Chapter 4 Software Development
page 4-7
4-1-10. Device RequestsDevice 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.
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 Return status for specified recipient
1CLEAR_FEA TURE Clear or disable a specific feature
3SET_FEATURE Set or enable a specific feature
5SET_ADDRESS Set device address for al future device accesses
6GET_DESCRIPTOR Return the specified descriptor, if it exists