Information Technology Reference
In-Depth Information
• The ability to detect and respond to the class-specific Bulk Only Mass
Storage Reset and Get Max LUN requests. (A device with a single logical
unit can stall the Get Max LUN request.)
• Support for the USB mass-storage class's protocol for receiving and
responding to commands required for the mass-storage interface's sub-
class and peripheral device type.
The device firmware doesn't have to support a file system. The USB trans-
fers just read and write blocks of data at logical block addresses in the storage
media. The device doesn't have to know or care about the contents of the
data blocks. The host software translates requests to read and write files and
directories into requests to read and write to blocks at specific LBAs.
Choosing a Device Controller
A mass-storage device can use just about any full- or high-speed device con-
troller chip. Low-speed chips aren't an option because they aren't allowed to
do bulk transfers. Some device controllers are designed specifically for use in
mass-storage devices. These controllers provide dedicated interfaces and
other support for accessing popular media types, including flash memory
and hard drives.
Controllers with Support for Flash Memory
Standard Microsystems Corporation (SMSC) has the USB2228 flash media
controller with support for communicating with MultiMediaCards, SD
Cards, CompactFlash, and other flash-memory card types. The chip can
also access raw flash-memory chips via a generic memory and I/O interface.
The chip includes an 8051-compatible microcontroller and 15 general-pur-
pose I/O pins. The USB interface supports the control endpoint and two
bulk endpoints required for a mass-storage device. Internal FETs can switch
power to memory cards. There is hardware support for ECC error correc-
tion and SD-Card security commands.
The microcontroller can run code from ROM or from external memory. If
running code from ROM, an external EEPROM can provide product-spe-
cific and chip-specific information such as a serial number and configura-
tion data.
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