Designing a Universal Serial Bus (USB) Device Using the Cypress ...

Cypress USB Thermometer Application Note
An IN or OUT can also be used as part of a larger sequence of transactions called a Control
Transfer. In a control transfer (see below), INs or OUTs are used both to transfer data and to
return status to the host.
Control Transfers
A host needs to be able to communicate with a device to control it or obtain its status. This is
done through the device’s Endpoint 0 using control transfers. A control transfer is simply a special
series of transactions in a specific sequence between a host and a control endpoint (Endpoint 0).
Control transfers are used by a host to enumerate a device or change its state in any way, or
obtain its status. Control transfers may also be used to send data to a device (although this is not
the original intention of a control transfer).
A control transfer has at least two transaction stages (a SETUP stage and a Status stage). It may
also have an optional data stage, which consists of one or more IN or OUT transactions.
The SETUP stage is illustrated in Figure 5.
There are three types of Control Transfers: a Control Read, a Control Write, and a No-data
Control.
The three types are illustrated in Figure 6.
SETUP Stage
The SETUP stage (See Figure 5) consists of three packets: the SETUP token, eight bytes of
data, and the handshake packet.
A “SETUP” is a special type of “OUT” that gives a specific command to the USB device to do
something.
Idle
Token
SETUP
Data
DATA0
Handshake
ACK
Idle
Host
Function
Figure 5
Control SETUP Transaction
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