Python spidev

~ Davis Miller

Python's spidev library is a powerful tool for interfacing with devices using the Serial Peripheral Interface (SPI) protocol on Linux-based systems, such as the Raspberry Pi. This guide delves deep into spidev, covering everything from installation and configuration to advanced usage scenarios, complete with numerous practical examples to help you master SPI communication in Python.

Introduction to SPI and spidev

What is SPI?

Serial Peripheral Interface (SPI) is a synchronous serial communication protocol used for short-distance communication, primarily in embedded systems. SPI enables high-speed data transfer between a master device (typically a microcontroller or a Raspberry Pi) and one or more slave devices (such as sensors, displays, and memory modules).

Key Characteristics of SPI:

  • Full-Duplex Communication: Data can be sent and received simultaneously.
  • Master-Slave Architecture: One master controls one or more slaves.
  • Multiple Slaves: Supports multiple slave devices with separate Chip Select (CS) lines.
  • Four-Wire Interface:
    • MOSI (Master Out Slave In): Data line for master to send data to slaves.
    • MISO (Master In Slave Out): Data line for slaves to send data to the master.
    • SCLK (Serial Clock): Clock signal generated by the master to synchronize data transmission.
    • CS (Chip Select): Line used to select individual slave devices.

What is spidev?

spidev is a Python library that provides bindings for the Linux SPI device interface. It allows Python programs to communicate with SPI devices by providing methods to configure the SPI bus and transfer data.

Key Features of spidev:

  • Simple API: Easy-to-use methods for configuring SPI parameters and transferring data.
  • Flexibility: Supports various SPI modes, speeds, and word sizes.
  • Integration: Ideal for Raspberry Pi and other Linux-based single-board computers.

Prerequisites

Before diving into using spidev, ensure you have the following:

  1. Hardware:
    • A Linux-based single-board computer (e.g., Raspberry Pi).
    • SPI-compatible peripheral devices (e.g., sensors, displays, ADCs).
    • Connecting wires (e.g., jumper cables) and, if necessary, a breadboard.
  2. Software:
    • Operating System: Raspberry Pi OS or any other Linux distribution with SPI support.
    • Python: Python 3.x installed.
    • Permissions: Sufficient privileges to access SPI devices (usually requires root or specific group memberships).
  3. Basic Knowledge:
    • Familiarity with Python programming.
    • Understanding of SPI communication principles.

Installing and Setting Up spidev

Enabling SPI on Raspberry Pi

If you're using a Raspberry Pi, SPI is disabled by default. Follow these steps to enable it:

Access Raspberry Pi Configuration:

sudo raspi-config

Navigate to Interface Options:

  • Use arrow keys to select "Interface Options" and press Enter.

Enable SPI:

  • Select "SPI" and press Enter.
  • Choose "Yes" to enable SPI.

Finish and Reboot:

  • Navigate to "Finish" and select "Yes" to reboot your Raspberry Pi.
sudo reboot

Installing the spidev Library

Update Package Lists:

sudo apt update

Install Python Development Headers:

sudo apt install python3-dev python3-pip

Install spidev via pip:

pip3 install spidev

Alternatively, you can install spidev using apt:

sudo apt install python3-spidev

Verify Installation:
Open a Python shell and try importing spidev:

import spidev
print(spidev.__version__)

If no errors occur and a version number is printed, the installation was successful.

Setting Permissions for SPI Devices

SPI devices are typically accessible via /dev/spidevX.Y, where X is the SPI bus number and Y is the device (CS) number.

Add User to spi Group:

sudo usermod -aG spi $(whoami)

Reboot or Re-login:
For the group changes to take effect, reboot your system or log out and log back in.

sudo reboot

Verify Group Membership:

groups

Ensure spi is listed among the groups.

Basic Usage of spidev

This section covers the fundamental operations using the spidev library: opening an SPI connection, configuring it, transferring data, and closing the connection.

Opening and Configuring SPI Connection

Import spidev and Initialize SPI:

import spidev

# Create an SPI object
spi = spidev.SpiDev()

# Open SPI bus 0, device 0
spi.open(0, 0)

Note: The bus and device numbers (0, 0) may vary based on your hardware configuration.

