USB System Architecture

bulk/control OUT transaction. This, of course, indicates that the transaction has completed normally, and that the next command can be processed by the host.

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, no completion status is yet available. The host issues the complete split transaction some time 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 accept the bulk/control data, likely because there is no buffer space for the current 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.

Bulk/Control Split IN Transaction Sequence

Bulk and Control Transactions are grouped during this discussion as they were for the bulk/control OUT transaction, because the sequence of events is the same for both transfer types. Only one important difference exists between bulk and control transfer sequence and operation:

1.Control transfers don’t include the endpoint direction check for buffer matching, as is done with other transfer types.

Bulk/Control IN Start Split Sequence

The bulk/control IN start-split transaction consists of the complete-split packet and the IN token packet as illustrated in Figure 17-30 on page 333. The possible hub responses are discussed in the following sections.

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Chapter 17: 2.0 Hubs During LS/FS Transactions

Figure 17-30: Bulk/Control IN Start-Split Sequence

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Start Split with Packet Error. If, during the start-split transaction, any of the packets sent from the host incur a packet error, no response is returned to the host. The host times out when no packet is returned, causing it to increment the error count for this endpoint. Two possible actions are taken by the host 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.

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