Intel® Stratix® 10 Hard Processor System Technical Reference Manual

ID 683222
Date 11/28/2022
Public

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18.6.2.2. Host Transaction

When configured as a host, the USB OTG controller pipes the USB transactions through one of two request queues (one for periodic transactions and one for nonperiodic transactions). Each entry in the request queue holds the SETUP, IN, or OUT channel number along with other information required to perform a transaction on the USB link. The sequence in which the requests are written to the queue determines the sequence of transactions on the USB link.

The host processes the requests in the following order at the beginning of each frame or microframe:

  1. Periodic request queue, including isochronous and interrupt transactions
  2. Nonperiodic request queue (bulk or control transfers)

The host schedules transactions for each enabled channel in round‑robin fashion. When the host controller completes the transfer for a channel, the controller updates the DMA descriptor status in the system memory.

For OUT transactions, the host controller uses two transmit FIFO buffers to hold the packet payload to be transmitted. One transmit FIFO buffer is used for all nonperiodic OUT transactions and the other is used for all periodic OUT transactions.

For IN transactions, the USB host controller uses one receive FIFO buffer for all periodic and nonperiodic transactions. The controller holds the packet payload from the USB device in the receive FIFO buffer until the packet is transferred to the system memory. The receive FIFO buffer also holds the status of each packet received. The status entry holds the IN channel number along with other information, including received byte count and validity status.

For generic hub operations, the USB OTG controller uses SPLIT transfers to communicate with slower‑speed devices downstream of the hub. For these transfers, the transaction accumulation or buffering is performed in the generic hub, and is scheduled accordingly. The USB OTG controller ensures that enough transmit and receive buffers are allocated when the downstream transactions are completed or when accumulated data is ready to be sent upstream.