Project 10: Stepper Motor Sequencer

Drive a stepper motor with full-step and half-step sequences.

Quick Reference

Attribute	Value
Difficulty	Level 2: Intermediate
Time Estimate	8-12 hours
Main Programming Language	Python (Alternatives: C)
Alternative Programming Languages	C
Coolness Level	Level 3: Genuinely Clever
Business Potential	4. The “Open Core” Infrastructure
Prerequisites	Motor basics, driver boards
Key Topics	state machines, step sequences, torque vs speed

1. Learning Objectives

By completing this project, you will:

Explain the core question: How can you control motion without feedback sensors?
Implement the full hardware read/write path with correct configuration.
Handle at least two failure modes with clear error messages.
Validate output against a deterministic demo.

2. All Theory Needed (Per-Concept Breakdown)

Stepper Sequencing and State Machines

Fundamentals Stepper motors move in discrete steps by energizing coils in a sequence. It is not enough to know the high-level description; you must understand the exact sequence of configuration steps, the expected signals, and the hardware limits. GPIO outputs through a driver board is only reliable when you respect step timing, torque limits, and coil current. The goal is to build a mental model that connects software intent to physical reality, so you can reason about failures and verify results with measurements. You should be able to explain what each signal means, which register or API controls it, and how the device responds to configuration changes.

In embedded work, this conceptual clarity is the difference between trial-and-error and engineering. If you can predict how the system should behave, you can diagnose why it doesn’t. That is why this fundamentals section emphasizes not just definitions, but the sequence of actions and the reasons behind them.

Deep Dive into the concept Configuration comes first. Define full-step and half-step sequences and choose a safe step delay. In practice, you should start with conservative settings and validate each step before moving on. A wrong mode, wrong address, or wrong pin function often produces silent failures. This project forces you to verify the interface at the protocol level—reading an ID register, observing a waveform, or confirming a response—before trusting higher-level logic.

The next layer is the protocol or signaling format itself. With GPIO outputs through a driver board, every byte, pulse, or edge has meaning. You should be able to map software commands to the on-the-wire representation and back again. That means understanding register maps, frame formats, or pulse widths, and knowing which values are valid or reserved. When you can describe the precise shape of the data, you can validate correctness with a logic analyzer or raw byte logs.

Timing is the second pillar. Too-fast stepping causes missed steps; add acceleration ramps. Linux is not a real-time OS, so you must decide whether user-space timing is sufficient or whether hardware support is required. When you need deterministic behavior, you should use hardware peripherals or kernel-space timing. This project includes a deterministic golden demo so you can measure timing and compare against expectations.

Electrical constraints are unavoidable. Use ULN2003 or similar driver and external supply; share ground. These are not theoretical concerns; violating voltage or current limits can damage the board or produce unreliable signals. This project explicitly integrates safe wiring patterns, such as level shifting, driver boards, or separate power rails, and requires you to document them in your lab notes.

Reliability depends on error handling. Plan for NACKs, framing errors, timeouts, noisy inputs, or disconnected devices. A robust system retries, backs off, and logs clear diagnostic information so failures can be reproduced. In this project, you will implement explicit timeouts and sanity checks so that errors become visible events, not silent data corruption.

Debugging and validation complete the loop. Mark the shaft and count steps per revolution to verify sequences. The goal is to correlate what your code thinks is happening with what the hardware is actually doing. If you can see the waveform, log the raw bytes, and reproduce the golden demo, you can trust your system. If you cannot, you must adjust your assumptions and re-check each layer.

A deeper look at Stepper Motor Sequencer starts with sequencing. Even simple hardware interactions require a strict order: configure the interface, validate the device, perform the transaction, and only then interpret results. The key topics here ({key_topics}) each have parameters that must be chosen deliberately, such as bus speed, pin mode, edge polarity, or timing period. When these are wrong, failures can look random. The discipline is to set conservative defaults, verify each step with a minimal test (like reading a device ID or toggling a pin), and then increase complexity gradually. This mirrors real-world bring-up procedures on embedded boards, where one wrong assumption can waste hours.

Failure modes deserve special attention. Wiring errors, missing pull-ups, incorrect voltage levels, and pinmux conflicts are more common than software bugs. At the protocol layer, you may see NACKs, framing errors, or corrupted samples caused by wrong timing. At the OS layer, permission errors, device file contention, or missing overlays can block access to the hardware. A robust implementation anticipates these failures: it checks return codes, times out cleanly, and reports exactly what went wrong. In production systems, these checks are the difference between an intermittent field failure and a diagnosable incident.

How this fits on projects This concept is the foundation for this project and determines whether your implementation is reliable or fragile.

Definitions & key terms

Full-step: Two coils energized
Half-step: Alternate single/double coils
Step rate: Steps per second

Mental model diagram

State machine -> coil pattern -> rotor step

How it works (step-by-step)

Define sequence table
Output pattern
Delay
Advance index

Minimal concrete example

seq=[0b1000,0b1100,0b0100,0b0110,...];

Common misconceptions

Steppers never miss steps
GPIO can drive coils directly

Check-your-understanding questions

Why does torque drop at high speed?
Why use a driver board?

Check-your-understanding answers

Rotor can’t align fast enough.
GPIO cannot supply coil current.

