Chapter 8: Error Recovery

the packet has been corrupted or that the sender is not properly generating stuffed bits, or that the receiver is not decoding the NRZI data correctly.

Packet-Related Error Handling

Any one of the packet-related errors results in the same device behavior. In every instance, the receiver of a corrupted packet must ignore the packet and not respond. The error handling method, however, varies depending on the transaction and which packet of a given transaction is corrupted.

Token Packet Errors

IN Packet Errors. Consider an IN packet during which some form of packet error occurs. Since an error is detected in the token packet, it is ignored by the target device and no response is returned to the host. The host expects either a data or handshake packet to be returned in response to the IN token. However, since the target does not respond, the host will detect a bus time-out and the transaction fails. The host is then responsible for retrying the failed transaction.

OUT or SETUP Packet Errors. If an OUT or SETUP packet is being broadcast by the host when a packet error is detected, the host will follow the token packet with a data packet. The target may decode the data packet without error, but may not be able to verify that it is the intended recipient (due to an address CRC error), or may not be able to detect the meaning of the data packet (due to a PID check error). Since the target does not respond to the OUT token nor the following data packet, the host detects a bus time-out and knows and that the transaction has failed. The host then reschedules the transaction.

Data Packet Errors

During OUT or SETUP Transactions. Errors occurring within data

packets cause the receiving device to discard the data and not respond to the sender. During an OUT transaction, the target, having detected a data packet error, will not respond with a handshake. The resulting time-out tells the host of the failed write. The host then retries the transaction later.

During IN Transactions. During IN transactions, valid data is returned by the target but the host receives a corrupted data packet. Since the host does not respond with an ACK handshake, the target is informed that the host did not receive the data. The host then must retry the transaction.

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