How to Log All Data to SD Card with Raspberry Pi Pico

This article will guide you through the process of using an SD card with your Raspberry Pi Pico, delving into the SPI communication standard and the SD library, and providing a practical example of data logging and retrieval.

 SPI (Serial Peripheral Interface)

SPI, a synchronous serial communication protocol at the heart of this data exchange, enables efficient communication between devices. SPI operates with a dedicated clock signal, ensuring both the sending and receiving devices are perfectly synchronized. This synchronized data transfer occurs over three key wires:

• MOSI (Master Out Slave In): The master device transmits data to the slave device.

• MISO (Master In Slave Out): The slave device sends data back to the master.

• SCK (Clock): This signal, generated by the master, controls the timing of data transfer.

A fourth wire, CS (Chip Select) or SS (Slave Select), is often used to handle communication with multiple slave devices. This line dynamically selects the specific slave device the master wants to communicate with at a given time.

Timing and Modes:

SPI communication is characterized by its timing, governed by two essential parameters:

• CPOL (Clock Polarity): This defines whether the clock signal is idle high or low. A CPOL of 0 indicates a low idle state, while a CPOL of 1 indicates a high idle state.

• CPHA (Clock Phase): This determines when data is transmitted relative to the clock signal's transitions. For CPHA = 0, the first data bit is sent immediately after the CS line becomes active. However, for CPHA = 1, the first data bit is sent on the first clock edge after CS activation.

The interplay of CPOL and CPHA defines four distinct SPI modes, each with its specific timing characteristics. This variety of modes allows flexibility in communication protocols and enables compatibility with various devices.

FAT32 File System: Organizing Data

Before diving into the coding aspect, let's address the file system that governs how data is organized on the SD card: FAT32 (File Allocation Table 32). This file system offers compatibility across various operating systems and provides a straightforward way to manage files. Its key features include:

• Maximum File Size: Up to 4 GB per file.

• Volume Size: Supports volumes up to 2 TB.

• 32-bit Entries: This structure allows for efficient access and management of files and directories.

The simplicity and wide compatibility of FAT32 make it an ideal choice for SD card projects, ensuring data integrity and ease of access.

Wiring Your Raspberry Pi Pico for SD Card Communication

To connect your SD card module to the Raspberry Pi Pico, you'll need to establish a clear and concise wiring connection. Here's a breakdown of the required connections:

• SD Card Module — Raspberry Pi Pico:

  • GND — GND

  • +3.3V — 3V3OUT

  • CS — SPI0 CSn

  • MOSI — SPI0 TX

  • SCK — SPI0 SCK

  • MISO — SPI0 RX

Preparing Your SD Card

Before utilizing the SD card module, you need to format it to the FAT32 file system. This process ensures your SD card is ready to accept data and maintain compatibility. Here's how to format your SD card using your computer:

1. Connect the SD Card: Insert your SD card into your computer's SD card slot or an external SD card reader.

2. Access Disk Management: On Windows, navigate to "This PC," right-click the SD card drive, and select "Format."

3. Select FAT32: Ensure "FAT32" is chosen as the file system.

4. Start Formatting: Click "Start" and confirm the formatting process, understanding that all data on the SD card will be erased.

Coding Your Raspberry Pi Pico for SD Card Interaction

With the hardware and SD card formatted, we can now delve into the coding process. Here's a guide to writing sketches in the Arduino IDE for both writing data to and reading data from your SD card:

1. Data Logging Sketch: "PicoSDCardDataLogger"

This sketch demonstrates how to log data to a file named "dataLog.txt" on your SD card.

a. Set Up Your Development Environment

1. Install the SD Library: Open the Arduino IDE and navigate to "Sketch" > "Include Library" > "Manage Libraries..." Search for "SD" and install the library.

2. Select the Board Manager: Choose "Tools" > "Board" > "Raspberry Pi Pico — Arduino Mbed OS RP2040 Boards."

3. Create a New Sketch: Select "File" > "New" and name it "PicoSDCardDataLogger."

b. Code Snippet:

#include <SD.h>

const int chipSelect = 17; // GP pin ID on Pico for chip select

// Sample data logging values
const String dataValues[] = {
  "<D: 12.5, 23.1, 34.2>",
  "<D: 13.6, 22.5, 31.9>",
  "<D: 11.4, 25.6, 30.0>",
  "<D: 14.1, 24.3, 29.8>",
  "<D: 10.9, 22.0, 32.5>"
};

int randomIndex; // A random index to retrieve data from the array

File dataLog; // Declare the log file to be used in the loop

void setup() {
  Serial.begin(115200); // Initiate the serial monitor
  while(!Serial); // Wait for the serial monitor to initialize

  while (!SD.begin(chipSelect)) {
    Serial.println("Failed to initialize SD card.");
  }
  Serial.println("SD card initialized successfully.");
}

void loop() {
  // Open a file in the SD card
  // Only one file can be open at a time
  // Specifying only the name or having a single forward
  // slash means the file will be located in the root directory
  File dataLog = SD.open("dataLog.txt", FILE_WRITE);

