Chapter 17: 2.0 Hubs During LS/FS Transactions

Isochronous IN Complete Split

A complete-split transaction causes the High-Speed Handler to search the Com- plete-Split Buffer for a matching entry. If a matching entry is found, the transaction translator returns either data or a handshake packet depending on the entry’s status.

Figure 17-22 on page 318 illustrates the sequence of events and lists the possible results of a complete-split isochronous IN transaction. The complete spit transaction consists of the complete split packet, followed by the IN token packet, and normally ends with the transfer of data to the host. One to six completesplit packets may be required to transfer all of the data from the isochronous endpoint to the host. Data is returned using the MDATA packet when more data will follow, and the transaction ends with the normal DATA0 packet. However, other responses may be issued to the host when errors occur. Each of the possible responses is discussed in the following sections.

Complete Split Packet Error. The first error that can occur is with the delivery of the complete split packet. If a packet error is detected by the HighSpeed Handler, it ignores the transaction and no data is returned to the host. The host expecting a data packet to be returned following the IN token packet, times out when no data is returned, and notifies the device-specific software of the error. Because USB does not attempt any retry during isochronous transactions, the data received from the isochronous endpoint will be lost.

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USB System Architecture

Figure 17-22: Sequence of Complete-Split Transaction During Isochronous IN

Complete Split

CSplit Token Packet

IN Token Packet

Complete Split with MDATA. This packet is returned to the host when valid data is to be transferred and when the endpoint is still returning more data to the Complete Split Buffer.

Complete Split with DATA0. This data packet indicates to the host that no more data will be returned for this isochronous IN transaction.

Complete Split with NYET. The Not Yet packet is returned to the host when no matching entries are found in the Complete Split Buffer. This can occur when the transaction translator recognizes that the transaction is still in progress on the lowor full-speed bus, and too little or no data has yet accumulated in the Complete Split Buffer. If fewer than three bytes of the low-/full- speed data packet have been received at the end of a microframe, the transaction translator must respond with a NYET to the corresponding high-speed complete-split.

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