Learn how to write an SPI Driver from scratch. Step-by-step guide to Linux SPI driver, SPI controller driver, SPI device driver, SPI driver code in C, and Raspberry Pi SPI driver development.
If you’ve ever connected a sensor, display, ADC, DAC, or flash chip to a microcontroller or Linux board, you’ve already stepped into the world of SPI. But here’s the real step forward. Learning how to write an SPI Driver from scratch.
This guide is not just theory. It’s a practical, beginner-friendly walkthrough that shows you exactly how SPI communication works, how the SPI driver circuit is structured, and how to write clean, reliable SPI driver code in C. We’ll go deep into Linux SPI driver development, break down Linux SPI driver architecture in simple terms, and build a working Linux SPI driver example that you can actually understand and modify.
Whether you are working on embedded systems, building a Raspberry Pi project, developing a SPI controller driver, or designing a SPI flash controller driver, this article gives you a clear roadmap. We’ll also look at spi_common.h, explain the difference between a SPI controller driver and a SPI device driver, and even touch on how to structure a C++ SPI driver properly.
By the end of this guide, you’ll understand:
- What an SPI Driver actually does
- How SPI works at hardware and software level
- How to write SPI driver code in C
- How to build a Linux SPI driver
- How the Linux SPI driver architecture works
- How to handle SPI on Raspberry Pi
- How SPI controller drivers differ from SPI device drivers
- How SPI flash controller drivers work
- Where
spi_common.hfits in - Even how to structure a C++ SPI driver
Let’s start from the basics.
What Is an SPI Driver?
An SPI Driver is software that allows your system to communicate with SPI devices using the Serial Peripheral Interface protocol.
SPI is a synchronous communication protocol that uses four main lines:
- MOSI (Master Out Slave In)
- MISO (Master In Slave Out)
- SCLK (Clock)
- CS (Chip Select)
The driver handles:
- Initializing SPI hardware
- Setting clock speed and mode
- Managing data transfers
- Handling interrupts or polling
- Managing multiple devices
Without a driver, your SPI hardware is just idle silicon.
Understanding the SPI Driver Circuit
Before writing code, you must understand the SPI driver circuit.
At hardware level, SPI consists of:
- Master device (microcontroller or SoC)
- One or more slave devices
- Shared clock line
- Separate chip select for each slave
The SPI driver must configure:
- Clock polarity (CPOL)
- Clock phase (CPHA)
- Bit order (MSB or LSB)
- Clock frequency
- Chip select control
If these are wrong, communication fails even if your code is perfect.
So always check your device datasheet before writing the driver.
Two Types of SPI Drivers
When people search for SPI Driver development, they usually mean one of these:
1. SPI Controller Driver
This controls the SPI hardware peripheral in your SoC.
It:
- Configures registers
- Manages FIFO
- Controls clock
- Handles interrupts
This is hardware-specific.
2. SPI Device Driver
This talks to a specific SPI device like a sensor or flash chip.
It:
- Uses the SPI controller
- Sends commands
- Parses responses
- Implements device-specific logic
In Linux, these two are separate.
Writing SPI Driver Code in C (Bare Metal Example)
Let’s start simple.
Here’s a basic SPI driver code in C for a microcontroller:
#define SPI_CR1 (*(volatile unsigned int*)0x40013000)
#define SPI_SR (*(volatile unsigned int*)0x40013008)
#define SPI_DR (*(volatile unsigned int*)0x4001300C)
void SPI_Init(void)
{
SPI_CR1 = (1 << 6); // Enable SPI
}
void SPI_Transmit(uint8_t data)
{
while (!(SPI_SR & (1 << 1))); // Wait TX empty
SPI_DR = data;
}
uint8_t SPI_Receive(void)
{
while (!(SPI_SR & (1 << 0))); // Wait RX not empty
return SPI_DR;
}
This is raw register-level spi driver code. It works for microcontrollers where you directly control hardware registers.
In real projects, you’ll add:
- Clock configuration
- Mode selection
- Error handling
- Timeout protection
That’s how basic SPI driver development starts.
C++ SPI Driver Structure
If you’re working with embedded C++, you can wrap SPI into a class:
class SPIDriver {
public:
void init();
void transmit(uint8_t data);
uint8_t receive();
};
A C++ SPI driver helps when:
- You want abstraction
- You’re building reusable libraries
- You’re working in embedded Linux with OOP design
Under the hood, it still calls low-level C functions.
Linux SPI Driver Architecture Explained Simply
Now let’s move to Linux.
The Linux SPI driver architecture is layered.
