Digital Protocols: How do SD-Cards work?

Secure Digital Input/Output EMC

Have you ever wondered how data exchange between your SD card and your PC takes place? Here is the answer:

SD cards use the SDIO protocol. SDIO stands for Secure Digital Input Output. An interesting fact is that SD cards are backward compatible with SPI. Basically, SDIO is just an extension of SPI, allowing data to be transmitted in both directions. With an SPI reader, you can also read and write SD cards. Over time, more pins have been added for transmission through the development of SD cards. They all use the same SDIO protocol.

The following image shows the pinout of a standard SD card.

1. SPI Card Select CS

2. SPI Serial Data In COPI

3. GND

4. Power

5. SPI Serial Clock SCLK

6. GND

7. SPI Serial Data Out CIPO

8. NC (Not Connected)

9. NC (Not Connected)

Search online for the other standards MMC, miniSD, or microSD. You will see that the pinouts have changed, but the principle is always the same.

Embedded Multimedia Cards eMMCs use the same protocol. eMMCs are surface mount memory chips. They are closely related to SD cards as they use the same protocol. However, instead of the usual 4 pins for data transmission, up to 8 pins are used here.

If you ever have an eMMC in front of you, you can control it with a simple SD card reader, which you can get at any PC store.

Much fun!