USB System Architecture

High Speed Handler

The High-Speed Handler interfaces the transaction translator to the high-speed bus. It receives high-speed packets and performs packet error checks including CRC. The handler makes start split entries into the non-periodic buffers and searches the buffers for a matching entry when it receives a complete split transaction.

Non-periodic Buffers

The specification requires a transaction translator to implement two buffers for managing split bulk/control transactions. Each buffer handles a single transaction at a time and is used to store start split and complete split information. The buffers are filled by the host when it issues a start split transaction. The low-/ full-speed buffer fetches start split information and generates the specified transaction. Data or completion status is accumulated in the same buffer and becomes the complete split data that the host retrieves when it issues the complete split transaction.

Low-/Full-Speed Handler

This handler fetches start-split information from the non-periodic buffers and starts the specified transaction on the target port. During IN transactions the handler transfers data to the buffer, and during OUT transactions it places the handshake packet or other status information into the buffer.

The low-/full-speed handler checks for packet errors and is required to perform local retries of transactions that fail. Thus, no time-out response status needs to be monitored. The typical “three strike and you’re out” retry mechanism is employed by the handler.

Bulk/Control Split OUT Transaction Sequence

Bulk and control transactions are grouped during this discussion because they have the same characteristics and use the same packet sequences. However, two important differences exist between the bulk and control transfer sequences:

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

2.The setup transaction of a control transfer may have a different complete split because the ACK handshake is the only legal response to this transaction.

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