Bus Pirate SPI Traffic Analysis with SD Card

1. Introduction to SPI and the Bus Pirate

SPI (Serial Peripheral Interface) is a protocol used to transfer data between microcontrollers and peripherals, like SD cards. The Bus Pirate can capture and analyze SPI traffic, making it ideal for debugging or reverse-engineering data exchanges with SD cards.

2. Required Components

• Bus Pirate (v3.6 or later)
• SD card with SPI mode support
• SD card adapter or breakout board (if needed)
• Connecting wires or breadboard
• Computer with terminal software (e.g., PuTTY or Tera Term)

3. Connecting the SD Card to the Bus Pirate

To capture SPI traffic from the SD card, connect it as follows:

• CLK (Clock) on the Bus Pirate to the SD card’s CLK pin
• MOSI on the Bus Pirate to the SD card’s DI pin
• MISO on the Bus Pirate to the SD card’s DO pin
• CS (Chip Select) on the Bus Pirate to the SD card’s CS pin
• Connect GND between the Bus Pirate and the SD card

Open terminal software, set the baud rate to 115200, and connect to the Bus Pirate's serial port.

4. Setting the Bus Pirate to SPI Mode

With the Bus Pirate connected, enter SPI mode by typing:

m 5

Next, configure SPI settings:

• Clock Polarity and Phase: 1 (match SD card requirements)
• Output Type: 3.3V
• Speed: 100 kHz or slower, depending on the SD card's requirements
• Chip Select: H (active low)

The Bus Pirate is now configured to monitor SPI traffic.

5. Sending SD Card Initialization Commands

Most SD cards require initialization in SPI mode. To initialize, send the following commands:

[0x40 0x00 0x00 0x00 0x00 0x95]

This command sends CMD0 (GO_IDLE_STATE) to set the SD card into SPI mode. The response from the SD card should be 0x01 (idle).

6. Reading Data from the SD Card

After initializing, you can read data from the SD card:

[0x51 0x00 0x00 0x00 0x00 0xFF]

This command sends CMD17 (READ_SINGLE_BLOCK) to read a single data block. The SD card responds with a start token (0xFE), followed by data bytes.

After sending the command, observe the terminal for the data response. This data can help identify the structure and contents of the SD card’s sectors.

7. Interpreting the SPI Traffic

Each SD card response includes:

• Response Token: Indicates the result of the command (e.g., 0x00 for success)
• Start Token: 0xFE, indicating the start of data in a read command
• Data: 512 bytes for each block read
• CRC: Error-checking code at the end of each response

Use the Bus Pirate’s [ and ] commands to capture raw data or analyze response timing.

8. Troubleshooting Common Issues

• No Response from SD Card: Check wiring and ensure the SD card supports SPI mode.
• Incorrect Response Tokens: Verify that the initialization sequence is correct and the Bus Pirate settings match the SD card’s requirements.
• Slow Response: Adjust the SPI clock speed for faster transactions, if the SD card supports it.

9. Conclusion

This experiment demonstrates how to analyze SPI traffic to an SD card using the Bus Pirate. By sending commands and observing responses, you can understand the data exchange process and troubleshoot SPI communication.