USB System Architecture

Isochronous IN Split Transaction Sequence

Isochronous IN transactions consist of a single start-split transaction and from one to six complete-split transactions used to return data to the host. The possible error conditions and responses are discussed in the following two sections.

Isochronous IN Start Split

Figure 17-21 on page 316 illustrates the start-split transaction sequence that consists of the start-split packet and the IN token packet. When the transaction is received without errors, the High-Speed Handler enters the transaction into the Start-Split Buffer and the Full-Speed Handler fetches and executes the transaction at an appropriate time. Data is accumulated in the Complete-Split Buffer and the host fetches it via a complete-split transaction.

If a packet error is detected in the start-split packet, the transaction is discarded because the information specifying the destination hub and port cannot be trusted. No check is performed on the IN token packet. The target device will detect errors associated with the IN token and will respond by not returning data. This condition is covered in the following section on complete split errors.

Figure 17-21: Sequence of Packets in Start-SplitTransaction During Isochronous IN

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