Appendix

Appendix A:

Standard Device

Requests

Overview

All USB devices must respond to a variety of requests called “standard” requests. These requests are used for configuring a device, for controlling the state of its USB interface, and with other miscellaneous features. Device requests are issued by the host using the control transfer mechanism. Prior to configuration, a device responds to its default address of zero. This permits configuration software to request the contents of any device’s descriptors (from endpoint zero) using device address zero during configuration.

Additionally, devices may also support class-specific requests. Except for hub devices, these class requests are defined by the device-class specifications and are not included within the main portion of the specification. Similarly, a device may support vendor-specific requests that pertain to a given vendor’s implementation.

Control transfers, used to transmit device requests, consist minimally of a setup stage and a status stage, but may also include a data stage, depending on the type of request being performed. The setup stage consists of setup transactions as illustrated in Figure A-1. The eight byte data payload of a setup transaction defines the type of request being issued by the host. This appendix discusses only the standard device requests. Refer to the particular device class chapter for information related to class-specific requests.

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USB System Architecture

Figure A-1: Format of Setup Transaction that Specifies the Device Request Being Performed

Standard Device Requests

When a control transfer is initiated, the setup stage of the transaction specifies the particular request to be performed by the device. The format of the setup data is shown inTable A-1, while Tabl eA-2 on pag e438 defines the contents of the setup data for each of the standard request types.

Note that in Figure A-1 bits 5 and 6 of offset zero are 00b indicating that the specific request is a standard request type. The second byte (the request field) defines which standard request is to be performed, while the definitions of the other fields are dependent upon the request specified. For example, the “Clear Feature” request defines the “value” field as the feature selector. Figure A-3 lists the specific features that the request applies to. The following sections describe each of the standard requests.

Note that the last column in Table A-2, labeled “Data,” indicates whether the request requires a data stage during the control transfer. For example, the “Get Configuration” request uses the data stage to transfer the configuration value. Many requests, however, can be performed without a data stage.

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Appendix A: Standard Device Requests

Table A-1: Format of Data Payload during SetupTransactions

If the request contains fields with illegal values or values not supported, the endpoint to which the request is directed will automatically enter the stalled state. Host software must clear the stall condition using the “Clear Stall” request. A control endpoint must continue accepting the setup transaction, even if it is stalled. If the default control endpoint fails to respond to a setup transaction, the device must be reset to clear the condition.

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USB System Architecture

Table A-2: Standard Device Requests

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