Week 3: Binary Protocol & State Machine

STM32F446RE-based LoRa sensor network with binary serialization, CRC integrity checking, and reliable transmission state machine.

Overview

This repository builds on Week 2's text-based LoRa communication to implement a production-grade binary protocol with:

• Postcard serialization for efficient binary encoding
• CRC-16 integrity checking for data validation
• ACK/retry state machine for reliable delivery
• Sequence number tracking for duplicate detection
• Performance comparison between text and binary protocols

What's New in Week 3

From Week 2 (Text Protocol)

Payload: "T:27.1H:56.0G:74721#0003" (24 bytes)
AT Command: "AT+SEND=2,24,T:27.1H:56.0G:74721#0003" (39 bytes total)
No integrity checking, no acknowledgments, no retries

Week 3 (Binary Protocol)

Payload: Binary struct (12-16 bytes) + CRC-16 (2 bytes)
Transmission: Automatic retries on failure
Reception: ACK sent back to transmitter
Integrity: CRC validation on every packet
Efficiency: ~40% size reduction vs text

Hardware Configuration

Same dual-node setup from Week 2:

Node 1 - Remote Sensor Node (Main Binary)

• MCU: STM32F446RET6 (Cortex-M4F @ 84 MHz, HSI clock)
• Board: NUCLEO-F446RE
• Radio: REYAX RYLR998 LoRa Module (UART4 @ 115200 baud)
• Primary Sensor: SHT31-D (Temperature & Humidity - Golden Reference)
• Secondary Sensor: BME680 (Gas Resistance, Temperature, Humidity, Pressure)
• Display: SSD1306 OLED 128x64 I2C
• Power: Split-rail (Elegoo MB102 for LoRa, Nucleo for Logic)
• Debug: LED on PA5
• ST-Link Probe: 0483:374b:0671FF3833554B3043164817

Node 2 - Gateway/Receiver (node2 Binary)

• MCU: STM32F446RET6 (Cortex-M4F @ 84 MHz, HSI clock)
• Board: NUCLEO-F446RE
• Radio: REYAX RYLR998 LoRa Module (UART4 @ 115200 baud)
• Sensor: BMP280 (Temperature & Pressure - local reference)
• Display: SSD1306 OLED 128x64 I2C
• Power: USB-powered via ST-Link
• Debug: LED on PA5
• ST-Link Probe: 0483:374b:066DFF3833584B3043115433

Pin Configuration

Peripheral	Protocol	Pin(s)	Function
LED	GPIO	PA5	Status indicator (toggles @ 1 Hz)
I2C1 SCL	I2C	PB8	Sensor & Display bus clock
I2C1 SDA	I2C	PB9	Sensor & Display bus data
UART4 TX	UART	PC10	LoRa module transmit
UART4 RX	UART	PC11	LoRa module receive

Week 3 Objectives

Day 1: Binary Protocol Design

• Define message types (Data, ACK, NACK)
• Create Serde-compatible structs
• Implement CRC-16 calculation
• Design packet framing format

Day 2: Postcard Integration

• Integrate postcard serialization
• Test serialization/deserialization on desktop
• Port to embedded (no_std)
• Measure serialized payload sizes

Day 3: State Machine Implementation

• Design TX state machine (Idle → Sending → WaitACK → Retry/Success)
• Design RX state machine (Listen → Validate → SendACK)
• Implement timeout handling
• Add retry logic with backoff

Day 4: Integration & Testing

• Replace text protocol with binary in Node 1
• Replace text parsing with binary in Node 2
• End-to-end testing
• Measure packet success rate

Day 5: Performance Analysis

• Compare payload sizes (text vs binary)
• Measure round-trip latency
• Test retry behavior with packet loss
• Document efficiency gains

Day 6: Optimization & Edge Cases

• Handle duplicate packets (sequence numbers)
• Test boundary conditions
• Optimize buffer sizes
• Add comprehensive logging

Day 7: Documentation & Review

• Performance comparison report
• Update PROTOCOL.md with binary format spec
• Complete NOTES.md with learnings
• Week 3 review and planning for Week 4

Performance Results (Week 3 Complete)

Binary Protocol Efficiency:

• Payload Size: 10 bytes (8 data + 2 CRC) vs 25 bytes text = 60% reduction
• Round-trip Latency: <1 second (observed in successful transmissions)
• Timeout Behavior: 2s × 3 attempts = 6 seconds total before giving up
• Success Rate: 70-80% in real-world conditions (64+ packets tested)

State Machine Validation:

• ✅ Graceful timeout handling with configurable retry count (MAX_RETRIES = 3)
• ✅ Automatic recovery when network conditions improve
• ✅ No crashes or hangs on repeated failures
• ✅ Clean state transitions (Idle ↔ WaitingForAck)

Real-World Testing:

• Tested with natural packet loss (packets #3, #4, #10, #13 failed during extended run)
• System continued operating through failures
• Immediate recovery on successful ACK reception
• CRC validation: 100% of received packets validated successfully

Range Test Results (2025-12-27):

Distance	RSSI	SNR	Success Rate	Environment
15m	-45 dBm	13 dB	100%	Through walls, no LoS
30m	-62 dBm	13 dB	99%	Wall obstacles
60m	-72 dBm	12 dB	99%	Wall obstacles
100m	-82 dBm	11 dB	99%	Downhill, trees, no LoS
150m	-91 dBm	4 dB	98%	Buildings & trees, no LoS
400m	-100 dBm	-2 dB	98%	Buildings & trees, no LoS
600m	-107 dBm	-6 dB	95%	Buildings & trees, no LoS

Key Findings:

• ✅ 600m range achieved through suburban obstacles (no line of sight)
• ✅ 95% success rate at 600m near LoRa sensitivity limit (-107 dBm)
• ✅ >98% success rate up to 400m even with negative SNR
• ✅ LoRa spread spectrum works below noise floor (functional at -6 dB SNR)
• ✅ Predictable signal degradation (~10 dBm per doubling of distance)

Range Test Visualization:

Graph shows RSSI, SNR, and packet loss vs distance. Generate with python3 plot_range_test.py

Week 3 Status: ✅ COMPLETE - All core objectives achieved, binary protocol operational with reliable delivery state machine, 600m range validated.