  // If the file is OK, write to it
  if (dataLog) {
    randomIndex = random(0, 5); // A random index between 0 and 4
    dataLog.println(dataValues[randomIndex]); // Write a random data line to the file
    dataLog.close(); // Close the file after we're done with it
    Serial.println(dataValues[randomIndex]); // Display the same line in the serial monitor to compare the integrity
  } else {
    Serial.println("Failed to open dataLog.txt."); // Display error message in the serial monitor
  }

  delay(1000); // Wait for a second between loggings
}
    

c. Explanation

• Library Inclusion: The #include <SD.h> line brings in the SD library, providing the necessary functions for SD card interaction.

• Chip Select: const int chipSelect = 17; defines the GP pin connected to the CS line on the SD card module.

• Data Values: The dataValues array simulates sensor data, holding strings representing different data points.

• Setup Function: The setup() function initializes the serial monitor and attempts to initialize the SD card using SD.begin(chipSelect).

• Loop Function: The loop() function opens the "dataLog.txt" file for writing, generates a random data line, writes it to the file, and displays it on the serial monitor.

2. Data Reading Sketch: "PicoSDCardFileReader"

This sketch reads the data stored in the "dataLog.txt" file on your SD card and displays it on the serial monitor.

a. Set Up Your Development Environment

1. Install the SD Library: Ensure you have the SD library installed as described in the previous section.

2. Select the Board Manager: Choose "Tools" > "Board" > "Raspberry Pi Pico — Arduino Mbed OS RP2040 Boards."

3. Create a New Sketch: Select "File" > "New" and name it "PicoSDCardFileReader."

b. Code Snippet:

#include <SD.h>

const int chipSelect = 17; // GP pin ID on Pico for chip select

File dataLog; // Declare the file we're gonna read from

void setup() {
  Serial.begin(115200); // Initiate the serial monitor
  while(!Serial); // Wait for the serial monitor to initialize

  while (!SD.begin(chipSelect)) {
    Serial.println("Failed to initialize SD card.");
  }
  Serial.println("SD card initialized successfully.");

  dataLog = SD.open("dataLog.txt"); // Open the file named dataLog.txt

  if (dataLog) { // If dataLog is open
    Serial.println("Reading from dataLog.txt");
    while (dataLog.available()) { // While there are bytes that can be read
      Serial.print((char) dataLog.read()); // Read bytes from the dataLog, cast bytes to char, print to serial monitor
    }
    dataLog.close(); // Close the file after we're done with it.
  } else {
    Serial.println("Failed to open dataLog.txt");
  }
}

void loop() {} // No code in the loop function as we only read once
    

c. Explanation:

• Library Inclusion: The #include <SD.h> line brings in the SD library, providing the necessary functions for SD card interaction.

• Chip Select: const int chipSelect = 17; defines the GP pin connected to the CS line on the SD card module.

• File Declaration: File dataLog; declares a variable to hold the file object.

• Setup Function: The setup() function initializes the serial monitor, initializes the SD card using SD.begin(chipSelect), opens the "dataLog.txt" file, and reads the contents, displaying them on the serial monitor.

• Loop Function: The loop() function is empty since we are only reading the data once.

Verifying Your Project

Once you've uploaded both sketches to your Raspberry Pi Pico, you can verify the data flow:

1. Monitor the Serial Monitor: Observe the serial monitor during the "PicoSDCardDataLogger" sketch execution. It should display the randomly generated data values.

2. Check the SD Card: After running the "PicoSDCardDataLogger" sketch, connect the SD card to your computer. You should find the "dataLog.txt" file containing the logged data.

3. Run the "PicoSDCardFileReader" sketch: The serial monitor should display the data read from the "dataLog.txt" file, confirming that the reading operation is successful.

Conclusion

This guide provides a comprehensive foundation for using an SD card module with your Raspberry Pi Pico, allowing you to store and retrieve data with ease. Understanding the SPI communication protocol, the FAT32 file system, and the SD library empowers you to create sophisticated electronics projects that utilize persistent data storage. As you delve further into the capabilities of the SD library, you'll discover a vast array of functionalities that can elevate your projects to new heights. Experiment, innovate, and let your Raspberry Pi Pico projects reach their full potential with the power of SD card storage.