Information Technology Reference
In-Depth Information
Table 1-1: Common USB mass storage devices use a variety of storage media.
Device
Storage Media
Local CPU Interface
to Media
Removable Media?
Hard drive
Hard disk
ATA
No
CD drive
CD
ATA + ATAPI
Yes
DVD drive
DVD
ATA + ATAPI
Yes
Flash drive
Flash memory
Local CPU data bus
No
Flash-memory-card
reader/writer
Flash memory
SPI,
MultiMediaCard bus,
SD-Card bus
Ye s
Other Considerations
A storage device isn't the solution for every application, however.
•
Mass-storage firmware is complex. A USB mass-storage device must sup-
port the USB protocols required for all USB devices as well as class-spe-
cific mass-storage protocols. If the device firmware needs to create, read,
or write to files and directories on its own (not via the USB interface),
the firmware must also support a file system. For some applications, a
different USB class or a vendor-specific protocol would require less time
and expense to implement.
•
USB mass-storage devices transfer data using bulk transfers. These pro-
vide the fastest transfers on an otherwise idle bus but have no guaranteed
timing or bus bandwidth. If your device needs precise timing in transfer-
ring data, the mass-storage class isn't appropriate.
•
A storage device should have one mass-storage master at a time. The mas-
ter, or mass-storage host, is the computer that reads and writes to the
storage media. Special-purpose mass-storage devices can function as mas-
ters on their own and can also permit a PC or other USB host to func-
tion as the master. If one master adds, deletes, or changes a file and the
other master isn't aware of the changes, confusion or worse problems can
result. Devices that support two masters can have a manual or electronic
switch to enable one master at a time, or a device can use firmware proto-
cols to inform the host when the media's contents have changed. For
some designs, another approach without this added complexity makes
more sense.
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