USB System Architecture

Complete Split with ACK. The transaction translator returns the ACK packet when the target device has returned an ACK handshake during the interrupt 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 information is yet available.

Two possible actions are taken by the host in this event:

1.The host issues the complete split transaction in a subsequent microframe.

2.If the NYET handshake occurs during the final microframe of the 1ms

frame, then the error count is incremented. If the error count is <3, the startsplit transaction is attempted again. If the error count is ≥ 3, the endpoint is halted, and device-specific software is notified.

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 interrupt data, likely because it has no room for the current packet. The host retries the split interrupt OUT transaction at the next polling interval for this endpoint.

Complete Split with STALL. When the lowor full-speed transaction ends with a STALL handshake, the STALL handshake is also returned to the host during the complete-split transaction. This notifies the host that the endpoint has incurred a serious error. Host software reports the error to the devicespecific software for handling.

Complete Split with ERR. This packet notifies the host that the transaction to the target device has failed due to a failed handshake. The transaction is retired by the transaction translator, and the host increments the error count. If the error count is <3, the transaction is rescheduled during the next polling interval. If the error is ≥ 3, the host halts the endpoint.

Interrupt IN Split Transaction Sequence

Interrupt IN split transactions consist of the split-start transaction and conclude with one or two complete split transactions. As with the previous examples, a variety of conditions may affect the normal sequence. These possibilities are described in the following sections.

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