Delkin Blog

Understanding the SD Card Interface

 

Secure Digital cards, or SD cards, are widely used in consumer and industrial markets. The SD card interface is simple and widely compatible, making it an easy choice for designers and OEMs. The Secure Digital Association determines the design specifications, and these specs are available only to members of the group. For members, the exact design protocols of the physical interface can be downloaded, which can be helpful to OEMs who are implementing SD cards into their systems. SD cards are available in both SLC NAND flash industrial formats and commercial MLC NAND flash designs. Here is what you need to know about the SD card interface.

 

Intelligent Controller

The flash management operations in SD cards are handled by an intelligent controller, and data transfers occur serially in 512-byte blocks between the host system and the card. For cards with capacities of 2Gbytes or lower, the defined file system is FAT12/16. Cards with capacities of 4Gbytes or more use FAT32.

 

This is just like CF and PC cards, which are familiar to many OEMs. However, the pinout for SD cards is different than with those designs. SD cards use nine contact pads instead of pins for contact. For legacy compatibility with MMC cards, the 9-pad is placed below the 1-pad and is positioned out of alignment and with a greater width. From top to bottom, the pinout looks like this:

8: DAT1

7: DAT0/DO

6: Vss2

5: CLK

4: Vcc

3: Vss1

2: CMD/DI

1: DAT3/CS

9: DAT2

 

Voltages

Although lower voltages are available, the standard voltage range of SD cards is 2.7-3.3V. The lower voltage models are generally designed specifically for consumer electronic devices that require this kind of reduced voltage range. Standard voltages are used for industrial grade SD cards.

 

Bus Information

SD cards operate with a default bus protocol of SD mode. However, if a simpler or slower bus is needed, the card can be configured for SPI mode. While in SPI mode, SD cards operate in one-bit bus widths. In SD mode, the cards use a serial clock mode for bus widths of between one and four bits.

 

When the SD card is put into SD or SPI mode, it cannot be switched to the other mode while power is applied. However, the modes can be switched during a power cycle as needed.

 

Delkin’s product team can answer all of your questions about the SD card interface and other industrial grade embedded storage options. Contact us today for more information or to request an engineering sample.

 

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