (enabled/disabled, present/absent, healthy/faulty).

3.1.2 Data Quality & Calibration Requirements

SR-DQC-001: Automatic Sensor Detection
The system SHALL automatically detect the presence of sensors based on hardware detection signals during initialization and runtime.

SR-DQC-002: Sensor Type Enforcement
The system SHALL enforce sensor-slot compatibility to prevent incorrect sensor installation and usage.

SR-DQC-003: Sensor Failure Detection
The system SHALL detect sensor failures including communication errors, out-of-range values, and non-responsive sensors.

SR-DQC-004: Machine Constants Management
The system SHALL maintain persistent Machine Constants (MC) data including:

  • Installed sensor type definitions
  • Sensor calibration parameters
  • Communication configuration parameters
  • System identity parameters

SR-DQC-005: Calibration Parameter Application
The system SHALL apply calibration parameters from MC data to raw sensor readings to produce calibrated values.

3.1.3 Communication Requirements

SR-COM-001: Main Hub Communication
The system SHALL provide bidirectional encrypted communication with a Main Hub to:

  • Send sensor data using CBOR encoding
  • Send diagnostic information
  • Receive configuration updates
  • Receive firmware updates

SR-COM-002: Secure Communication Protocols
The system SHALL use mTLS 1.2 with X.509 certificates for all Main Hub communication.

SR-COM-003: On-Demand Data Broadcasting
The system SHALL transmit the most recent sensor dataset upon request from the Main Hub within 5 seconds.

SR-COM-004: Peer Communication
The system SHALL support limited peer-to-peer communication with other Sub-Hubs using ESP-NOW for:

  • Connectivity checks
  • Time synchronization support
  • Basic status exchange

SR-COM-005: Communication Fault Tolerance
The system SHALL continue autonomous operation during Main Hub communication failures for up to 24 hours.

3.1.4 Persistence & Data Management Requirements

SR-DATA-001: Persistent Sensor Data Storage
The system SHALL store sensor data persistently on SD Card using FAT32 file system with wear-aware batch writing.

SR-DATA-002: Data Persistence Abstraction
The system SHALL provide a unified Data Persistence (DP) component that abstracts storage media access for all persistent data operations.

SR-DATA-003: Safe Data Handling During Transitions
The system SHALL ensure all critical data is safely persisted before:

  • Firmware updates
  • Configuration updates
  • System teardown
  • Reset or restart operations

SR-DATA-004: Data Integrity Protection
The system SHALL implement data integrity mechanisms including checksums and atomic write operations to prevent data corruption.

SR-DATA-005: Storage Capacity Management
The system SHALL manage storage capacity by implementing circular logging with configurable retention periods.

3.1.5 Firmware Update (OTA) Requirements

SR-OTA-001: OTA Update Negotiation
The system SHALL implement an OTA handshake mechanism with the Main Hub to acknowledge update availability and signal readiness.

SR-OTA-002: Firmware Reception and Storage
The system SHALL securely receive firmware images and store them temporarily on SD Card before activation.

SR-OTA-003: Firmware Integrity Validation
The system SHALL validate firmware integrity using SHA-256 checksums before activation.

SR-OTA-004: Safe Firmware Activation
The system SHALL implement A/B partitioning with automatic rollback capability for safe firmware activation.

SR-OTA-005: OTA State Management
The system SHALL coordinate OTA operations with the system state machine to ensure safe transitions and data preservation.

3.1.6 Security & Safety Requirements

SR-SEC-001: Secure Boot
The system SHALL implement Secure Boot V2 to ensure only authenticated firmware is executed.

SR-SEC-002: Flash Encryption
The system SHALL implement Flash Encryption using AES-256 to protect sensitive data and intellectual property.

SR-SEC-003: Certificate Management
The system SHALL manage X.509 certificates for mTLS communication with secure storage and validation.

SR-SEC-004: Security Violation Handling
The system SHALL detect and respond to security violations including:

  • Boot verification failures
  • Certificate validation failures
  • Unauthorized access attempts

3.1.7 Diagnostics & Health Monitoring Requirements

SR-DIAG-001: Diagnostic Code Management
The system SHALL implement structured diagnostics with:

  • Unique diagnostic codes (format: 0xSCCC)
  • Severity levels (INFO, WARNING, ERROR, FATAL)
  • Root cause hierarchy
  • Timestamp information

SR-DIAG-002: Diagnostic Data Storage
The system SHALL persistently store diagnostic events in a circular log with configurable retention.