Configure SPI Parameters:

# Set maximum speed in Hz
spi.max_speed_hz = 50000  # 50 kHz

# Set SPI mode (0 to 3)
spi.mode = 0

# Set bits per word
spi.bits_per_word = 8

SPI Modes:
SPI modes define the clock polarity and phase. There are four modes:

ModeClock Polarity (CPOL)Clock Phase (CPHA)
00 (Low)0 (Sample on rising edge)
10 (Low)1 (Sample on falling edge)
21 (High)0 (Sample on falling edge)
31 (High)1 (Sample on rising edge)

Ensure that the SPI mode matches the requirements of your peripheral device.

Complete Configuration Example:

import spidev

spi = spidev.SpiDev()
spi.open(0, 0)  # Open bus 0, device 0
spi.max_speed_hz = 1000000  # 1 MHz
spi.mode = 1
spi.bits_per_word = 8

Transferring Data

spidev provides methods to transfer data to and from SPI devices:

  • xfer2: Sends and receives data in one transaction.
  • readbytes: Reads a specified number of bytes from the SPI device.

Example: Sending and Receiving Data with xfer2

# Define the data to send (list of integers)
send_data = [0x01, 0x02, 0x03]

# Transfer data and receive response
received_data = spi.xfer2(send_data)

print("Sent:", send_data)
print("Received:", received_data)

Explanation:

  • xfer2 sends the bytes in send_data to the SPI device.
  • Simultaneously, it reads the same number of bytes from the device.
  • The received data is stored in received_data.

Example Output:

Sent: [1, 2, 3]
Received: [4, 5, 6]

Note: The actual received data depends on the connected SPI device's behavior.

Closing the SPI Connection

After completing SPI transactions, it's good practice to close the connection:

spi.close()

Advanced Features and Configurations

Beyond basic data transfer, spidev offers advanced configurations to optimize communication with SPI devices.

SPI Modes

As previously mentioned, SPI modes define clock polarity and phase. Selecting the correct mode is crucial for proper communication.

Setting SPI Mode:

spi.mode = 3  # Set to SPI mode 3

Verifying SPI Mode:

You can retrieve the current mode:

current_mode = spi.mode
print(f"Current SPI mode: {current_mode}")

Multiple SPI Devices

A single SPI bus can support multiple devices using separate Chip Select (CS) lines. Each device on the bus has a unique CS line, identified by the device number.

Opening Multiple Devices:

# Open device 0 on bus 0
spi1 = spidev.SpiDev()
spi1.open(0, 0)

# Open device 1 on bus 0
spi2 = spidev.SpiDev()
spi2.open(0, 1)

Configuring Each Device Independently:

# Configure spi1
spi1.max_speed_hz = 500000  # 500 kHz
spi1.mode = 0

# Configure spi2
spi2.max_speed_hz = 1000000  # 1 MHz
spi2.mode = 3

Transferring Data with Multiple Devices:

# Send data to spi1
data1 = [0xAA, 0xBB]
received1 = spi1.xfer2(data1)
print("Received from spi1:", received1)

# Send data to spi2
data2 = [0xCC, 0xDD]
received2 = spi2.xfer2(data2)
print("Received from spi2:", received2)

Handling Chip Select (CS) Lines

The CS line is used to select which SPI device the master communicates with. Managing CS lines correctly ensures that the master and slave devices communicate without interference.

Automatic CS Handling:

By default, spidev automatically manages the CS line based on the device number provided during the open call.

Manual CS Control:

For scenarios requiring more control over CS lines (e.g., sharing SPI bus with non-spidev devices), you can disable automatic CS and handle it manually using GPIO.

Disable Hardware CS:

spi.no_cs = True

Use GPIO for CS:

import RPi.GPIO as GPIO
import time

CS_PIN = 8  # GPIO pin number

GPIO.setmode(GPIO.BCM)
GPIO.setup(CS_PIN, GPIO.OUT)

def select_device():
    GPIO.output(CS_PIN, GPIO.LOW)  # Active low

def deselect_device():
    GPIO.output(CS_PIN, GPIO.HIGH)

# Example usage
select_device()
spi.xfer2([0x01, 0x02, 0x03])
deselect_device()

GPIO.cleanup()

Note: Ensure that the chosen GPIO pin does not conflict with other SPI functions.

Reading and Writing Data

Writing Data:

Use xfer2 to send data to the SPI device.

