Standard Device Requests, Device Request Types

Chapter 4 Software Development

4-1-10. Device Requests

Device requests represent commands from the host to USB functions using control transfers. The USB function parses the request, performs the necessary action, and returns the appropriate response.

There are three types of device requests.

	Table 4.1.6. Device Request Types

Standard	Requests that all USB devices must support because they hold the key to hot plugging
	(a.k.a. dynamic insertion and removal)
Class-specific	Requests required by the USB Device Class Specifications for the general application
Vendor-specific	Requests implemented by the vendor for accessing functionality particular to the USB
	device

A control transfer starts with a SETUP token packet followed by an 8-byte data packet containing the following fields. Note that the meaning of the wValue and wIndex fields depends on the request type.

Table 4.1.7. Control Transfer Data Packet

bmRequestType	1byte	Transfer direction, request type, and endpoint address
bRequest	1byte	Request number
wValue	2byte	Value (meaning depends on the request type)
wIndex	2byte	Index or offset (which depends on the request type)
wLength	2byte	Number of bytes transferred during the Data stage

A control transfer consists of up to three stages.

1.Setup stage: The data packet following the SETUP token packet is an 8-byte command from the host to the device. The device parses the command and, as necessary, prepares to send or receive data.

2.Data stage: The pipe transfers the specified data in the direction specified by the command. Note that commands that do not involve data transfer skip this stage.

3.Status stage: The host initiates a transaction, in the direction opposite the immediately preceding data transfer, for reporting command success or failure from the device to the host.

4-1-11. Standard Device Requests

The standard device requests are common ones to all USB devices. Most important are those that support hot plugging (a.k.a. dynamic insertion and removal). Detailed descriptions are in Section 9.- 4 of the USB version 1.1 specifications.

Table 4.1.8. Standard Device Requests

No. Request Name

Description

0GET_STATUS

1CLEAR_FEA TURE

3 SET_FEATURE

5SET_ADDRESS

6GET_DESCRIPTOR

Return status for specified recipient Clear or disable a specific feature Set or enable a specific feature

Set device address for al future device accesses Return the specified descriptor, if it exists

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