Chapter 17: 2.0 Hubs During LS/FS Transactions

Two possible actions are taken by the host when it receives NYET:

1.The host issues the complete split transaction in the next microframe if data transfers are still pending.

2.If the NYET handshake occurs during the final microframe of the 1 ms frame, the transaction ends and the error is reported to device software.

Complete Split with ERR. The ERROR packet is returned to the host when an error has occurred on the low-speed/full-speed bus. The host then notifies device-specific software of the error.

Handling CRC16 During Split Isochronous IN Transactions

Only one CRC16 is returned at the end of the data packet from the target isochronous endpoint. The full-speed handler is required to check this parity and report the CRC error by allowing the isochronous complete-split to time out. In many cases, an isochronous IN data packet transfer crosses several microframe boundaries, requiring as many complete split transactions to fetch the contents of the data packet returned by the endpoint. When more data must be sent, the transaction translator issues an MDATA packet to tell the host that another com- plete-split is required. In this case, the high-speed handler must generate CRC16 for each high-speed data packet sent to the host. Also, even if the MDATA packets are transferred successfully, subsequent transactions can fail, thereby causing the entire packet to fail.

Interrupt Split OUT Transaction Sequence

The interrupt OUT split transaction sequence begins with a single start-split transaction that delivers OUT data to the transaction translator. The transaction ends when completion status information is returned to the host via the com- plete-split transaction. Characteristics of these transactions are:

•Interrupt transactions are performed at either fullor low-speed.

•Maximum packet size for the interrupt data payload is 64 bytes for fullspeed transactions and 8 bytes for low-speed transactions.

•Transactions will always complete in one or two microframes.

Interrupt OUT Start Split Sequence

Figure 17-23 illustrates the packet sequence of an interrupt OUT split transaction, consisting of the start SPLIT packet, OUT token, and DATA0/1 packet. The data packet will have a maximum data payload of 64 bytes.

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