USB System Architecture

The transaction translator performs error checks on the start-split transaction packets. If no errors are detected, the transaction is entered into the Start-Split Buffer and the transaction is performed to the lowor full-speed device.

If packet errors are encountered, the transaction translator ignores the transaction and the transaction is not performed. This error is detected during the com- plete-split transaction sequence.

Figure 17-23: Interrupt OUT Start-Split Packet Sequence

Interrupt OUT Complete Split Sequence

The host delivers the complete split transaction to verify that data was successfully delivered to the interrupt OUT endpoint. When the transaction translator receives the complete split transaction it searches the Complete Split Buffer for a matching entry. The response depends on the status of the entry and whether an entry is found. Figure 17-24 illustrates the complete split sequence for an inter-

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Chapter 17: 2.0 Hubs During LS/FS Transactions

rupt OUT transaction. The following sections discuss each of the possible responses to the complete split transaction and their possible causes.

Figure 17-24: Interrupt OUT Complete-Split Packet Sequence

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Complete Split Packet Error. If packet errors are detected by the hub, the transaction is discarded and the error is reported to the host. The error is reported via the usual USB method of not responding. When the host times out, it recognizes that a packet error has occurred and takes two possible actions:

1.It increments the error count for this endpoint, and if the count is <3, it immediately issues the complete-split transaction again.

2.It increments the error count, and if the count is ≥ 3, host software halts endpoint processing and reports the error to device-specific software.

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