# Send a write command followed by data
write_command = [0x02, 0x00, 0x10, 0xFF]  # Example: Write to address 0x0010 with data 0xFF
spi.xfer2(write_command)

Reading Data:

To read data, send a read command and retrieve the response.

# Send a read command
read_command = [0x03, 0x00, 0x10, 0x00]  # Example: Read from address 0x0010
response = spi.xfer2(read_command)

# The response may contain the requested data
print("Data Read:", response)

Note: The exact commands depend on the SPI device's protocol.

Setting and Getting SPI Attributes

You can set various SPI attributes and retrieve their current values.

Setting Attributes:

spi.max_speed_hz = 1000000  # 1 MHz
spi.mode = 1
spi.bits_per_word = 8

Getting Attributes:

current_speed = spi.max_speed_hz
current_mode = spi.mode
bits = spi.bits_per_word

print(f"Speed: {current_speed} Hz, Mode: {current_mode}, Bits per word: {bits}")

Working with Raw Bytes

spidev operates with lists of integers representing bytes (0-255). For more complex data handling, convert between bytearrays and lists.

Sending a Bytearray:

data = bytearray([0xDE, 0xAD, 0xBE, 0xEF])
received = spi.xfer2(list(data))
print("Received:", received)

Receiving as Bytes:

received_bytes = bytes(received)
print("Received Bytes:", received_bytes)

Configuring Delay Between Transactions

Some SPI devices require a delay between transactions.

spi.delay = 1000  # Delay in microseconds

Note: The delay attribute specifies the delay after the SPI transaction.

Configuring LSB/MSB First

Set the bit order for data transmission.

MSB First (Default):

spi.lsbfirst = False

LSB First:

spi.lsbfirst = True

Note: Ensure that the bit order matches the SPI device's requirements.

Practical Examples

To solidify your understanding of spidev, let's explore several practical examples involving common SPI devices.

Example 1: Interfacing with MCP3008 Analog-to-Digital Converter

The MCP3008 is an 8-channel 10-bit ADC commonly used with Raspberry Pi for reading analog sensors.

Hardware Setup

  • Connections:
MCP3008 PinRaspberry Pi GPIO Pin
VDD3.3V
VREF3.3V
AGNDGND
DGNDGND
CLKGPIO11 (SCLK)
DOUTGPIO9 (MISO)
DINGPIO10 (MOSI)
CS/SHDNGPIO8 (CE0)

Python Code to Read Analog Value

import spidev
import time

# Initialize SPI
spi = spidev.SpiDev()
spi.open(0, 0)  # Open bus 0, device 0
spi.max_speed_hz = 1350000

def read_channel(channel):
    """
    Reads data from the specified ADC channel (0-7).
    """
    if channel < 0 or channel > 7:
        raise ValueError("Channel must be between 0 and 7")

    # MCP3008 protocol: start bit + single/diff + channel + two zero bits
    cmd = [1, (8 + channel) << 4, 0]
    response = spi.xfer2(cmd)
    # Convert the response to a single integer
    data = ((response[1] & 3) << 8) + response[2]
    return data

def convert_to_voltage(data, vref=3.3):
    """
    Converts ADC data to voltage.
    """
    voltage = (data * vref) / 1023
    return voltage

try:
    while True:
        # Read channel 0
        adc_value = read_channel(0)
        voltage = convert_to_voltage(adc_value)
        print(f"ADC Value: {adc_value}, Voltage: {voltage:.2f} V")
        time.sleep(1)
except KeyboardInterrupt:
    spi.close()
    print("\nSPI connection closed.")

Explanation

  1. SPI Initialization:
    • Opens SPI bus 0, device 0 (CE0).
    • Sets maximum speed to 1.35 MHz, suitable for MCP3008.
  2. read_channel Function:
    • Sends a 3-byte command to initiate a read on the specified channel.
    • Receives a 3-byte response, extracts the 10-bit ADC value.
  3. convert_to_voltage Function:
    • Converts the ADC value (0-1023) to voltage based on the reference voltage (3.3V).
  4. Loop:
    • Continuously reads from channel 0 every second.
    • Prints ADC value and corresponding voltage.

Sample Output:

ADC Value: 512, Voltage: 1.65 V
ADC Value: 600, Voltage: 1.94 V
ADC Value: 480, Voltage: 1.56 V
…

Example 2: Controlling an SPI-based LCD Display

Many LCD displays, such as the ST7735-based TFT screens, use SPI for communication. This example demonstrates how to initialize and control such a display.

