Information Technology Reference
In-Depth Information
Figure 1-7: SD Cards are available in three form factors. From left to right:
original SD Card, miniSD Card, and microSD Card.
ure 1-7). The form factors are similar to the options available for MultiMe-
diaCards. A card that performs I/O functions such as modem, GPS device,
or network interface is called an SDIO Card.
The optional write-protect switch is a sliding tab on the side of the card. If
the tab is in the lock position, the host must write-protect the contents. The
switch by itself doesn't offer protection. The firmware accessing the card is
responsible for reading the state of the switch and protecting the contents
when appropriate. SD-Card connectors include a pin that enables reading
the switch state. The miniSD and microSD Cards don't have write-protect
switches but can be inserted in an SD-Card adapter that contains a switch.
Interfacing
An SD Card can use SPI or the SD-Card bus. The SD-Card bus can use a
bus width of one or four bits. The SD-Card bus can have shorter timeout
values and doesn't require a clock frequency of 400 kHz or less on power up.
A typical SD Card uses 65 mA to read and 75 mA to write and has a
low-power sleep mode when the card isn't being accessed.
Protocols
SD Cards use the same commands and protocols defined by the MultiMedi-
aCard specification. SD Cards also support a series of commands that are
specific to SD Cards. These commands support security functions and
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