Chapter 7: Packets & Transactions

Figure 7-24: OUT Transaction During Isochronous Transfer

Isochronous endpoint does not return a handshake pack et

Setup Transactions/Control Transfers

Control transfers are used to issue commands (also called requests) to USB devices. For example, control transfers are performed during USB device configuration to read the standard descriptors and assign a unique address to a device. Control transfers always begin with a setup transaction, called the setup stage. The setup stage defines the nature of the control transfer. Some control transfers include a data stage consisting of one or more IN or OUT transactions that are used to deliver the payload of the control transfer. Whether data is sent to or received from the device is defined by the setup stage. The final stage of a control transfer is the status stage. This stage confirms that the requested operation has been completed successfully. Control transfers exist in two basic forms:

•Transfers consisting of a setup stage and status stage.

•Transfers consisting of a setup stage, data stage, and status stage.

The data phase of a setup transaction contains 8 bytes of information as defined in Table 7-3. This information defines the request being issued to the device and provide all the information needed by the device to fulfill the request.

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Table 7-3: Format of Setup Transaction Data Phase

Two Stage Control Transfer

A two stage control transfer consists solely of the setup and status stages as illustrated in Figure 7-25. In this instance the 8 bytes delivered during the setup transaction contains all the information needed to perform the specified request (e.g., remote wake-up request). The status stage consists of an IN transaction to verify that the request has been successfully processed.

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Chapter 7: Packets & Transactions

Figure 7-25: Format of a Two Stage Control Transfer

Three Stage Control Transfer with IN Data Stage

Figure 7-26 illustrates a control transfer that requires data be returned from the target device back to the host. As an example, a control transfer may be performed when the host system issues a request to read a device descriptor. The first stage of the transfer consists of the setup transaction that defines the nature of the control transfer. The setup transaction consists of the setup token, data packet, and handshake. During setup transactions data is always sent to the target device to specify the type of request. Following the setup phase, the host initiates one or more IN transactions, prompting the target to return the requested data. The host completes the control transfer by using an OUT transaction to request verification that the device endpoint has successfully returned the contents of the descriptor. Note that the target device indicates successful completion of the request by issuing an ACK handshake during the OUT transaction. The OUT data packet issued by the host has a length of zero.

Figure 7-26: Control Transfer Requesting Data from Target

Three Stage Read Control Transfer

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Three Stage Control Transfer with OUT Data Stage

Figure 7-27 illustrates the format of a control transfer in which a command is issued to a control endpoint. The setup transaction defines the request being issued and is followed by the data stage (one or more OUT transactions). The host then issues an IN token to the control endpoint to obtain completion status. The target returns a data packet with a length of zero to indicate that it has successfully processed the initial control request.

Figure 7-27: Control Transfer Issuing a Command to a Target’s Control Endpoint

Control Transfers With Errors

The host system attempts retries to ensure that a control transfer completes successfully. The action taken by the target and host depends on the nature of the error condition and when the error occurs during the transfer. See Chapter 8, entitled "Error Recovery," on page 167 for further details.

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