Motorola USB08 - Manual (Page 64)

Designer Reference Manual USB08 Evaluation Board
64 Universal Serial Bus (USB) Interface MOTOROLA
Universal Serial Bus (USB) Interface
If the device does not hold any data ready for sending, it sends a busy
handshake NAK instead of the data packets.
IN Transaction (Device Busy)
While the actual data communication via the stream pipes is based on
simple IN and OUT transactions, the management of the device uses
more complex control transfers via the control endpoint 0. These control
transfers are secured using a double handshake. Control transfers
consist of two or three transaction stages, like those shown here.
2-Stage Control Transfer (No Data)
3-Stage Control Transfer (Host Read)
The setup stage starts with a SETUP transaction (see above). The data
stage is necessary if data has to be sent from the device to the host and
consists of several IN transactions. The status stage serves for the back
confirmation that the information was processed correctly. In the status
stage, empty DATA1 packets are sent.
Host Device
IN (ADDR,EP) NAK
Setup Stage Status Stage
SETUP,DATA0,ACK IN,DATA1,ACK
Setup Stage Data Stage Status Stage
SETUP,DATA0,ACK IN,DATA1,ACK (...) OUT,DATA1,ACK