Hardware Setup

  • Connections:
LCD PinRaspberry Pi GPIO Pin
VCC3.3V
GNDGND
SCLGPIO11 (SCLK)
SDAGPIO10 (MOSI)
RESGPIO25
DCGPIO24
CSGPIO8 (CE0)
BL3.3V

Python Code to Initialize and Draw on LCD

Note: This example uses the Adafruit_ILI9341 library for demonstration purposes. Replace with the appropriate library for your LCD.

Install Required Libraries:

pip3 install adafruit-circuitpython-ili9341
pip3 install pillow

Python Code:

import time
import digitalio
from PIL import Image, ImageDraw, ImageFont
import board
import busio
import adafruit_ili9341

# SPI Configuration
spi = busio.SPI(board.SCK, board.MOSI)
cs = digitalio.DigitalInOut(board.CE0)
dc = digitalio.DigitalInOut(board.D24)
rst = digitalio.DigitalInOut(board.D25)

# Initialize the display
display = adafruit_ili9341.ILI9341(spi, cs=cs, dc=dc, rst=rst, baudrate=24000000)

# Create blank image for drawing.
width, height = display.width, display.height
image = Image.new("RGB", (width, height))
draw = ImageDraw.Draw(image)

# Draw a black filled box to clear the image.
draw.rectangle((0, 0, width, height), outline=0, fill=(0, 0, 0))

# Load a TTF font.
font = ImageFont.truetype("/usr/share/fonts/truetype/dejavu/DejaVuSans.ttf", 24)

# Draw some shapes.
draw.rectangle((50, 50, 100, 100), outline=(255, 0, 0), fill=(255, 0, 0))
draw.line((0, 0) + image.size, fill=(255, 255, 255))
draw.line((0, image.size[1], image.size[0], 0), fill=(255, 255, 255))

# Draw text.
draw.text((10, 10), "Hello, SPI LCD!", font=font, fill=(255, 255, 255))

# Display image.
display.image(image)

# Keep the display on for 10 seconds
time.sleep(10)

# Clear display
draw.rectangle((0, 0, width, height), outline=0, fill=(0, 0, 0))
display.image(image)

Explanation

  1. Import Libraries:
    • digitalio, board, busio: For GPIO and SPI configuration.
    • adafruit_ili9341: Library specific to the ILI9341-based LCD.
    • PIL: For image creation and drawing.
  2. SPI and GPIO Configuration:
    • Initializes SPI with SCLK and MOSI.
    • Sets up CS, DC, and RST pins using GPIO.
  3. Display Initialization:
    • Creates an ILI9341 display object with the specified SPI and GPIO configurations.
    • Sets the baud rate to 24 MHz for faster communication.
  4. Image Creation:
    • Creates a blank RGB image with the same dimensions as the display.
    • Initializes a drawing context.
  5. Drawing Shapes and Text:
    • Draws a red square, white diagonal lines, and a "Hello, SPI LCD!" message.
    • Utilizes the DejaVuSans font.
  6. Displaying the Image:
    • Sends the image to the LCD for rendering.
  7. Cleanup:
    • Waits for 10 seconds before clearing the display.

Sample Output:

An SPI-based LCD display will show a red square, white diagonal lines, and the text "Hello, SPI LCD!" in white font.

Example 3: Communicating with an EEPROM (24LC256)

EEPROMs like the 24LC256 provide non-volatile storage, allowing you to read and write data.

Hardware Setup

  • Connections:
EEPROM PinRaspberry Pi GPIO Pin
VCC3.3V
GNDGND
SDAGPIO10 (MOSI)
SCLGPIO11 (SCLK)
WPGND

Note: 24LC256 uses I2C, but for demonstration, assume it's SPI-based. Alternatively, use an SPI EEPROM like 25LC256.

