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2.2 SD Card File System
The SD memory card is a flash memory card which was originally designed to be used
with audio and video devices as higher speeds and higher storage became necessary.
Within the SD memory card there is a microcontroller that is designed to work directly
with the flash memory chips. This microcontroller keeps current information on the
internal workings of the SD card, as well as provides an interface for the user to the
flash chips. Voltage levels for SD cards from different companies may vary. The
common voltage range is 3.3 volts which works well with most microcontrollers. The
communication format and pin configuration of an SD memory card is that of a simple
SPI connection. The SPI pins consist of Slave Select (SS), Serial Data Clock (SCK),
Master In Slave Out (MISO), Master Out Slave In (MOSI), voltage at the Common
Collector (Vcc), and Ground (Gnd). This is a common SPI interface found with most
microcontrollers. There are also additional data lines that can be used if the designer
desires to use the SD bus format of communication. Although higher data rates can
be attained, the programming for this format of communication proves to take more
memory and more processor use than the SPI connection.
The SD card is designed to be able to allow hot insertion. This is when an SD card can
be inserted into the host device, while the device is on, without creating any problems.
This is accomplished by a hardwired connection that is made by pin connectors when
the card is inserted. The host device will see the signal and know to start
communication with the card.
The memory organization of an SD card can be formatted as FAT 16, FAT 32, or
exFAT. In the FAT 16 format the memory is divided up into groups of 512 byte
sections called sectors. These sectors are then organized in groups called clusters. The
number of sectors per cluster can vary with card size. As an example a 2 GB memory
card may have 64 sectors per cluster. These sectors can be used to store data that
can be erased and rewritten frequently. Although most memory is used for raw data,
there are sectors which maintain the file system of the card that must not be
tampered with in order to maintain proper card operation.
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