USB System Architecture

Figure 7-19: IN Transaction During Isochronous Transfer

OUT Transactions

A typical OUT transaction consists of the token packet, data packet, and handshake packet. However, in some instances an OUT transaction may not include all three packets. OUT transactions can be employed when performing bulk, control, and isochronous transfers. Several conditions may occur during an OUT transaction:

•data is sent without errors.

•data is sent with errors.

•the target is temporarily unable to accept data.

•the target is unable to accept data without an existing error condition being cleared.

•an isochronous transfer is taking place; thus, data is sent to the target, but no handshake follows the data phase.

Each of these conditions is discussed and illustrated in the following paragraphs.

OUT Transaction Without Data Packet Errors

Figure 7-20 illustrates a normal OUT transaction in which data is successfully sent to the target device without error. The target device, having received the data without error returns an ACK handshake to the root hub. Such OUT transactions can take place during bulk, interrupt, or control transfers.

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Chapter 7: Packets & Transactions

Figure 7-20: OUT Transaction Without Errors

OUT Transaction with Errors

Figure 7-21 assumes that the same transaction described in the preceding paragraph is performed except that in this instance, the target device detects errors when receiving the data. The target device discards the data and does not respond with a handshake. The host will detect a bus time-out since the expected handshake packet does not appear in time (see “Bus Time-Out” on page 172). The host must retry the OUT transaction at a later time.

Figure 7-21: OUT Transaction with Data Packet Errors

OUT Transaction — Target Unable to Accept Data

The transaction illustrated in Figure 7-22 results if the target device is temporarily unable to accept data. The target receives the data without error, but cannot accept it (e.g., due to a busy or buffer full condition). In response, the target returns NAK during the handshake phase of the transaction. This informs the host that the target did not accept the data and that the transaction must be retried later.

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USB System Architecture

Figure 7-22: OUT Transaction to Target That is Unable to Accept Data

OUT Transaction With Target Stalled

Figure 7-23 illustrates a transaction that results from the target device being unable to receive data due to an internal error condition. The target receives the OUT token recognizing that it is addressed; however, when the host sends the data packet, the target device is unable to accept the data and returns a STALL to the host during the handshake phase of the transaction. The STALL condition is reported to the host software, which must clear the error condition within the target.

Figure 7-23: OUT Transaction to Stalled Endpoint

OUT Transaction During Isochronous Transfer

Figure 7-24 illustrates an OUT transaction performed during an isochronous transfer. Whether data is received with or without errors by the target device, no handshake phase is performed.

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