Python Code to Write and Read Data

Assuming an SPI EEPROM like the 25LC256:

Hardware Setup:

EEPROM PinRaspberry Pi GPIO Pin
VCC3.3V
GNDGND
SI (MOSI)GPIO10 (MOSI)
SO (MISO)GPIO9 (MISO)
SCKGPIO11 (SCLK)
CSGPIO8 (CE0)

Python Code:

import spidev
import time

# Initialize SPI
spi = spidev.SpiDev()
spi.open(0, 0)  # Bus 0, Device 0 (CE0)
spi.max_speed_hz = 500000
spi.mode = 0

def write_eeprom(address, data):
    """
    Writes data to the EEPROM at the specified address.
    """
    # EEPROM Write Enable (WREN) command: 0x06
    spi.xfer2([0x06])
    time.sleep(0.1)  # Short delay

    # EEPROM Write (WRITE) command: 0x02
    cmd = [0x02, (address >> 8) & 0xFF, address & 0xFF] + data
    spi.xfer2(cmd)
    time.sleep(0.05)  # Write cycle time

def read_eeprom(address, length):
    """
    Reads data from the EEPROM starting at the specified address.
    """
    # EEPROM Read (READ) command: 0x03
    cmd = [0x03, (address >> 8) & 0xFF, address & 0xFF] + [0x00] * length
    response = spi.xfer2(cmd)
    # The first three bytes are dummy bytes, followed by the data
    return response[3:]

try:
    # Example: Write "Hello" to address 0x0000
    write_address = 0x0000
    write_data = [ord(c) for c in "Hello"]
    write_eeprom(write_address, write_data)
    print("Data written to EEPROM.")

    # Read back the data
    read_length = 5
    read_data = read_eeprom(write_address, read_length)
    read_string = ".join([chr(b) for b in read_data])
    print(f"Data read from EEPROM: {read_string}")
except Exception as e:
    print("Error:", e)
finally:
    spi.close()

Explanation

  1. SPI Initialization:
    • Opens SPI bus 0, device 0 (CE0).
    • Sets speed to 500 kHz and mode to 0.
  2. write_eeprom Function:
    • Write Enable (WREN): Sends the 0x06 command to enable writing.
    • Write Command (WRITE): Sends the 0x02 command followed by the 16-bit address and data bytes.
    • Delays: Incorporates delays to accommodate write cycle times.
  3. read_eeprom Function:
    • Read Command (READ): Sends the 0x03 command followed by the 16-bit address.
    • Dummy Bytes: Appends dummy bytes (0x00) to receive data.
    • Data Extraction: Skips the first three bytes (command and address) to retrieve the actual data.
  4. Usage:
    • Writes the string "Hello" to address 0x0000.
    • Reads back 5 bytes from the same address.
    • Prints the retrieved string.

Sample Output:

Data written to EEPROM.
Data read from EEPROM: Hello

Note: Ensure that the EEPROM's write cycle time is respected to prevent data corruption.

Example 4: Reading Data from an SPI Temperature Sensor (e.g., MCP9808)

The MCP9808 is a high-accuracy temperature sensor with SPI interface.

Hardware Setup

  • Connections:
MCP9808 PinRaspberry Pi GPIO Pin
VDD3.3V
GNDGND
SCKGPIO11 (SCLK)
SDIGPIO10 (MOSI)
SDOGPIO9 (MISO)
CSGPIO8 (CE0)

Python Code to Read Temperature

import spidev
import time

# Initialize SPI
spi = spidev.SpiDev()
spi.open(0, 0)  # Bus 0, Device 0 (CE0)
spi.max_speed_hz = 1000000  # 1 MHz
spi.mode = 0

def read_temperature():
    """
    Reads temperature from MCP9808 sensor.
    """
    # MCP9808 Read Temperature Register (0x05)
    read_cmd = [0x05, 0x00]  # Command to read temperature
    response = spi.xfer2(read_cmd + [0x00, 0x00])  # Send read command and receive 2 bytes

    # Extract temperature from response
    temp_raw = (response[2] << 8) | response[3]
    temp_c = (temp_raw & 0x0FFF) / 16.0
    if temp_raw & 0x1000:
        temp_c -= 256  # Handle negative temperatures

    return temp_c

try:
    while True:
        temperature = read_temperature()
        print(f"Temperature: {temperature}°C")
        time.sleep(2)
except KeyboardInterrupt:
    spi.close()
    print("\nSPI connection closed.")

Explanation

  1. SPI Initialization:
    • Opens SPI bus 0, device 0 (CE0).
    • Sets speed to 1 MHz and mode to 0.
  2. read_temperature Function:
    • Read Command: Sends the 0x05 register address to read temperature.
    • Response: Receives two bytes containing temperature data.
    • Data Processing:
      • Combines the two bytes to form a 12-bit temperature value.
      • Converts the raw value to Celsius.
      • Handles negative temperatures if applicable.
  3. Loop:
    • Continuously reads and prints the temperature every 2 seconds.
    • Gracefully handles keyboard interruption.

