SAMD21 RTIC Security Sensor

Embedded security sensor firmware for SAMD21 microcontroller using RTIC framework.

Overview

This firmware implements a security sensor system that monitors door state and vibration, communicates via LoRa (SX1278) with encrypted messages, and includes watchdog protection and key management.

Features

• ✅ Door sensor monitoring: Real-time door state detection via GPIO interrupt
• ✅ Vibration detection: Tamper detection using SW420 vibration sensor
• ✅ LoRa communication: Long-range wireless communication via SX1278
• ✅ Encrypted communication: ChaCha20Poly1305 encryption using Sigil protocol
• ✅ Hardware RNG: ATECC608x for Sigil handshake and encrypted event nonces
• ✅ Watchdog protection: 16-second watchdog timer with automatic reset detection
• ✅ Retry logic: Automatic retry for LoRa operations with 500ms delay
• ✅ Error handling: Comprehensive error handling with LED signaling

Hardware Requirements

• MCU: SAMD21G18A
• LoRa Module: SX1278
• Sensors:
  • Door sensor (magnetic switch) on PA06
  • Vibration sensor (SW420) on PA09
• Indicators:
  • Status LED on PA21
  • Buzzer on PA19 (GPIO, active buzzer)
  • Error LED on PA14
  • Init LED on PA18

Pin Configuration

Function	Pin	Description
LoRa SPI SCK	PB11	SPI clock
LoRa SPI MOSI	PB10	SPI data out
LoRa SPI MISO	PA12	SPI data in
LoRa NSS	PA15	SPI chip select
LoRa RST	PA20	Reset pin
LoRa DIO0	PA07	Interrupt pin (EIC Ch7)
Door Sensor	PA06	Door state (EIC Ch6)
Vibration	PA09	Vibration detection (EIC Ch9)
Entropy ADC	PA02	Analog noise source
Status LED	PA21	Alarm indicator
Buzzer	PA19	Active buzzer (GPIO on/off)
Error LED	PA14	Error indicator
Init LED	PA18	Initialization indicator

Architecture

Main Tasks

Task	Priority	Description
on_eic	4	External interrupt handler (LoRa DIO0, door, vibration)
watchdog_feed	2	Periodic watchdog feeding (every 4s)
process_lora_message	1	LoRa message processing (async)
send_sensor_event	1	Door state change event transmission (async)
install	1	Installation message sending (every 10s until validated)
idle	-	Main loop with WFI for power saving

Module Structure

src/
├── main.rs              # Main application and RTIC tasks
├── (moved)              # Business-logic config/state live in `contact_sensor`
├── device_id/           # Device UID reading (safe wrapper)
│   └── mod.rs
├── helpers/             # Helper modules
│   ├── alarm/           # Alarm control (LED/buzzer)
│   ├── lora/            # LoRa message handling
│   └── timeout/         # Timeout management utilities
├── tasks/               # RTIC tasks
│   └── install/         # Installation message sending
└── types/               # Type definitions
    └── mod.rs

Communication Protocol

The system uses the Sigil protocol for encrypted LoRa communication:

• Install messages: Broadcast to all gateways using derived install key
• Validate requests: Uses DEFAULT_KEY for initial validation
• Event messages: Uses validated session key or derived session key
• Encryption: ChaCha20Poly1305 with 12-byte nonces

Note: the firmware no longer depends on hkdf/sha2 directly; ECIES (HKDF-SHA256 + ChaCha20Poly1305) for the Sigil install handshake is handled inside the sigil crate.

Message Flow

1. Installation:

   • Sensor sends InstallSensorRequest every 10 seconds for 5 minutes
   • Continues until gateway responds with validation

2. Validation:

   • Gateway sends ValidateRequest using DEFAULT_KEY
   • Sensor responds with ValidateResponse containing session key
   • Session key stored in current_key with expiration

3. Event Reporting:

   • Door state changes trigger Event messages
   • Uses validated key if available, otherwise derived session key
   • Automatic retry on failure (1 retry, 500ms delay)

Security

Key Management

• Master Key: Stored in src/config/mod.rs (⚠️ hardcoded for development)
• Session Keys: Derived per-communication for enhanced security
• Install Keys: Derived without device_id for gateway compatibility

⚠️ Production Security Notes

• Master key should be loaded from Secure Element (Atecc608x) in production
• Never commit master keys to version control
• Use environment variables or secure storage for keys
• Implement key expiration verification (structure in place, logic pending)

Watchdog Timer

• Timeout: 16 seconds
• Feed interval: Every 4 seconds (4× safety margin)
• Clock: GCLK4 at 1024 Hz (separate from RTC)
• Reset detection: Reads RCAUSE register, lights error LED if watchdog reset

Building and Flashing

Prerequisites

• Rust toolchain (stable or beta)
• thumbv6m-none-eabi target: rustup target add thumbv6m-none-eabi
• OpenOCD or J-Link for flashing

Build

# Development build
cargo build --target thumbv6m-none-eabi

# Release build
cargo build --release --target thumbv6m-none-eabi

Flash

./deploy.sh                    # firmware @ 0x2000 (bootloader slot, default)
./deploy.sh false              # firmware @ 0x0 (bare-metal)
./deploy_direct.sh             # shortcut for ./deploy.sh false
./deploy_tmp.sh [true|false]   # deep_sleep bench

Manual (with bootloader slot):

cargo objcopy --release --bin samd21 --target thumbv6m-none-eabi -- -O binary firmware.bin
openocd -f samd21.cfg -c "init; reset halt; program firmware.bin 0x2000 verify reset; reset run; exit"

Debug (VS Code / Cursor)

1. Install extensions: rust-analyzer, Cortex-Debug (see .vscode/extensions.json).
2. Connect J-Link (SWD), close any running OpenOCD session.
3. Select Debug (OpenOCD) and press F5.

See .vscode/README.md for troubleshooting (missing SVD, OpenOCD on macOS, J-Link fallback).

Testing

See docs/TESTING_WITH_EMBEDDED_HAL_MOCK.md for testing strategy.

Unit Tests

cargo test --target thumbv6m-none-eabi

Validation Script

./scripts/run_tests.sh

Documentation

• Code Review: See revue.md for detailed code analysis and improvements
• Interrupt Optimizations: See OPTIMISATIONS_INTERRUPTIONS.md
• Testing Strategy: See docs/TESTING_WITH_EMBEDDED_HAL_MOCK.md
• Pragmatic Testing: See docs/TESTS_PRAGMATIC_APPROACH.md

Dependencies

• atsamd-hal: SAMD21 hardware abstraction layer
• rtic: Real-Time Interrupt-driven Concurrency framework
• embedded-hal: Hardware abstraction traits
• sigil: Communication protocol library (local)
• lora: LoRa SX1278 driver (local)
• tools: Utility library (local, encryption, random, MAC)

License

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Contributing

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