Motorola USB08 Reference Manual page 63

USB08 Evaluation Board
MOTOROLA
A USB transaction is a series of packets to transmit information between
a host and a device. A transaction is always initiated by sending a token
packet (SETUP, IN, or OUT). This packet is always sent by the host
because devices cannot initiate a USB transaction. The token packet
contains the address and the desired endpoint of the device.
SETUP and OUT packets are supplemented by a DATA packet from the
host, which contains up to eight bytes of data. The packets DATA0 and
DATA1 are always sent in an alternating sequence. This procedure is
called data toggle and serves for error protection. Following the data
packet, the device answers with a handshake packet. If the device could
receive the data, it sends an ACK packet. If the device was not
immediately ready, it sends a NAK packet signalling to the host that the
packet should be sent again at a later time. In the event of an error, the
device sends a STALL packet.
SETUP Transaction
Host
SETUP (ADDR,EP)
OUT Transaction
Host
OUT (ADDR,EP)
Regarding IN transactions, the data packet is sent by the device and the
host closes the transaction with a handshake packet.
IN Transaction
Host
IN (ADDR,EP)
Universal Serial Bus (USB) Interface
Universal Serial Bus (USB) Interface
Host Device
DATA0 (D1..D8) ACK
Host Device
DATA0 (D1..D8) ACK
Device Host
DATA0 (D1..D8) ACK
USB Basics
Designer Reference Manual
63