Building

Build Node 1 (Sensor Node)

cargo build --release

Build Node 2 (Gateway)

cargo build --release --bin node2

Flashing

Flash Node 1

cargo build --release && probe-rs run --probe 0483:374b:0671FF3833554B3043164817 --chip STM32F446RETx target/thumbv7em-none-eabihf/release/wk3-binary-protocol

Flash Node 2

cargo build --release --bin node2 && probe-rs run --probe 0483:374b:066DFF3833584B3043115433 --chip STM32F446RETx target/thumbv7em-none-eabihf/release/node2

Shell Aliases (Recommended)

# Node 1
alias n1='cargo build --release && probe-rs run --probe 0483:374b:0671FF3833554B3043164817 --chip STM32F446RETx target/thumbv7em-none-eabihf/release/wk3-binary-protocol'

# Node 2
alias n2='cargo build --release --bin node2 && probe-rs run --probe 0483:374b:066DFF3833584B3043115433 --chip STM32F446RETx target/thumbv7em-none-eabihf/release/node2'

Binary Protocol Specification

Message Types

#[derive(Serialize, Deserialize, Debug)]
pub enum MessageType {
    SensorData,
    Ack,
    Nack,
}

#[derive(Serialize, Deserialize, Debug)]
pub struct SensorDataPacket {
    pub seq_num: u16,
    pub temperature: i16,  // Centidegrees (e.g., 2710 = 27.1°C)
    pub humidity: u16,     // Basis points (e.g., 5600 = 56.0%)
    pub gas_resistance: u32,
    pub crc: u16,
}

Packet Format

[Length (1 byte)][Message Type (1 byte)][Payload (N bytes)][CRC-16 (2 bytes)]

Example SensorDataPacket:

• Length: 1 byte (e.g., 14)
• Type: 1 byte (0x01 = SensorData)
• Sequence: 2 bytes (u16)
• Temperature: 2 bytes (i16)
• Humidity: 2 bytes (u16)
• Gas: 4 bytes (u32)
• CRC: 2 bytes (u16)
• Total: ~14 bytes vs 24 bytes text

Dependencies

Key crates:

• stm32f4xx-hal = "0.23.0" - Hardware abstraction layer
• cortex-m-rtic = "1.1" - RTIC framework
• serde = "1.0" - Serialization framework (no_std)
• postcard = "1.0" - Binary serialization format
• crc = "3.0" - CRC calculation
• sht3x = "0.1.1" - SHT31 sensor driver
• bme680 = "0.6.0" - BME680 sensor driver
• ssd1306 = "0.8.4" - OLED display driver
• shared-bus = "0.3.1" - I2C bus sharing
• heapless = "0.8" - Stack-allocated data structures
• defmt-rtt = "0.4" - Logging via RTT

See Cargo.toml for complete dependency list.

Project Structure

wk3-binary-protocol/
├── src/
│   ├── main.rs          # Node 1 firmware (binary TX)
│   └── bin/
│       └── node2.rs     # Node 2 firmware (binary RX)
├── Cargo.toml           # Dependencies with Week 3 additions
├── memory.x             # Linker script for STM32F446
├── README.md            # This file
├── NOTES.md             # Learning insights
├── TROUBLESHOOTING.md   # Issues and solutions
└── PROTOCOL.md          # Binary protocol specification

Success Criteria

Week 3 Complete When:

• Binary protocol replaces text protocol
• CRC validation working on every packet
• ACK/NACK mechanism implemented
• Retry logic handles packet loss gracefully
• Sequence numbers prevent duplicate processing
• Performance comparison documented
• Payload size reduced by >30% vs text
• Packet delivery rate >95% (maintained from Week 2)

Foundation from Week 2

This repository starts with the working Week 2 codebase:

• ✅ Multi-sensor integration (BME680 + SHT31-D)
• ✅ Shared I2C bus with mutex management
• ✅ OLED display with sensor data
• ✅ LoRa transmission working end-to-end
• ✅ RTIC timing patterns established
• ✅ Simple, reliable UART interrupt handlers

Week 2 Achievement: Text-based LoRa communication with 100% packet delivery at close range, RSSI -20 to -22 dBm, SNR 13 dB.

Week 3 Goal: Same reliability with binary protocol, adding integrity checking and automatic retries.

Learning Resources

• Postcard Documentation: https://docs.rs/postcard/
• Serde no_std Guide: https://serde.rs/no-std.html
• CRC Theory: https://en.wikipedia.org/wiki/Cyclic_redundancy_check
• State Machine Patterns in Embedded Rust
• RTIC Shared Resources (from Week 2)

References

• Week 2 Repository: wk2-lora-sensor-fusion
• Overall Plan: 4-Month Plan
• Weekly Details: PLAN_WEEKLY.md

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Author: Antony (Tony) Mapfumo Part of the 12-Week IIoT Systems Engineer Transition Plan Week 3 of 12 - Binary Protocol & State Machine