SR-DIAG-003: Diagnostic Session Support
The system SHALL provide diagnostic sessions allowing authorized engineers to:

  • Retrieve diagnostic data
  • Inspect system health status
  • Clear diagnostic records

SR-DIAG-004: Layered Watchdog System
The system SHALL implement multiple watchdog levels:

  • Task-level watchdogs for critical tasks
  • Interrupt watchdog for system responsiveness
  • RTC watchdog for ultimate system recovery

3.1.8 System Management Requirements

SR-SYS-001: System State Machine
The system SHALL implement a finite state machine with the following states:

  • INIT, BOOT_FAILURE, RUNNING, WARNING, FAULT
  • OTA_PREP, OTA_UPDATE, MC_UPDATE, TEARDOWN
  • SERVICE, SD_DEGRADED

SR-SYS-002: State-Aware Operation
The system SHALL restrict feature operations based on current system state according to defined state transition rules.

SR-SYS-003: Controlled Teardown
The system SHALL execute a controlled teardown sequence that:

  • Stops sensor acquisition
  • Flushes all critical data
  • Ensures storage consistency
  • Prepares for target state transition

SR-SYS-004: Local Human-Machine Interface
The system SHALL provide local status indication using:

  • OLED display (128x64, I2C interface)
  • Three-button navigation (Up, Down, Select)
  • Menu system for diagnostics and sensor status

SR-SYS-005: Engineering Access
The system SHALL support authenticated engineering sessions for:

  • Log inspection
  • Machine constants inspection and update
  • Controlled debugging operations

3.1.9 Power & Fault Handling Requirements

SR-PWR-001: Brownout Detection
The system SHALL detect power supply brownout conditions and initiate controlled shutdown procedures.

SR-PWR-002: Power-Loss Recovery
The system SHALL implement power-loss recovery mechanisms to restore operation after power restoration.

SR-PWR-003: Fault Classification
The system SHALL classify faults into categories:

  • Sensor faults, Communication faults, Storage faults
  • Power faults, Security faults, Software faults, Hardware faults

SR-PWR-004: Fault Escalation
The system SHALL implement fault escalation procedures based on severity and frequency.

3.1.10 Hardware Abstraction Requirements

SR-HW-001: Sensor Abstraction Layer
The system SHALL implement a Sensor Abstraction Layer (SAL) that provides uniform interfaces for different sensor types.

SR-HW-002: Hardware Interface Abstraction
The system SHALL abstract hardware interfaces (GPIO, I2C, SPI, UART, ADC) through driver layers to enable portability.

SR-HW-003: GPIO Discipline
The system SHALL implement GPIO discipline with defined ownership and access control for hardware resources.

3.2 Non-Functional Requirements

3.2.1 Performance Requirements

SR-PERF-001: Sensor Acquisition Timing
The system SHALL complete sensor acquisition cycles within 1 second for all enabled sensors.

SR-PERF-002: Communication Response Time
The system SHALL respond to Main Hub requests within 5 seconds under normal operating conditions.

SR-PERF-003: Memory Usage
The system SHALL operate within 80% of available SRAM (409.6KB) and Flash (6.4MB) capacity.

SR-PERF-004: Storage Performance
The system SHALL achieve minimum 10KB/s write performance to SD Card for data logging.

3.2.2 Reliability Requirements

SR-REL-001: System Availability
The system SHALL achieve 99.9% availability during normal operating conditions.

SR-REL-002: Mean Time Between Failures
The system SHALL achieve MTBF of 8760 hours (1 year) under specified environmental conditions.

SR-REL-003: Fault Recovery
The system SHALL automatically recover from transient faults within 30 seconds.

SR-REL-004: Data Integrity
The system SHALL maintain data integrity with error rate < 1 in 10^6 operations.

3.2.3 Safety Requirements

SR-SAFE-001: Fail-Safe Operation
The system SHALL fail to a safe state (sensor data marked invalid) upon detection of critical faults.

SR-SAFE-002: Sensor Data Validation
The system SHALL validate sensor data ranges and mark out-of-range values as invalid.

SR-SAFE-003: Watchdog Protection
The system SHALL implement multiple watchdog mechanisms to detect and recover from software failures.

3.2.4 Security Requirements

SR-SEC-005: Authentication
The system SHALL authenticate all external communication using certificate-based mutual authentication.

SR-SEC-006: Data Encryption
The system SHALL encrypt all sensitive data at rest using AES-256 encryption.

SR-SEC-007: Secure Communication
The system SHALL encrypt all network communication using TLS 1.2 or higher.

SR-SEC-008: Access Control
The system SHALL implement role-based access control for engineering and diagnostic functions.

