SD Card SPI Mode Initialization: A Detailed Guide

While SD cards natively use a proprietary 4-bit SD bus, virtually all cards also support a secondary SPI (Serial Peripheral Interface) mode. This makes them easily accessible to a wide range of microcontrollers. However, the initialization sequence required to bring a card from a power-off state to a ready state in SPI mode is strict and often a source of significant challenges for developers.

This guide provides a detailed, step-by-step process for initializing SD, SDHC, and SDXC cards in SPI mode, focusing on the critical command handshake.

Prerequisites & SPI Configuration

Before beginning the sequence, ensure your hardware is configured correctly:

• SPI Bus: Connect MOSI, MISO, and SCK lines. Both MOSI and MISO require pull-up resistors (typically 10kΩ to 50kΩ).
• Chip Select (CS): Connect the card's CS line to a GPIO on the host microcontroller. A pull-up resistor is also required.
• Clock Speed: During initialization, the SPI clock frequency (**SCK**) must be in the range of **100 kHz to 400 kHz**. After successful initialization, the clock speed can be increased (e.g., up to 25 MHz or higher, depending on the card).

The Initialization Sequence

The process must be followed in this exact order. Any deviation can cause the card to enter an unresponsive state.

Step 1: Power-Up & Dummy Clocks

After applying power (VCC), the host must wait for the card's internal power-up sequence to complete (typically a few milliseconds). During this time, the host must perform two actions:

• Set the **MOSI** and **CS** lines to high (logic '1').
• Send at least **74 clock pulses** on the **SCK** line. Sending 80 pulses (10 bytes of 0xFF) is a common and safe practice.

This procedure ensures the card is ready to receive its first command.

Step 2: Enter SPI Mode (CMD0)

To switch the card to SPI mode, the host must send the CMD0 (GO_IDLE_STATE) command.

• Pull **CS** low.
• Send the 6-byte CMD0 command frame: 0x40 0x00 0x00 0x00 0x00 0x95.
• Keep sending clock pulses and read **MISO**. The card should respond with an **R1 response**. A value of 0x01 indicates the card is in an idle state and has successfully entered SPI mode.

Step 3: Verify Interface Conditions (CMD8)

This step is critical for identifying modern cards (SDv2 and later). The host sends CMD8 with a check pattern.

• Send the CMD8 command frame: 0x48 0x00 0x00 0x01 0xAA 0x87.
• If the card responds: It's a **SDv2 or later** card. The response will be 5 bytes (R7 format), and the host must verify that the last 4 bytes match the sent pattern (0x00 0x00 0x01 0xAA).
• If the card does not respond (or responds with an illegal command error): It's an older **SDv1 or MMC card**. The host must then proceed with the SDv1 initialization sequence. This guide focuses on the modern SDv2 sequence.

Step 4: The Initialization Handshake (ACMD41)

This is the core initialization loop. The host repeatedly polls the card until it reports that it's ready. This is achieved by sending ACMD41, which itself is a two-step command.

1. Send CMD55 (APP_CMD). This command tells the card that the *next* command will be an Application-Specific Command (ACMD).
2. Send ACMD41 (SD_SEND_OP_COND). The host also sets the HCS bit in this command to indicate it supports high-capacity cards.
3. Read the **R1 response** from ACMD41.
   • If the response is 0x01, the card is still busy initializing. The host must repeat steps 1 and 2.
   • If the response is 0x00, the card is out of its idle state and ready for operation. The loop can be terminated.

Step 5: Check Card Capacity (CMD58) - Optional

After the ACMD41 loop succeeds, it's good practice to send CMD58 (READ_OCR) to read the OCR register. The response contains the Card Capacity Status (CCS) bit. If this bit is 1, the card is a high-capacity card (SDHC/SDXC). If it's 0, it's a standard capacity card (SDSC). This information is crucial for addressing memory blocks correctly later on.

Conclusion

The SD card SPI initialization sequence is a precise, multi-step process that requires strict adherence to timing and command order. By methodically applying dummy clocks, resetting the card with CMD0, verifying the card version with CMD8, and executing the CMD55/ACMD41 handshake, a host microcontroller can reliably bring any compatible card into an operational state. Once initialized, the SPI clock speed can be increased, and standard block read/write commands can be issued.

For further details, always refer to the official "Physical Layer Simplified Specification" provided by the SD Association, as it is the definitive source for command structures and state transitions.