Drone RX Firmware v2.0 - Professional Modular Architecture

Advanced receiver firmware for micro quadcopter based on XIAO ESP32-S3 with professional modular design, cascaded PID control, and ESP-NOW telemetry.

🚁 Overview

This is a complete refactored drone receiver firmware that transformed from a single 437-line Arduino sketch into a professional, modular embedded system. The firmware implements sophisticated flight control algorithms while maintaining 100% behavioral compatibility with the original code.

Key Features

• Dual Control Modes: ANGLE mode (roll/pitch) + RATE mode (yaw)
• Advanced PID Control: Cascaded PIDs with anti-windup and conditional integration
• ESP-NOW Communication: Bi-directional (RC uplink + telemetry downlink)
• X-Frame Mixer: Optimized for CW/CCW motor arrangement with anti-clip
• Professional Safety: Arming logic, failsafe, throttle gating
• Modular Architecture: 23 files with clean separation of concerns

📡 Hardware Configuration

Microcontroller

• Board: Seeed Studio XIAO ESP32-S3
• CPU: ESP32-S3 @ 240 MHz
• Framework: Arduino + ESP-IDF APIs
• Language: C++17 with PlatformIO

Motors (4× Brushed Coreless)

Pin Mapping:
├── M1 (Front-Left, CW)  → GPIO 7, LEDC Ch0
├── M2 (Front-Right, CCW)→ GPIO 4, LEDC Ch1  
├── M3 (Rear-Left, CW)   → GPIO 3, LEDC Ch2
└── M4 (Rear-Right, CCW) → GPIO 1, LEDC Ch3

PWM: 12 kHz, 10-bit resolution (0-1023)

IMU (MPU6050)

Interface: I²C @ 400 kHz
├── SDA → GPIO 6
└── SCL → GPIO 5

Configuration:
├── Gyroscope: ±500 dps range
├── Accelerometer: ±2g range  
├── DLPF: ~42 Hz bandwidth
└── Calibration: 500-sample gyro bias on boot

Radio (ESP-NOW)

Mode: Bidirectional
├── Uplink: RC command reception
├── Downlink: Telemetry transmission @ 2 Hz
├── Channel: 1 (configurable)
└── Range: ~100m line-of-sight

🛠️ Quick Start

Build Requirements

# Install PlatformIO
pip install platformio

# Clone and build
git clone <repository-url>
cd drone-rx-firmware
pio run

# Upload to board  
pio run -t upload

# Monitor serial output
pio device monitor -b 115200

Hardware Setup

1. Connect XIAO ESP32-S3 via USB-C
2. Wire MPU6050: SDA→GPIO6, SCL→GPIO5, VCC→3.3V, GND→GND
3. Connect Motors: GPIO 7,4,3,1 to motor controllers
4. Power: 1S LiPo (3.7V) with appropriate current capability

Configuration

1. Set Controller MAC: Edit CTRL_MAC[] in telemetry.cpp
2. Tune PIDs: Modify constants in config.h
3. Adjust Parameters: Flight limits, deadbands, etc. in config.h

🎮 RC Packet Format

The firmware expects RC packets in this exact binary format:

struct RcPacket {
    uint8_t  magic;      // 0xA5
    uint8_t  version;    // 1  
    uint16_t seq;        // Sequence number
    uint16_t thr;        // Throttle: 0..1000
    int16_t  yaw;        // Yaw: -1000..1000
    int16_t  pitch;      // Pitch: -1000..1000
    int16_t  roll;       // Roll: -1000..1000  
    uint8_t  flags;      // bit0 = ARM (1=armed)
    uint8_t  rssi_hint;  // Optional RSSI
    uint16_t crc;        // CRC-16/X.25
} __attribute__((packed));

📊 Telemetry Downlink

Real-time flight data transmitted at 2 Hz to ground station:

struct TelemetryPacket {
    uint8_t  magic;           // 0xB5
    uint8_t  version;         // 1
    uint16_t seq;             // Sequence  
    uint32_t ms;              // Timestamp
    
    // Setpoints (degrees × 10 for precision)
    int16_t  setAngleRoll_x10, setAnglePitch_x10;
    int16_t  setYawRate_dps;
    
    // Current attitude (degrees × 10)
    int16_t  roll_deg_x10, pitch_deg_x10;
    
    // Angular rates (deg/s)
    int16_t  rollRate_dps, pitchRate_dps, yawRate_dps;
    
    // PID outputs
    int16_t  outRoll, outPitch, outYaw;
    
    // Motor mixer values (0-1000)
    uint16_t m1, m2, m3, m4;
    
    // Status
    uint16_t thr;        // Throttle
    uint8_t  armed;      // Armed state
    uint8_t  linkAlive;  // RC link status
    
    uint16_t crc;        // CRC-16/X.25
} __attribute__((packed));

🎛️ Control Architecture

Flight Control Flow

RC Input → Deadband → Setpoints
                          ↓
IMU → Attitude Estimation (Complementary Filter)
                          ↓  
Angle PID → Rate Setpoint → Rate PID → Mixer → Motors
                          ↑
                    Gyro Rates

Control Modes

• Roll/Pitch: ANGLE mode with cascaded control
  • Outer loop: Angle PID (degrees → deg/s)
  • Inner loop: Rate PID (deg/s → motor output)
• Yaw: Direct RATE mode with conditional integration
• Throttle: Direct pass-through with arming ramp

