USB System Architecture

depending on the error count:

1.If the error count is <3, the start-split is retried at some time in the future.

2.If the error count is ≥ 3, the endpoint is halted.

The transaction translator discards the packet when the error is detected and no entry is made in the buffer.

Start Split with ACK. The normal response to a start-split packet is the ACK handshake packet that tells the host that the start split was accepted by the transaction translator. The start split may be accepted in two ways:

1.the start split transaction is entered into one of the non-periodic buffers.

2.the start split matches an old entry in one of the buffers. The old entry may be present in the buffer because of an error detected by the host during the acknowledgement of the start split transaction. In this case, the host would re-issue the start split that the buffer had just received. In the event of a match with a previous entry, ACK is returned but the data is discarded.

When a new entry is made, the transaction translator initiates the transaction on the low-/full-speed bus when it has time. Once the host gets confirmation that the start split was accepted by the transaction translator, it must not issue additional start splits to the same endpoint until the current transaction completes.

Start Split with NAK. A NAK packet is returned to the host if all the nonperiodic buffers are currently in use. This notifies the host that the start split was not accepted and the host retries the transaction later.

Bulk/Control OUT Complete Split Sequence

Figure 17-29 on page 331 illustrates the packet sequence and possible transaction translator responses to an OUT complete split transaction targeting a bulk or control endpoint. Each of these responses is described in the following sections.

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Chapter 17: 2.0 Hubs During LS/FS Transactions

Figure 17-29: Bulk/Control OUT Complete-Split Sequence

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Complete Split Packet Error. Packet errors detected by the hub result in the complete split transaction being 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.The host increments the error count for this endpoint, and if the count is <3, it retries the complete-split transaction again, but can issue other split transaction to other endpoints before retrying this one.

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

Complete Split with ACK. The transaction translator returns the ACK packet when the target device has returned an ACK handshake during the

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