Information Technology Reference
In-Depth Information
device firmware to access files on its own only when the device isn't attached
to and enumerated by a USB host. For example, a data logger can collect
data in the field and store the data in files. When the device is brought in
from the field and attached to a USB host, the firmware no longer collects
and stores data on its own and instead responds to commands from the USB
host. Device firmware can use the presence of VBUS or successful enumera-
tion of an attached device to determine whether the firmware can access
files.
Reset Behavior
A device generates a UNIT ATTENTION condition after experiencing a
reset. The SCSI Architectural Model specification describes three conditions
that correspond to different reset types. The conditions are power on, hard
reset, and logical-unit reset. A device sets an ADDITIONAL SENSE
CODE value to indicate which type of reset occurred.
A power-on condition exists after power is applied. The power-on condition
causes a hard-reset condition. A transport protocol can define other events
that cause a device to enter the hard-reset condition. A hard reset in turn
causes the device to enter the logical-unit reset condition. Other logical-unit
reset events can cause the device to enter the logical-unit reset condition.
Unlike the power-on and hard reset, a logical-unit reset doesn't reset
mode-page and other parameters to default values.
The USB specifications define two reset types for mass-storage devices: the
USB port reset and the Bulk-Only Mass Storage Reset request. The USB
specifications don't map these resets to SCSI reset types and thus don't spec-
ify SCSI behaviors on resetting. Devices typically map the USB port reset to
the SCSI hard reset. Some devices map the bulk-only mass-storage reset to
the hard reset, while others map the bulk-only reset to the logical-unit reset.
Problems can result if a host's expectations don't match a device's behavior
after a reset.
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