USB System Architecture

ceeds to the periodic transfers via the HCCA register. Note that interrupts are scheduled differently from the other transfer types. The HCCA register increments at the end of each frame to point at the next linked list of interrupt TDs, which are scheduled for completion during the current frame. The end of the interrupt list always links to the beginning of the isochronous EDs. Once the isochronous transfers have completed, the controller is free to continue processing control and bulk transfers where it left off, if sufficient time remains within the current frame.

The OHC links transfer descriptors to the “Done” queue after it has completed successfully or if an error has occurred when performing the TD. The OHCD can then check the done queue for completion status information.

The ED and TD List Structure

All transfers are scheduled using the standard TD list structure as illustrated in Figure D-3. The head pointer is the OHC register for control and bulk transfers. A different interrupt head pointer is selected for each frame as the HCCA register value increments. The interrupt head pointer is located in the HCCA memory area. Isochronous EDs simply link from the last interrupt ED in the current interrupt list. TDs are enqueued to each ED as IRPs are requested by USB device drivers. An endpoint that is currently idle will not have any TDs enqueued.

Interrupt and Isochronous Transfer Processing

Processing the interrupt and isochronous ED list begins with the interrupt head pointer for the current frame. The list is traversed sequentially, until one transaction associated with the first TD has been performed for each endpoint in the list.

Control and Bulk Transfer Processing

At the beginning of a frame the control and bulk queues are processed until the “remaining” field of the HcFmRemaining register is less than or equal to the “start” field of the HcPeriodicStart register. More control transfers are performed than bulk transfers based on a ratio of control to bulk transfers specified within the “ControlBulkServiceRatio” field of the HcControl register. The controller performs these transfers based on a round robin sequence tied to the ratio (e.g., three control transfer followed by one bulk transfer). If sufficient time remains within the frame after completing the periodic transfers, the controller returns to the control and bulk lists, and continues processing where it left off.

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