Mastering SPI Communication with Raspberry Pi and Arduino

Serial Peripheral Interface (SPI) is a common communication protocol used in embedded systems for interfacing microcontrollers with various peripherals like sensors, memory devices, and other microcontrollers. In this post, we will explore how to set up and debug an SPI communication link between a Raspberry Pi and an Arduino, two of the most popular platforms in electronics and hobbyist projects.

Theoretical Overview

SPI operates based on a master-slave architecture, where the master device initiates the communication and provides the clock signal, while one or more slave devices receive the clock and respond accordingly. Key features of SPI include:

• Full-duplex communication: Simultaneous data transmission and reception.
• High-speed data transfer: Suitable for situations requiring fast data exchange.
• Separate data lines: MOSI (Master Out Slave In) and MISO (Master In Slave Out).

Understanding these basics is crucial for effectively implementing and troubleshooting SPI connections.

Design and Implementation

Hardware Setup

1. Raspberry Pi: Acts as the master device.
2. Arduino Uno: Functions as the slave device.
3. Connections:
   • Connect Raspberry Pi’s GPIO 10 (MOSI) to Arduino’s Digital Pin 11 (MOSI).
   • Connect GPIO 9 (MISO) to Digital Pin 12 (MISO).
   • Connect GPIO 11 (SCLK) to Digital Pin 13 (SCK).
   • Connect a common ground.

Software Setup

Raspberry Pi Configuration

import spidev
spi = spidev.SpiDev()
spi.open(0, 0)  # Open bus 0, device 0 (CS0)
spi.max_speed_hz = 500000  # Set speed to 500 kHz

def send_data(data):
    response = spi.xfer([data])
    return response

Arduino Sketch

#include <SPI.h>

volatile byte received_data = 0;

void setup() {
  pinMode(MISO, OUTPUT);
  SPCR |= _BV(SPE);
  SPI.attachInterrupt();
}

ISR(SPI_STC_vect) {
  received_data = SPDR;  // Capture data from SPI Data Register
  SPDR = received_data;  // Send back the received data
}

void loop() {
  // Data handling logic can be added here
}

Debugging the Setup

When establishing SPI communication, common issues include misconfigured pins and incorrect clock settings. Here’s how to troubleshoot:

1. Check Connections: Ensure all SPI and ground connections are secure and correct.
2. Logic Analyzer: Use a logic analyzer to verify that the signals (MOSI, MISO, SCLK) are active and conform to expected patterns.
3. Simple Echo Test: Implement a simple echo functionality where the Arduino sends back whatever it receives. This confirms basic send/receive capability.

# Raspberry Pi test code
test_data = 0xAA  # Send hexadecimal AA
response = send_data(test_data)
print(f"Received: {response}")

Results and Observations

When the setup is correct, you should observe the Raspberry Pi successfully sending data to the Arduino and receiving the same data back. This test confirms that the hardware and software are configured correctly, and the SPI communication is functioning as expected.

Conclusion

Setting up SPI communication between a Raspberry Pi and an Arduino involves careful attention to wiring, configuration parameters, and debugging steps. By following the detailed setup and troubleshooting tips outlined in this post, you can establish a robust SPI communication link suitable for numerous applications, from simple DIY projects to complex embedded systems.

Stay tuned for more posts on leveraging different communication protocols and interfacing techniques to enhance your electronics projects.