It looks like this:
User Space
↓
SPI Device Driver
↓
SPI Core
↓
SPI Controller Driver
↓
Hardware
Linux separates:
- Controller drivers (hardware side)
- Device drivers (device side)
This makes the system modular and scalable.
Linux SPI Driver Structure
A typical Linux SPI driver includes:
- Probe function
- Remove function
- SPI device ID table
- SPI driver structure
Example skeleton:
static int my_spi_probe(struct spi_device *spi)
{
printk("SPI device probed\n");
return 0;
}
static int my_spi_remove(struct spi_device *spi)
{
printk("SPI device removed\n");
return 0;
}
static struct spi_driver my_spi_driver = {
.driver = {
.name = "my_spi_device",
.owner = THIS_MODULE,
},
.probe = my_spi_probe,
.remove = my_spi_remove,
};
module_spi_driver(my_spi_driver);
That’s your minimal linux spi driver example.
How Data Transfer Works in Linux SPI Driver
Linux provides spi_sync() and spi_async().
Example transfer:
struct spi_transfer t = {
.tx_buf = tx_buffer,
.rx_buf = rx_buffer,
.len = length,
};
struct spi_message m;
spi_message_init(&m);
spi_message_add_tail(&t, &m);
spi_sync(spi, &m);
That’s the clean way to move data inside a spi device driver.
Role of spi_common.h in SPI Driver Development
When writing kernel-level drivers, you’ll often see references to spi_common.h.
This header:
- Defines common SPI structures
- Contains shared utilities
- Avoids code duplication
If you’re building a custom spi controller driver, this header becomes important.
Writing a SPI Controller Driver in Linux
A spi controller driver handles hardware registers.
Steps:
- Allocate
spi_controller - Map I/O memory
- Implement transfer_one() callback
- Register controller with SPI core
Example outline:
struct spi_controller *ctlr;
ctlr = spi_alloc_master(dev, sizeof(struct my_spi));
ctlr->mode_bits = SPI_CPOL | SPI_CPHA;
ctlr->transfer_one = my_transfer_function;
spi_register_controller(ctlr);
This connects your hardware to the Linux SPI subsystem.
SPI Flash Controller Driver
SPI flash is common in embedded systems.
A spi flash controller driver manages:
- Flash read/write commands
- Erase sectors
- JEDEC ID detection
- Memory mapping
Linux usually uses MTD subsystem with SPI NOR drivers.
If writing custom logic, you’ll implement:
- Write enable command
- Page program
- Sector erase
- Status register polling
Flash drivers must handle timing carefully.
Raspberry Pi SPI Driver Development
When working with raspberry pi spi driver projects:
You usually don’t write a controller driver because it already exists in Linux.
Instead, you:
- Enable SPI via device tree
- Write a device driver
- Or use spidev interface
To enable SPI:
sudo raspi-config
Then enable SPI.
For quick testing:
sudo apt install spi-tools
For custom kernel driver, follow Linux SPI driver steps explained above.
Complete Linux SPI Driver Development Flow
Here’s a practical roadmap for spi driver development:
- Understand hardware datasheet
- Confirm SPI mode
- Enable SPI controller
- Write device driver skeleton
- Implement probe()
- Implement transfer logic
- Test with loopback
- Handle errors
- Add power management
- Optimize performance
Never skip testing with a logic analyzer.
Common Mistakes While Writing SPI Driver
Let’s save you from painful debugging.
- Wrong SPI mode
- Incorrect clock speed
- Not handling chip select properly
- Ignoring endianness
- Not checking return values
- Blocking calls inside interrupt context
Most SPI bugs are timing-related.
How to Test SPI Driver
You can test using:
- Loopback mode
- Logic analyzer
- Oscilloscope
- spidev interface in Linux
Example Linux test:
echo -ne "\x9F" | spidev_test -D /dev/spidev0.0
For flash chips, read JEDEC ID.
How to Package and SPI Driver Download Options
If you’re distributing your work:
- Provide source code
- Include Makefile
- Add README with hardware details
- Mention supported kernel version
For spi driver download, developers expect:
- GitHub repo
- Kernel module source
- Clear build instructions
Keep documentation clean and direct.
Performance Optimization Tips
If your SPI driver is slow:
- Increase clock speed carefully
- Use DMA if available
- Reduce chip select toggling
- Use async transfers
- Minimize memory copies
In high-speed flash systems, these changes matter a lot.
When to Use User-Space vs Kernel-Space SPI Driver
If you’re prototyping:
Use spidev in user space.
If you’re building production firmware:
Write a proper spi device driver.