Sample Output:

Temperature: 25.50°C
Temperature: 25.75°C
Temperature: 26.00°C
…

Note: Ensure the MCP9808 sensor is properly wired and configured.

Example 5: Driving a SPI Motor Controller (e.g., L293D)

Motor controllers like the L293D can be controlled via SPI to manage motor speed and direction.

Hardware Setup

  • Connections:
L293D PinRaspberry Pi GPIO Pin
VCC15V
VCC2External Motor Power (e.g., 12V)
GNDGND
IN1GPIO10 (MOSI)
IN2GPIO9 (MISO)
IN3GPIO8 (CE0)
IN4GPIO7 (CE1)
EN1GPIO25
EN2GPIO24

Note: The L293D is actually an H-bridge IC typically controlled via GPIO. For demonstration, assume a hypothetical SPI-based motor controller.

Python Code to Control Motor Direction and Speed

import spidev
import time

# Initialize SPI
spi = spidev.SpiDev()
spi.open(0, 0)  # Bus 0, Device 0 (CE0)
spi.max_speed_hz = 500000  # 500 kHz
spi.mode = 0

def set_motor(direction, speed):
    """
    Sets motor direction and speed.
    direction: 'forward' or 'reverse'
    speed: 0 to 255
    """
    if direction == 'forward':
        dir_byte = 0x01
    elif direction == 'reverse':
        dir_byte = 0x02
    else:
        dir_byte = 0x00  # Stop

    speed_byte = speed & 0xFF
    cmd = [dir_byte, speed_byte]

    spi.xfer2(cmd)
    print(f"Motor set to {direction} with speed {speed}.")

try:
    # Move motor forward at speed 200
    set_motor('forward', 200)
    time.sleep(5)

    # Move motor reverse at speed 150
    set_motor('reverse', 150)
    time.sleep(5)

    # Stop motor
    set_motor('stop', 0)
    time.sleep(2)
except Exception as e:
    print("Error:", e)
finally:
    spi.close()
    print("SPI connection closed.")

Explanation

  1. SPI Initialization:
    • Opens SPI bus 0, device 0 (CE0).
    • Sets speed to 500 kHz and mode to 0.
  2. set_motor Function:
    • Direction Byte:
      • 0x01 for forward.
      • 0x02 for reverse.
      • 0x00 for stop.
    • Speed Byte: Value between 0 and 255 to control motor speed.
    • Command: Sends a 2-byte command with direction and speed.
    • Transmission: Uses xfer2 to send the command.
  3. Usage:
    • Sets the motor to forward at speed 200, waits 5 seconds.
    • Sets the motor to reverse at speed 150, waits 5 seconds.
    • Stops the motor, waits 2 seconds.
    • Closes the SPI connection gracefully.

Sample Output:

Motor set to forward with speed 200.
Motor set to reverse with speed 150.
Motor set to stop with speed 0.
SPI connection closed.

Note: The actual command bytes (0x01, 0x02, etc.) depend on the motor controller's protocol. Refer to the device's datasheet for accurate command definitions.

Error Handling and Troubleshooting

Working with SPI devices can sometimes lead to errors. Understanding common issues and their solutions is crucial for smooth operation.

Common Errors

PermissionError: [Errno 13] Permission denied
Cause: Insufficient permissions to access SPI device files (e.g., /dev/spidev0.0).
Solution:

Ensure your user is part of the spi group.

sudo usermod -aG spi $(whoami)
sudo reboot

Alternatively, run your script with sudo:

sudo python3 your_script.py
FileNotFoundError: [Errno 2] No such file or directory: '/dev/spidevX.Y'

Cause: SPI interface not enabled or incorrect bus/device numbers.
Solution:

  • Enable SPI on your device (e.g., Raspberry Pi) as described earlier.

Verify the correct SPI bus and device numbers.

ls /dev/spidev*
  • Adjust the open(bus, device) parameters accordingly.

IOError: [Errno 16] Device or resource busy
Cause: Another process is using the SPI device.
Solution:

  • Ensure no other scripts or services are accessing the SPI device.
  • Reboot the system to reset SPI device states.

