LM3S5762 Datasheet, PDF (630/841 Pages) Texas Instruments – Stellaris® LM3S5762 Microcontroller

Universal Serial Bus (USB) Controller
The USB controller maintains a frame counter. If the target Device is a full-speed device, the USB
controller automatically sends an SOF packet at the start of each frame and increments the frame
counter. If the target Device is a low-speed device, a K state is transmitted on the bus to act as a
keep-alive to stop the low-speed device from going into SUSPEND mode.
After the SOF packet has been transmitted, the USB Host controller cycles through all the configured
endpoints looking for active transactions. An active transaction is defined as a receive endpoint for
which the REQPKT bit is set or a transmit endpoint for which the TXRDY bit and/or the FIFONE bit is
set.
An isochronous or interrupt transaction is started if the transaction is found on the first scheduler
cycle of a frame and if the interval counter for that endpoint has counted down to zero. As a result,
only one interrupt or isochronous transaction occurs per endpoint every n frames, where n is the
interval set via the USB Host Transmit Interval Endpoint n (USBTXINTERVALn) or USB Host
Receive Interval Endpoint n (USBRXINTERVALn) register for that endpoint.
An active bulk transaction starts immediately, provided sufficient time is left in the frame to complete
the transaction before the next SOF packet is due. If the transaction must be retried (for example,
because a NAK was received or the target Device did not respond), then the transaction is not
retried until the transaction scheduler has first checked all the other endpoints for active transactions.
This process ensures that an endpoint that is sending a lot of NAKs does not block other transactions
on the bus. The controller also allows the user to specify a limit to the length of time for NAKs to be
received from a target Device before the endpoint times out.
16.3.2.5
USB Hubs
The following setup requirements apply to the USB Host controller only if it is used with a USB hub.
When a full- or low-speed Device is connected to the USB controller via a USB 2.0 hub, details of
the hub address and the hub port also must be recorded in the corresponding USB Receive Hub
Address Endpoint n (USBRXHUBADDRn) and USB Receive Hub Port Endpoint n
(USBRXHUBPORTn) or the USB Transmit Hub Address Endpoint n (USBTXHUBADDRn) and
USB Transmit Hub Port Endpoint n (USBTXHUBPORTn) registers. In addition, the speed at
which the Device operates (full or low) must be recorded in the USB Type Endpoint 0 (USBTYPE0)
(endpoint 0), USB Host Configure Transmit Type Endpoint n (USBTXTYPEn), or USB Host
Configure Receive Type Endpoint n (USBRXTYPEn) registers for each endpoint that is accessed
by the Device.
For hub communications, the settings in these registers record the current allocation of the endpoints
to the attached USB Devices. To maximize the number of Devices supported, the USB Host controller
allows this allocation to be changed dynamically by simply updating the address and speed
information recorded in these registers. Any changes in the allocation of endpoints to Device functions
must be made following the completion of any on-going transactions on the endpoints affected.
16.3.2.6
Babble
The USB Host controller does not start a transaction until the bus has been inactive for at least the
minimum inter-packet delay. The controller also does not start a transaction unless it can be finished
before the end of the frame. If the bus is still active at the end of a frame, then the USB Host controller
assumes that the target Device to which it is connected has malfunctioned, and the USB controller
suspends all transactions and generates a babble interrupt.
16.3.2.7
Host SUSPEND
If the SUSPEND bit in the USBPOWER register is set, the USB Host controller completes the current
transaction then stops the transaction scheduler and frame counter. No further transactions are
started and no SOF packets are generated.
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November 17, 2011
Texas Instruments-Production Data