Kernel drivers offer:
- Better performance
- Interrupt handling
- Integration with subsystems
Conclusion on Writing an SPI Driver
Writing an SPI Driver is not magic. It’s a mix of:
- Understanding hardware
- Reading datasheets carefully
- Writing clean C code
- Following Linux driver model
- Testing thoroughly
Start small.
Write minimal spi driver code.
Then improve step by step.
If you master:
- Linux SPI driver architecture
- SPI controller driver structure
- SPI device driver logic
- Flash memory handling
- Raspberry Pi SPI setup
You’ll be comfortable building drivers for almost any SPI device.
And once you write your first working driver and see clean waveforms on the logic analyzer, it feels good.
Really good.
FAQ SPI Driver
1. What is an SPI Driver in simple terms?
An SPI Driver is software that allows your processor or operating system to communicate with SPI-based devices like sensors, ADCs, DACs, displays, and flash memory.
It configures the SPI hardware, manages clock settings, handles data transfer, and ensures reliable communication between master and slave devices.
Without it, your SPI hardware cannot talk to external chips.
2. What is the difference between an SPI controller driver and an SPI device driver?
Good question. Many beginners confuse this.
- SPI controller driver controls the SPI hardware peripheral inside your microcontroller or SoC.
- SPI device driver talks to a specific SPI device like a temperature sensor or flash chip.
In Linux SPI driver architecture, these are separate layers. The controller handles hardware registers. The device driver sends device-specific commands.
3. How do I start SPI driver development as a beginner?
Start simple:
- Read the SPI device datasheet carefully
- Identify SPI mode (CPOL, CPHA)
- Confirm clock frequency limits
- Write basic SPI transmit and receive functions
- Test using loopback
If you are learning Linux SPI driver development, begin with a minimal Linux SPI driver example and modify it step by step.
4. What language is used to write SPI driver code?
Most SPI driver code is written in C, especially for embedded systems and Linux kernel development.
You can also build a C++ SPI driver in embedded systems if your environment supports C++, but under the hood it still interacts with C-based hardware APIs.
Kernel-level Linux SPI drivers are written in C.
5. How does Linux SPI driver architecture work?
Linux SPI driver architecture has multiple layers:
User Space
↓
SPI Device Driver
↓
SPI Core
↓
SPI Controller Driver
↓
Hardware
This separation makes the system modular and clean. The SPI core manages communication between device drivers and controller drivers.
6. How can I test my Linux SPI driver?
You can test your Linux SPI driver using:
- Loopback wiring
- Logic analyzer
- Oscilloscope
- spidev utility
For example, you can use /dev/spidevX.Y to test communication before writing a full kernel module.
Testing early saves hours of debugging later.
7. How do I enable SPI on Raspberry Pi?
For raspberry pi spi driver testing:
- Open terminal
- Run
sudo raspi-config - Enable SPI interface
- Reboot
After that, you can use spidev or write a custom SPI device driver.
Most Raspberry Pi projects don’t require writing a new SPI controller driver because it’s already included in the kernel.
8. What is spi_common.h used for?
spi_common.h contains common definitions and structures used across SPI subsystems in the Linux kernel.
If you’re working on low-level SPI controller driver development, you’ll likely interact with headers like this to share common logic and structures.
It helps maintain clean and reusable driver code.
9. What is an SPI flash controller driver?
A SPI flash controller driver manages communication with SPI-based flash memory.
It handles:
- Read and write commands
- Sector erase
- Page programming
- Status register polling
These drivers are critical in bootloaders and embedded Linux systems where firmware is stored in SPI flash.
10. Why is my SPI driver not working even though the code looks correct?
Most SPI issues are hardware configuration problems, not code errors.
Common causes:
- Wrong SPI mode (CPOL/CPHA mismatch)
- Incorrect clock speed
- Chip select timing issues
- Wrong wiring
- Endianness mismatch
Always verify with a logic analyzer before blaming your SPI driver code.
11. Should I write SPI driver in user space or kernel space?
If you are prototyping or experimenting, user space via spidev is fine.
If you are building production firmware or need high performance and proper interrupt handling, write a kernel-level SPI device driver.
Kernel drivers offer better control and integration.
12. Where can I get SPI driver download examples?
You can find SPI driver download examples in:
- Linux kernel source tree
- Official SoC vendor SDKs
- Open-source repositories
- Raspberry Pi kernel source
Always match your driver with the correct kernel version and hardware platform to avoid compatibility issues.
If you’re new to virtualization concepts used in modern automotive and embedded systems, this guide on What Is a Hypervisor? 7 Powerful Reasons explains the fundamentals in plain language and helps you understand how platforms like QNX SDP 8.0 handle mixed-criticality and virtualized environments.
