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USB CONTROLLER
10.3.6 CONTROL TRANSACTIONS
Control transfers are bursty, non-periodic, host software-initiated request/response communication, typically used
for command/status operations.
Control transfers allow access to different parts of a device. Control transfers are intended to support
configuration/command/status type communication flows between client software and its function. A control
transfer is composed of a Setup bus transaction moving request information from host to function, zero or more
Data transactions sending data in the direction indicated by the Setup transaction, and a Status transaction
returning status information from function to host. The Status transaction returns "success" when the endpoint has
successfully completed processing the requested operation.
Control transfers are supported via bi-directional communication flow over message pipes. As a consequence,
when a control pipe is configured, it uses both the input and output endpoint with the specified endpoint number.
10.3.7 ISOCHRONOUS TRANSACTIONS
In non-USB environments, isochronous transfers have the general implication of constant-rate, error-tolerant
transfers. In the USB environment, requesting an isochronous transfer type provides the requester with the
following:
— Guaranteed access to USB bandwidth with bounded latency
— Guaranteed constant data rate through the pipe as long as data is provided to the pipe
— In the case of a delivery failure due to error, no retrying of the attempt to deliver the data.
While the USB isochronous transfer type is designed to support isochronous sources and destinations, it is not
required that software using this transfer type actually be isochronous in order to use the transfer type.
An isochronous pipe is a stream pipe and is, therefore, always uni-directional. An endpoint description identifies
whether a given isochronous pipe's communication flow is into or out of the host. If a device requires bi-
directional isochronous communication flow two isochronous pipes must be used, one in each direction.
10.3.8 INTERRUPT TRANSACTIONS
The interrupt transfer type is designed to support those devices that need to send or receive small amounts of
data infrequently, but with bounded service periods. Requesting a pipe with an interrupt transfer type provides the
requester with the following:
— Guaranteed maximum service period for the pipe
— Retry of transfer attempts at the next period, in the case of occasional delivery failure due to error on the
bus.
An interrupt pipe is a stream pipe and is therefore always uni-directional. An endpoint description identifies
whether a given interrupt pipe's communication flow is into or out of the host.
10-6
S3C2500B
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