Project 6: SPI ADC (Analog-to-Digital Converter)

Read analog voltages via SPI using an MCP3008 and visualize the data.

Quick Reference

Attribute Value
Difficulty Level 3: Advanced
Time Estimate 12-18 hours
Main Programming Language C (Alternatives: Python)
Alternative Programming Languages Python
Coolness Level Level 3: Genuinely Clever
Business Potential 3. The “Service & Support” Model
Prerequisites SPI basics, bit masking, voltage dividers
Key Topics SPI mode, ADC conversion, bit packing

1. Learning Objectives

By completing this project, you will:

  1. Explain the core question: How do you read analog signals using a digital-only SoC?
  2. Implement the full hardware read/write path with correct configuration.
  3. Handle at least two failure modes with clear error messages.
  4. Validate output against a deterministic demo.

2. All Theory Needed (Per-Concept Breakdown)

SPI ADC Transfers and Bit Packing

Fundamentals SPI ADCs convert analog voltages to digital codes using timed serial transfers. 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. SPI via /dev/spidev0.0 is only reliable when you respect SPI mode, clock speed, and reference voltage stability. 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. Set SPI mode 0 for MCP3008, choose a conservative clock (100 kHz to 1 MHz), and define bits-per-word as 8. 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 SPI via /dev/spidev0.0, 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. CS timing and sample rate determine throughput; too-fast clocks cause corrupted samples. 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. ADC inputs must remain within 0-Vref. Use a stable Vref and proper grounding. 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. Read raw counts while sweeping a potentiometer. Use a scope to verify CS and SCLK edges. 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 SPI ADC (Analog-to-Digital Converter) 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.

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

  • CPOL/CPHA: Clock polarity/phase
  • Vref: Reference voltage
  • ADC code: Digital count output

Mental model diagram

Analog input -> ADC -> SPI bits -> digital value

How it works (step-by-step)

  1. Configure SPI mode
  2. Build command frame
  3. Transfer bytes
  4. Extract 10-bit value

Minimal concrete example

uint8_t tx[3]={0x01,0x80,0x00}; spi_transfer(tx, rx, 3);

Common misconceptions

  • Any SPI mode works
  • Vref doesn’t matter

Check-your-understanding questions

  1. How is a 10-bit value packed into 3 bytes?
  2. Why lower SPI speed helps stability?

Check-your-understanding answers

  1. Top bits are in rx[1], lower bits in rx[2].
  2. Slower clocks improve signal integrity.

Real-world applications

  • Analog sensors
  • Data acquisition

Where you’ll apply it

References

  • MCP3008 datasheet
  • spidev docs

Key insights SPI ADC reliability is defined by mode, clock, and clean reference voltage.

Summary Correct SPI framing and stable Vref turn analog voltages into accurate digital readings.

Homework/Exercises to practice the concept

  1. Read all 8 channels and log to CSV.

Solutions to the homework/exercises

  1. Loop channels 0-7 and store values with timestamps.

3. Project Specification

3.1 What You Will Build

Read MCP3008 channel and log voltage.

3.2 Functional Requirements

  1. Implement the primary hardware interaction
  2. Provide CLI configuration
  3. Log raw data and converted output
  4. 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

./spi_adc --channel 0 --rate 10

3.5 Data Formats / Schemas / Protocols

SPI frames and CSV samples.

3.6 Edge Cases

  • Wrong SPI mode
  • Clock too fast
  • Vref instability

3.7 Real World Outcome

Potentiometer sweep produces 0-3.3 V readings.

3.7.1 How to Run (Copy/Paste)

cd project-root
make
./spi_adc --channel 0 --rate 10

3.7.2 Golden Path Demo (Deterministic)

Run ./spi_adc --channel 0 --rate 10 with default wiring and verify output matches expected physical behavior.

3.7.3 If CLI: exact terminal transcript

$$ ./spi_adc --channel 0 --rate 10
[INFO] raw=512 voltage=1.65
$$ echo $$?
0

Failure Demo (Deterministic)

$$ ./spi_adc --channel 9
[ERROR] Invalid channel
$$ echo $$?
2

4. Solution Architecture

4.1 High-Level Design

Input -> Interface -> Logic -> Output

4.2 Key Components

| Component | Responsibility | Key Decisions | |———–|—————-|—————| | Interface layer | Configure and transact | Use correct mode/speed | | Parser/Logic | Interpret data | Validate ranges | | Output | Logs/actuation | Deterministic output |

4.3 Data Structures (No Full Code)

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

4.4 Algorithm Overview

Key Algorithm: Control/Read Loop

  1. Configure interface.
  2. Perform transaction.
  3. Validate output.
  4. 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 do you read analog signals using a digital-only SoC?”

5.4 Concepts You Must Understand First

  1. Electrical limits
  2. Interface configuration
  3. Timing constraints

5.5 Questions to Guide Your Design

  1. How will you verify the hardware response?
  2. What timeout is safe?
  3. 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

  1. Explain the key interface parameters.
  2. What failure modes did you handle?
  3. 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 | |——-|——|———| | SPI programming | The Linux Programming Interface | Ch. 49 | | Timing and IO | 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

| Decision | Options | Recommendation | Rationale | |———-|———|—————-|———–| | Interface mode | default, custom | default | Minimize variables | | Logging | stdout, file | stdout | Simpler debugging |

6. Testing Strategy

6.1 Test Categories

| Category | Purpose | Examples | |———-|———|———-| | Unit | Config parsing | CLI flags | | Integration | Hardware IO | On Pi | | Edge | Missing device | Error path |

6.2 Critical Test Cases

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

6.3 Test Data

Default config; invalid flag

7. Common Pitfalls & Debugging

7.1 Frequent Mistakes

| Pitfall | Symptom | Solution | |———|———|———-| | Wrong wiring | No response | Re-check pinout | | Wrong mode | Garbage data | Verify settings | | No timeouts | Hangs | Add timeout |

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