RC Submarine Controller Development Skill

Firmware development for a Raspberry Pi Pico-based RC submarine control system with active ballast, depth hold, pitch stabilization, and hardware failsafe protection.

Project Overview

• Platform: Raspberry Pi Pico (RP2040 dual-core ARM Cortex-M0+)
• Input: Standard 6-channel RC receiver (PWM)
• Sensors: MS5837 pressure sensor, MPU-6050 IMU, leak detector, battery monitor
• Actuators: Ballast pump, solenoid valve, control surface servos
• Architecture: Dual-core (Core 0: safety monitor, Core 1: control logic)

Quick Reference

Pin Assignments

See references/pinout.md for complete GPIO mapping.

Build Commands

# Configure
mkdir build && cd build
cmake -DPICO_SDK_PATH=~/pico-sdk ..

# Build
make -j4

# Flash (with Pico in BOOTSEL mode)
cp rc_sub_controller.uf2 /media/RPI-RP2

# Static analysis
cppcheck --enable=all --error-exitcode=1 src/

Project Structure

rc-sub-controller/
├── CMakeLists.txt
├── src/
│   ├── main.c              # Entry, dual-core setup
│   ├── config.h            # Pin assignments, constants
│   ├── types.h             # Shared data types
│   ├── drivers/            # Hardware abstraction
│   ├── control/            # PID, state machines
│   ├── safety/             # Core 0 safety monitor
│   └── util/               # Logging, CRC, assertions
├── pio/                    # PIO programs
└── test/                   # Unit tests

Coding Standards (Power of 10 Lite)

Apply these mandatory rules to all code:

1. No dynamic memory after init - Use static buffers only; no malloc/free in control loops
2. Fixed loop bounds - All loops must have provable iteration limits
3. Check all return values - Never ignore errors; propagate up call chain
4. Compile with -Wall -Werror - All warnings are errors
5. Run cppcheck - Zero errors before commit
6. Assert preconditions - Minimum 1 assert per function

Assert Macro

#define P10_ASSERT(cond) do { \
    if (!(cond)) { \
        log_fatal("ASSERT: %s:%d: %s", __FILE__, __LINE__, #cond); \
        trigger_emergency_blow(EVT_ASSERT_FAIL); \
    } \
} while(0)

Assert failures trigger emergency surface—never hang.

Dual-Core Architecture

┌─────────────────────────────────────────────────────────────────┐
│  CORE 0 (Safety)              │  CORE 1 (Control)              │
│  • Watchdog feeding           │  • RC input capture            │
│  • Fault detection            │  • Sensor polling              │
│  • Emergency blow             │  • PID loops                   │
│  • Battery/leak monitor       │  • State machine               │
│  • 100 Hz loop                │  • 50 Hz loop                  │
│  • CANNOT be blocked          │  • Can be complex              │
└─────────────────────────────────────────────────────────────────┘
                    Shared State (lock-free, atomic flags)

Core 0 runs independently—if Core 1 crashes, Core 0 still triggers emergency surface.

Driver Implementation Guide

RC Input (PIO-based PWM Capture)

Use PIO for accurate PWM measurement. See references/drivers.md for complete implementation.

// Output structure
typedef struct {
    uint16_t channels[6];  // 1000-2000 µs
    uint32_t timestamp_ms;
    bool valid;
} RcFrame_t;

// Convert to signed percent
int8_t pwm_to_percent(uint16_t pulse_us) {
    int16_t centered = (int16_t)pulse_us - 1500;
    int16_t scaled = (centered * 100) / 500;
    return (int8_t)CLAMP(scaled, -100, 100);
}

Pressure Sensor (MS5837)

I2C driver with temperature compensation. See references/drivers.md.

typedef struct {
    int32_t depth_cm;      // Depth in centimeters
    int32_t temp_c_x10;    // Temperature × 10
    uint32_t timestamp_ms;
    bool valid;
} DepthReading_t;

IMU (MPU-6050)

Complementary filter for pitch/roll. See references/drivers.md.

typedef struct {
    int16_t pitch_deg_x10;  // Pitch × 10 (0.1° resolution)
    int16_t roll_deg_x10;   // Roll × 10
    uint32_t timestamp_ms;
    bool valid;
} AttitudeReading_t;

Control System

PID Controller

Generic PID with anti-windup. See references/control.md.

typedef struct {
    float kp, ki, kd;
    float integral;
    float prev_error;
    float integral_limit;
    float output_min, output_max;
} PidController_t;

float pid_update(PidController_t* pid, float setpoint, float measurement, float dt);
void pid_reset(PidController_t* pid);

State Machine

See references/control.md for complete state machine specification.

