What Is The Correct Command Sequence For MicroSD Card Initialization In SPI?

Mr tony

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Sep 26, 2024

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What is the Correct Command Sequence for microSD Card Initialization in SPI?

MicroSD cards, a ubiquitous storage solution in embedded systems, rely on the SPI protocol for communication. While their small size packs a lot of storage capacity, understanding the correct sequence for initializing them in an SPI environment is crucial for reliable data access. This article delves into the specifics of the initialization process, breaking down the command sequence and essential steps involved in getting your microSD card ready for operation.

Understanding the SPI Interface

Before diving into the initialization sequence, it's essential to grasp the basics of SPI communication. SPI (Serial Peripheral Interface) is a synchronous serial communication protocol widely used in embedded systems. It utilizes four main lines:

• MOSI (Master Out Slave In): This line carries data from the master (typically the microcontroller) to the slave (the microSD card).
• MISO (Master In Slave Out): This line transmits data from the slave (microSD card) back to the master.
• SCK (Serial Clock): This line provides the clock signal that synchronizes data transmission between the master and slave.
• CS (Chip Select): This line is used to select the slave device, in this case, the microSD card.

The microSD Card Initialization Process

Initializing a microSD card in SPI involves a series of commands sent to the card. These commands are crucial for establishing communication and configuring the card's operation. The following is a step-by-step guide to the initialization process:

1. Select the microSD Card (CS Low)

The first step is to select the microSD card by pulling the CS line low. This signals to the card that it is being addressed for communication.

2. Send the Initialization Commands

After selecting the card, a sequence of specific commands is sent to initiate the communication protocol and configure the card's behavior. These commands are crucial for setting up the card's operating mode, including read/write operations, access rights, and data transfer rates.

2.1. CMD0: RESET (Go Idle)

This command is typically sent first to reset the microSD card and put it in an idle state, ready for further instructions.

Command Structure:

Example Implementation (C):

uint8_t spi_send_command(uint8_t cmd, uint32_t arg) {
  uint8_t cmd_buffer[6];
  cmd_buffer[0] = cmd | 0x40; // Start bit
  cmd_buffer[1] = (arg >> 24) & 0xFF;
  cmd_buffer[2] = (arg >> 16) & 0xFF;
  cmd_buffer[3] = (arg >> 8) & 0xFF;
  cmd_buffer[4] = arg & 0xFF;
  cmd_buffer[5] = 0x95; // CRC7 checksum

  for (int i = 0; i < 6; i++) {
    spi_transfer(cmd_buffer[i]);
  }

  return spi_read_response();
}

// ... in the main initialization function ...
spi_send_command(CMD0, 0); // Send CMD0 (Go Idle)

2.2. CMD1: SEND_OP_COND (Get Card Status)

This command is used to check the card's status and identify its supported features. The response from the card includes information about its voltage range and other capabilities.

Command Structure:

Example Implementation (C):

uint8_t spi_read_response(void) {
  uint8_t response = spi_transfer(0xFF);
  return response;
}

// ... in the main initialization function ...
spi_send_command(CMD1, 0);
uint8_t card_status = spi_read_response();
if ((card_status & 0x01) == 0x01) {
  // Card is ready
} else {
  // Wait for card to be ready
}

2.3. CMD8: SEND_IF_COND (Check Card's Supported Voltage)

This command is used to check if the microSD card supports the operating voltage (usually 2.7-3.6V) for your system.

Command Structure:

Example Implementation (C):

// ... in the main initialization function ...
spi_send_command(CMD8, 0x1AA);
uint8_t voltage_check = spi_read_response();
if (voltage_check == 0x01) {
  // Card supports the voltage
} else {
  // Card does not support the voltage
}

2.4. CMD55: APP_CMD (Application Specific Command)

This command is used to access application-specific commands, which are often used for advanced features on the microSD card.

Command Structure:

2.5. ACMD41: SD_SEND_OP_COND (Get Card Status)

This command is similar to CMD1 but is used specifically for SD cards (Secure Digital). It's sent after CMD55 to check the card's status and determine its operating voltage.

Command Structure:

Example Implementation (C):

// ... in the main initialization function ...
spi_send_command(CMD55, 0);
spi_send_command(ACMD41, 0x40000000); // Indicate voltage range
uint8_t card_status = spi_read_response();
if ((card_status & 0x01) == 0x01) {
  // Card is ready
} else {
  // Wait for card to be ready
}

3. Set the Block Size (Optional)

MicroSD cards can store data in blocks. The default block size can vary depending on the card, and you can set a preferred block size using the CMD16 command. This improves efficiency, especially when performing large data transfers.

Command Structure:

Example Implementation (C):

// ... in the main initialization function ...
spi_send_command(CMD16, 512); // Set block size to 512 bytes

4. Select the Data Transfer Mode (Optional)

Depending on your application's requirements, you might need to configure the microSD card for specific data transfer modes, such as High Speed (HS) or Ultra High Speed (UHS) mode. These modes offer higher data transfer rates, but they require specific command sequences and voltage levels for the microSD card to support them.

5. Deselect the microSD Card (CS High)

Once the initialization process is complete, the microSD card should be deselected by pulling the CS line high. This terminates the communication and prepares the card for data operations.

Key Points to Remember

• CRC7: Checksums are crucial for error detection during communication.
• Error Handling: Implement appropriate error handling mechanisms to address potential issues during initialization, such as incorrect responses from the card.
• Timing: Ensure proper timing for the SPI signals to avoid communication errors.
• Card Type: The commands and initialization steps can differ slightly between SD and SDHC/SDXC cards. Refer to the specific card specifications for details.

Conclusion

The initialization process for microSD cards in SPI communication is essential for establishing a reliable connection and accessing the data stored on the card. By understanding the command sequence and implementing the necessary steps, you can seamlessly integrate microSD cards into your embedded systems, enabling efficient data storage and retrieval capabilities.