SD Card Interface Overview
The simplified design requirements of a serial interface have made SD Cards an ideal solution for OEMs in consumer and industrial markets. This post is a brief primer on the SD Card interface itself.
The official SD Card specification is controlled by the SD Card Association (www.sdcard.org) and is available only to SD Association members. If you are interested in designing the SD Card into your OEM system, it is highly recommended you download the full SD Card document since it contains valuable information on the physical interface and command protocols.
The SD Card has an intelligent controller which handles the flash management operations. Data transfer between the OEM system and the card is transferred serially in 512 byte blocks. The defined file system for 2Gbyte and lower cards is FAT12/16 and for cards 4Gbytes and higher is it FAT32.
SD Cards operate at a standard voltage range of 2.7 - 3.3V. There are some low power SD Cards also available in the market for specific consumer electronics devices. Cactus SD Cards operate at the standard range.
The default bus protocol for the SD Card is SD mode, but the card can also be configured to operate in the much slower and simpler SPI mode. In SD mode, the card operates in clock serial mode with bus widths of 1-4 bits. In SPI mode, the card operates in 1-bit mode only.
In the normal mode, the clock speed is 0-25MHz and in the high speed mode it is 0-50MHz. Using the 4-bit bus width, this equates to a maximum throughput of up to 12.5Mbytes/sec for normal and 25Mbytes/sec for high speed mode. The Cactus SD Cards support up to high speed mode.
There are Ultra High Speed (UHS-I and UHS-II) SD cards which have much higher performance. They are targeted at high performance consumer video applications and can achieve a bus speed as high as 312Mbytes/sec.
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