States: INIT → SURFACE → DIVING → SUBMERGED_MANUAL
                                → SUBMERGED_DEPTH_HOLD
                          ↓
                     SURFACING → SURFACE
                          ↓
        Any state → EMERGENCY (on fault)

Safety System

Failsafe Triggers

Any of these conditions trigger emergency surface:

1. Signal loss - No valid RC frame for >3 seconds
2. Low battery - Voltage below threshold (e.g., 6.4V for 2S LiPo)
3. Leak detected - Water sensor triggers
4. Depth exceeded - Beyond configured max depth
5. Pitch exceeded - Beyond ±45° (configurable)

Emergency Blow Sequence

void trigger_emergency_blow(uint8_t reason) {
    // Atomic flag—cannot be undone except by power cycle
    g_emergency_active = true;
    
    // 1. Open vent valve (release air from ballast)
    valve_open();
    
    // 2. Run pump in reverse (expel water)
    pump_set_speed(-100);
    
    // 3. Full up on control surfaces
    servo_set_position(SERVO_BOWPLANE, +100);
    servo_set_position(SERVO_STERNPLANE, +100);
    
    // 4. Log the event
    log_event(EVT_EMERGENCY_BLOW, reason);
}

Task Checklist

Execute scripts/task_status.py to see current progress. Tasks are independent—complete in any order.

Foundation (F1-F5)

• F1: Project scaffold (CMakeLists.txt, directories)
• F2: Blink test (confirm toolchain)
• F3: Dual-core hello (both cores running)
• F4: Config header (pin assignments)
• F5: Types header (shared structs)

Drivers (D1-D8)

• D1: RC input driver (PIO PWM capture)
• D2: Pressure sensor (MS5837 I2C)
• D3: IMU driver (MPU-6050 I2C)
• D4: Pump driver (PWM output)
• D5: Valve driver (GPIO)
• D6: Servo driver (PWM output)
• D7: Battery monitor (ADC)
• D8: Leak detector (GPIO interrupt)

Control (C1-C5)

• C1: PID module
• C2: Depth controller
• C3: Pitch controller
• C4: Ballast state machine
• C5: Main state machine

Safety (S1-S4)

• S1: Safety monitor (Core 0)
• S2: Emergency blow sequence
• S3: Watchdog setup
• S4: Fault logging

Integration (I1-I5)

• I1: Wire everything
• I2: Bench test
• I3: Dry run
• I4: Wet test (bathtub)
• I5: Pond test

File Generation

When generating code files for this project:

1. Always include the standard header:

   /**
    * RC Submarine Controller
    * [filename] - [brief description]
    * 
    * Power of 10 compliant
    */
   

2. Use the type definitions from types.h - See references/types.md

3. Follow the naming conventions:

   • Functions: module_action() (e.g., pump_set_speed())
   • Types: CamelCase_t (e.g., RcFrame_t)
   • Constants: UPPER_CASE (e.g., MAX_DEPTH_CM)
   • Files: lowercase.c/.h

4. Run the validation script after generating code:

   scripts/validate_code.py path/to/file.c
   

Claude CLI Quick Start

If using Claude CLI (Claude Code), see CLAUDE.md for full instructions.

# One-time setup
./scripts/setup.sh              # Install all dependencies
python3 scripts/init_project.py . # Initialize project structure  
make install-hooks              # Setup git hooks

# Daily workflow
make ci-quick                   # Quick checks
make test                       # Run unit tests
make build                      # Build firmware

The CI system enforces Power of 10 rules automatically on commit.

Reference Documents

• CLAUDE.md - Claude CLI onboarding guide
• references/pinout.md - Complete GPIO pin assignments
• references/drivers.md - Driver implementation details
• references/control.md - PID and state machine specifications
• references/types.md - All type definitions
• references/safety.md - Safety system specifications
• references/hardware.md - BOM and wiring diagrams

CI System

• ci/run_ci.sh - Main CI runner (all checks)
• ci/p10_check.py - Power of 10 compliance checker
• ci/install_hooks.sh - Git hook installer
• test/run_tests.sh - Unit test runner
• test/test_harness.h - Minimal test framework

Scripts

• scripts/init_project.py - Initialize project directory structure
• scripts/validate_code.py - Check Power of 10 compliance
• scripts/task_status.py - Show task completion status
• scripts/generate_driver.py - Generate driver boilerplate

Related Documents

This skill supports development based on:

• RC_Submarine_Controller_Spec_NASA_JPL_v2.docx (full specification)
• RC_Submarine_Controller_Kickoff.docx (project plan)