Safety Features

• Arming Logic: Requires both link + ARM flag
• Soft Ramping: 0.5s ramp-up on ARM, instant cutoff on DISARM
• Failsafe: 200ms RC timeout → automatic disarm
• Throttle Gating: No stabilization below 5% throttle
• Anti-Windup: Conditional integration to prevent PID saturation

⚙️ PID Tuning

Default gains optimized for micro quadcopters:

// Angle PIDs (degrees → deg/s)
#define PID_ANGLE_ROLL_KP   3.0f
#define PID_ANGLE_ROLL_KD   0.05f

// Rate PIDs (deg/s → output)  
#define PID_RATE_ROLL_KP    0.12f
#define PID_RATE_ROLL_KD    0.002f

// Yaw with integral term
#define PID_RATE_YAW_KP     0.15f
#define PID_RATE_YAW_KI     0.02f
#define PID_RATE_YAW_KD     0.001f

Tuning Procedure

1. Start Conservative: Reduce all gains by 50%
2. Tune Rate PIDs First: Increase Kp until oscillations appear, back off 20%
3. Add Damping: Increase Kd to reduce oscillations
4. Tune Angle PIDs: Similar process for outer loop
5. Add Yaw Integration: Minimal Ki to eliminate steady-state error

🏗️ Project Architecture

Module Organization

firmware/
├── include/           # Header files (interfaces)
│   ├── config.h       # All configuration constants
│   ├── types.h        # Data structures & types
│   ├── pid.h          # PID controller classes  
│   ├── imu.h          # IMU sensor interface
│   ├── mixer.h        # Motor mixing algorithms
│   ├── safety.h       # Arming & failsafe logic
│   ├── telemetry.h    # Downlink telemetry
│   └── [8 more...]    # Other module interfaces
└── src/               # Implementation files  
    ├── main.cpp       # Main control loop
    ├── pid.cpp        # PID implementations
    ├── imu.cpp        # MPU6050 driver
    └── [7 more...]    # Module implementations

Design Principles

• Single Responsibility: Each module has one clear purpose
• Clean Interfaces: Well-defined APIs between modules
• Zero Heap: No dynamic allocation in main control loop
• ISR Safety: Radio callbacks only touch volatile buffers
• Configurable: All parameters centralized in config.h

🔧 Advanced Features

Motor Control

• Slew Limiting: 800 duty/s rate limit prevents voltage sag
• Idle Floor: 8% minimum PWM when armed for instant response
• Anti-Clip Mixer: Preserves control authority when motors saturate
• Motor Scaling: Per-motor trim for CW/CCW torque balance

Attitude Estimation

• Complementary Filter: 98% gyro integration + 2% accelerometer
• Configurable Mapping: Easy IMU orientation changes
• Bias Calibration: Automatic gyro offset correction

Communication

• Packet Validation: Magic bytes + version + CRC verification
• ISR-Safe Reception: Minimal processing in interrupt context
• Non-Blocking Transmission: Telemetry never blocks control loop

🐛 Debugging & Testing

Serial Commands

• T: Motor test sequence (20% power, 400ms each)

Debug Output (200ms interval)

ARM:1 THR:450 setAng R:15.2 P:-8.1 ang R:14.8 P:-7.9 
rate R:125.4 P:-89.2 Y:12.3 out R:45.2 P:-23.1 Y:8.7 
base:450.0 m:[523 389 427 495]

Common Issues

• IMU Fail: Check I2C wiring and 3.3V power
• ESP-NOW Fail: Verify controller MAC address
• Motor Issues: Test with 'T' command, check PWM connections
• No ARM: Ensure RC link active and ARM flag set

📈 Performance

Real-Time Specs

• Control Loop: ~500 Hz (2ms typical, max 20ms)
• IMU Reading: 400 kHz I2C, ~1ms
• Motor Update: 12 kHz PWM, hardware-generated
• Telemetry: 2 Hz downlink, <1ms processing
• Memory: <32KB flash, <4KB RAM

Latency Budget

RC Reception:    <5ms  (ESP-NOW + validation)
Control Calc:    ~2ms  (PID + mixer + filters)
Motor Output:    <1ms  (PWM register update)
Total Latency:   <8ms  (stick to motor response)

🚀 Future Enhancements

Planned Features

• Battery Monitoring: ADC-based voltage sensing
• Flight Modes: Acro, Horizon, GPS hold
• Blackbox Logging: SD card flight data recording
• Parameter Tuning: Runtime PID adjustment via telemetry
• Multiple Profiles: Switchable flight characteristics

Extension Points

• Additional Sensors: Barometer, magnetometer, GPS
• Advanced Control: Optical flow, position hold
• Smart Features: Return-to-home, waypoint navigation
• Connectivity: WiFi configuration, smartphone app

📜 License & Support

This firmware is designed for educational and research purposes.

Contributing

1. Fork the repository
2. Create feature branch (git checkout -b feature/amazing-feature)
3. Commit changes (git commit -m 'Add amazing feature')
4. Push branch (git push origin feature/amazing-feature)
5. Open Pull Request

Support

• 📖 Documentation: Check README.md and code comments
• 🐛 Issues: Use GitHub Issues for bug reports
• 💬 Discussions: GitHub Discussions for questions
• 📧 Direct: Contact maintainers for critical issues

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⚠️ Safety Notice: This is experimental flight control software. Always test in safe environments with proper safety equipment. Never fly over people or property without appropriate precautions.

🚁 Happy Flying! ✈️