Incorrect Data Transmission
Cause: Mismatch in SPI mode, speed, or wiring issues.
Solution:

  • Verify SPI mode matches the device's requirements.
  • Check SPI speed settings.
  • Inspect physical connections for loose or incorrect wiring.
  • Use an oscilloscope or logic analyzer to debug SPI signals.

Debugging Tips

Verify SPI Device Availability:

ls /dev/spidev*

Ensure the expected SPI devices are listed.

Check SPI Configuration:

import spidev

spi = spidev.SpiDev()
spi.open(0, 0)
print(f"Mode: {spi.mode}, Max Speed: {spi.max_speed_hz} Hz, Bits per word: {spi.bits_per_word}")
spi.close()

Confirm that SPI parameters are correctly set.

Use SPI Tools:
Install and use spidev utilities to test SPI communication.

sudo apt install spi-tools
spi_test -v -b 500000 -m 0 /dev/spidev0.0

Logging and Print Statements:
Incorporate logging or print statements in your code to trace execution and data values.

print("Sending data:", send_data)
print("Received data:", received_data)

Loopback Test:
For basic communication testing, perform a loopback test by connecting MOSI to MISO and verifying that sent data is received correctly.
Connections for Loopback:

  • Connect GPIO10 (MOSI) to GPIO9 (MISO).

Test Code:

import spidev

spi = spidev.SpiDev()
spi.open(0, 0)
spi.max_speed_hz = 500000
spi.mode = 0

send_data = [0xAA, 0xBB, 0xCC]
received = spi.xfer2(send_data)
print("Sent:", send_data)
print("Received:", received)

spi.close()

Expected Output:

Sent: [170, 187, 204]
Received: [170, 187, 204]

Note: 0xAA = 170, 0xBB = 187, 0xCC = 204.

Use GPIO Libraries for Additional Control:
Sometimes integrating spidev with GPIO libraries like RPi.GPIO can help manage CS lines or other peripherals.

Best Practices

Adhering to best practices ensures efficient and reliable SPI communication using spidev.

  1. Correct SPI Mode and Settings:
    • Always verify and match the SPI mode, speed, and bits per word with your device's specifications.
  2. Handle CS Lines Appropriately:
    • Ensure proper management of Chip Select lines, especially when dealing with multiple SPI devices.
  3. Use Short and Efficient Transactions:
    • Minimize the number of SPI transactions by batching data when possible.
    • Keep transactions short to reduce latency.
  4. Manage Resources Properly:

Always close SPI connections after use to free up resources.

spi.close()
  1. Implement Error Handling:

Incorporate try-except blocks to gracefully handle exceptions and ensure the SPI connection is closed properly.

try:
    # SPI operations
except Exception as e:
    print("Error:", e)
finally:
    spi.close()
  1. Optimize Data Formats:
    • Use appropriate data formats (e.g., integers, bytes) to match the SPI device's requirements.
  2. Ensure Electrical Compatibility:
    • Confirm that voltage levels between the Raspberry Pi and SPI device are compatible (e.g., both use 3.3V logic).
  3. Secure Physical Connections:
    • Use stable connections to prevent communication errors due to loose wires.
  4. Document Your Code:
    • Maintain clear and concise documentation within your code for future reference and maintenance.
  5. Stay Informed About spidev Updates:
    • Keep the spidev library updated to benefit from bug fixes and new features.
pip3 install –upgrade spidev

Conclusion

The Python spidev library is an essential tool for developers working with SPI devices on Linux-based systems like the Raspberry Pi. By providing a simple yet powerful interface for SPI communication, spidev enables seamless integration with a wide range of peripherals, from sensors and displays to memory modules and motor controllers.

This comprehensive guide has covered:

  • Fundamental Concepts: Understanding SPI and the role of spidev.
  • Installation and Configuration: Setting up spidev and enabling SPI interfaces.
  • Basic and Advanced Usage: Conducting SPI transactions, handling multiple devices, and managing CS lines.
  • Practical Examples: Real-world applications interfacing with ADCs, displays, EEPROMs, sensors, and motor controllers.
  • Error Handling and Best Practices: Ensuring reliable and efficient SPI communication.

By mastering spidev, you can unlock the full potential of SPI-enabled hardware, paving the way for innovative and responsive projects.

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