USB System Architecture

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 on 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 IN Complete Split Sequence

Figure 17-31 on page 335 illustrates the packet sequence and the possible responses that may be returned from the transaction translator. Note that the bulk/control IN sequence does not include the MDATA packet that is used during interrupt IN split transactions. Because timing and schedules are not as critical for bulk and control transfers there is no need to transfer data during every microframe. The transaction translator waits for the buffer to fill before returning data. Any complete split transaction issued by the host while the transaction is being performed results in a NYET packet response, indicating that the data packet is not yet ready for transfer.

The following sections describe the possible responses to the complete split transaction.

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Chapter 17: 2.0 Hubs During LS/FS Transactions

Figure 17-31: Bulk/Control IN 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.

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USB System Architecture

Complete Split with NYET. This packet is returned to the host when the transaction translator recognizes that the transaction is still in progress on the lowor full-speed bus. Thus, the IN data packet is not yet available. The host issues the complete-split transaction again sometime in the future. This is not considered an error, since there is no specific time during which the transaction must complete.

Complete Split with NAK. This packet is returned to the host by the transaction translator if the lowor full-speed transaction ended with a NAK handshake packet. NAK indicates that the endpoint was not able to return the bulk/control data because there is no buffer space to store the packet. The host retries the split OUT transaction at some time in the future based on its normal schedule.

Complete Split with STALL. The transaction translator returns a STALL handshake under three circumstances:

1.The low-/full-speed transaction ended with a STALL handshake.

2.The transaction translator does not find a local buffer with a corresponding transaction.

3.The transaction translator has attempted to perform the transaction three times due to errors and has failed.

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