Real-world applications

3D printers
CNC machines

Where you’ll apply it

See §5.4 and §5.10
Also used in: P16-industrial-iot-brain.md

References

Stepper motor datasheets
ULN2003 datasheet

Key insights Stepper control is a timing-sensitive state machine with mechanical limits.

Summary Correct sequencing and safe timing prevent missed steps and lost position.

Homework/Exercises to practice the concept

Implement acceleration ramp.

Solutions to the homework/exercises

Decrease delay gradually at start and increase at end.

3. Project Specification

3.1 What You Will Build

Drive stepper with sequence table.

3.2 Functional Requirements

Implement the primary hardware interaction
Provide CLI configuration
Log raw data and converted output
Handle error conditions

3.3 Non-Functional Requirements

Performance: Meets timing or throughput expectations for the device.
Reliability: Handles timeouts, disconnects, or missing devices safely.
Usability: Clear CLI flags and readable logs.

3.4 Example Usage / Output

python stepper.py --steps 512 --mode half

3.5 Data Formats / Schemas / Protocols

Step counts and direction logs.

3.6 Edge Cases

Too fast -> missed steps
Wrong sequence
No driver board

3.7 Real World Outcome

Motor rotates predictable angle per step count.

3.7.1 How to Run (Copy/Paste)

cd project-root
make
python stepper.py --steps 512 --mode half

3.7.2 Golden Path Demo (Deterministic)

Run python stepper.py --steps 512 --mode half with default wiring and verify output matches expected physical behavior.

3.7.3 If CLI: exact terminal transcript

$$ python stepper.py --steps 512 --mode half
[INFO] steps=512
$$ echo $$?
0

Failure Demo (Deterministic)

$$ python stepper.py --steps -1
[ERROR] Invalid step count
$$ echo $$?
2

4. Solution Architecture

4.1 High-Level Design

Input -> Interface -> Logic -> Output

4.2 Key Components

4.3 Data Structures (No Full Code)

struct Config { int mode; int rate; int pin; };

4.4 Algorithm Overview

Key Algorithm: Control/Read Loop

Configure interface.
Perform transaction.
Validate output.
Log or actuate.

Complexity Analysis: O(n) iterations.

5. Implementation Guide

5.1 Development Environment Setup

sudo apt-get update
sudo apt-get install -y build-essential

5.2 Project Structure

project-root/
├── src/
│   └── main.c
├── Makefile
└── README.md

5.3 The Core Question You’re Answering

“How can you control motion without feedback sensors?”

5.4 Concepts You Must Understand First

Electrical limits
Interface configuration
Timing constraints

5.5 Questions to Guide Your Design

How will you verify the hardware response?
What timeout is safe?
What is your retry strategy?

5.6 Thinking Exercise

Map each software step to a physical signal transition or bus event.

5.7 The Interview Questions They’ll Ask

Explain the key interface parameters.
What failure modes did you handle?
How did you verify timing?

5.8 Hints in Layers

Hint 1: Start with default bus speeds. Hint 2: Log raw bytes before parsing. Hint 3: Use a logic analyzer.

5.9 Books That Will Help

| Topic | Book | Chapter | |——-|——|———| | Motor control basics | Making Embedded Systems | Ch. 2 | | Timing and state machines | Making Embedded Systems | Ch. 6 |

5.10 Implementation Phases

Phase 1: Bring-up (3 hours)

Goals: Verify device presence. Checkpoint: First successful transaction.

Phase 2: Core loop (4-6 hours)

Goals: Stable operation. Checkpoint: Deterministic output.

Phase 3: Robustness (2-4 hours)

Goals: Error handling. Checkpoint: Clear logs and exit codes.

5.11 Key Implementation Decisions

6. Testing Strategy

6.1 Test Categories

6.2 Critical Test Cases

Golden path success
Bad argument -> exit 2
Device missing -> clear error

6.3 Test Data

Default config; invalid flag

7. Common Pitfalls & Debugging

7.1 Frequent Mistakes

7.2 Debugging Strategies

Use dmesg for kernel errors
Log raw data

7.3 Performance Traps

Excessive logging or busy loops can distort timing.

8. Extensions & Challenges

8.1 Beginner Extensions

Add a status LED
Add config file support

8.2 Intermediate Extensions

Add retry and backoff
Add CSV/JSON output

8.3 Advanced Extensions

Hardware timestamps
Kernel driver variant

9. Real-World Connections

9.1 Industry Applications

Prototyping
Diagnostics

libgpiod
spidev
mosquitto

9.3 Interview Relevance

Demonstrates interface and timing knowledge

10. Resources

10.1 Essential Reading

Raspberry Pi docs
Device datasheet

10.2 Video Resources

Interface tutorials

10.3 Tools & Documentation

i2c-tools, spidev, libgpiod

P01-sysfs-legacy-blink.md
P02-register-blink-mmio.md

11. Self-Assessment Checklist

11.1 Understanding

I can explain the interface parameters
I can reason about timing limits

11.2 Implementation

Hardware responds consistently
Errors handled

11.3 Growth

I can integrate this into larger systems

12. Submission / Completion Criteria

Minimum Viable Completion:

Basic hardware interaction works
Deterministic demo runs

Full Completion:

Error handling and logs
Documentation updated

Excellence (Going Above & Beyond):

Performance measurements
Extended features