3.2.5 Maintainability Requirements

SR-MAINT-001: Diagnostic Accessibility
The system SHALL provide comprehensive diagnostic information accessible through local and remote interfaces.

SR-MAINT-002: Configuration Management
The system SHALL support field configuration updates through authenticated channels.

SR-MAINT-003: Firmware Updateability
The system SHALL support secure over-the-air firmware updates with rollback capability.

3.2.6 Portability Requirements

SR-PORT-001: Hardware Abstraction
The system SHALL abstract hardware dependencies to enable porting to compatible microcontroller platforms.

SR-PORT-002: Standard Interfaces
The system SHALL use standard communication protocols (I2C, SPI, UART) for sensor interfaces.

3.3 Interface Requirements

3.3.1 Hardware Interfaces

SR-HW-IF-001: Sensor Interfaces
The system SHALL provide the following sensor interfaces:

  • 4x I2C interfaces for digital sensors
  • 2x SPI interfaces for high-speed sensors
  • 2x UART interfaces for serial sensors
  • 4x ADC channels for analog sensors

SR-HW-IF-002: Communication Interfaces
The system SHALL provide:

  • Wi-Fi 802.11n (2.4 GHz) interface
  • Optional Zigbee/LoRa interface for backup communication

SR-HW-IF-003: Storage Interfaces
The system SHALL provide:

  • SD Card interface (SPI-based)
  • Internal NVM (encrypted)

SR-HW-IF-004: User Interface
The system SHALL provide:

  • OLED display interface (I2C, 128x64 pixels)
  • 3x GPIO inputs for buttons (Up, Down, Select)

3.3.2 Software Interfaces

SR-SW-IF-001: Main Hub Protocol
The system SHALL implement the Main Hub communication protocol using:

  • MQTT over TLS 1.2 transport
  • CBOR message encoding
  • Defined message schemas for sensor data, diagnostics, and control

SR-SW-IF-002: Peer Communication Protocol
The system SHALL implement peer communication using ESP-NOW protocol with defined message formats.

SR-SW-IF-003: Diagnostic Interface
The system SHALL provide diagnostic interface supporting:

  • Log retrieval commands
  • System status queries
  • Configuration inspection commands

4. System Architecture Requirements

4.1 Architectural Constraints

SR-ARCH-001: Layered Architecture
The system SHALL implement a strict layered architecture with dependency flow from Application → Drivers → OSAL → HAL.

SR-ARCH-002: Hardware Abstraction
Application logic SHALL NOT access hardware directly and SHALL use driver abstractions exclusively.

SR-ARCH-003: State-Aware Execution
All system features SHALL be aware of current system state and respect state-based operation restrictions.

SR-ARCH-004: Event-Driven Communication
Internal component communication SHALL use event-driven publish/subscribe mechanisms.

SR-ARCH-005: Data Persistence Abstraction
All persistent data access SHALL be performed through the Data Persistence (DP) component.

4.2 Component Requirements

SR-COMP-001: State Manager Component
The system SHALL implement a State Manager (STM) component responsible for:

  • System state machine implementation
  • State transition validation and coordination
  • Teardown sequence management

SR-COMP-002: Event System Component
The system SHALL implement an Event System component providing:

  • Publish/subscribe event bus
  • Non-blocking event delivery
  • Event type management

SR-COMP-003: Sensor Manager Component
The system SHALL implement a Sensor Manager component responsible for:

  • Sensor lifecycle management
  • Data acquisition scheduling
  • Local filtering and validation

SR-COMP-004: Data Persistence Component
The system SHALL implement a Data Persistence (DP) component providing:

  • Storage media abstraction
  • Data serialization/deserialization
  • Wear-aware storage management

5. Verification Requirements

5.1 Verification Methods

Each system requirement SHALL be verified using one or more of the following methods:

  • Test (T): Verification by test execution
  • Analysis (A): Verification by analysis or calculation
  • Inspection (I): Verification by inspection or review
  • Demonstration (D): Verification by demonstration

5.2 Verification Levels

  • Unit Level: Individual component verification
  • Integration Level: Component interaction verification
  • System Level: End-to-end system verification
  • Acceptance Level: User acceptance verification

6. Traceability

This SRS provides the foundation for:

  • Software Requirements Specification (SWR-XXX requirements)
  • Feature specifications (F-XXX features)
  • Component specifications (C-XXX components)
  • Test specifications and procedures

All system requirements SHALL be traceable to:

  • Stakeholder needs and use cases
  • Feature definitions in Features.md
  • Architectural constraints in Cross-Feature Constraints.md

Document Status: Final for Implementation Phase
Next Review: After Software Requirements Specification completion