Designer Reference Manual
USB08 Evaluation Board
62
Universal Serial Bus (USB) Interface
MOTOROLA
Universal Serial Bus (USB) Interface
4.4 USB Basics
Concerning the electrical interface, the plug and the cable, as well as
questions on the bus topology we refer to the Universal Serial Bus
Specification Revision 1.1 standard reference. The volume of
information contained in this specification exceeds by far this manual.
Also some good introductions are offered by books, for example the
book by Kelm
(1)
.
NOTE:
Some terms and procedures from the USB specification, which are
important for the implementation of the reference design, will be
repeated here and described briefly.
Packets form the basic modules of the USB communication in the levels
above the electrical connection. Packets are atomic, for instance, they
cannot be interrupted or divided into sections. The packet types shown
in
are relevant for low-speed USB.
In addition to the packets shown in
, the bus traffic consists of
further quasi-static bus conditions (reset, suspend, resume) and the
“Keep-alive-EOP” (refer to Universal Serial Bus Specification
Revision 1.1 standard for more detailed information).
1. Kelm, H.J.: USB1.1; Franzis 2000
Table 4-1. Low-Speed USB Packet Types
Name
Group
Function
SETUP
Token
Starts a control transfer
IN
Token
Starts a data transfer to the host
OUT
Token
Starts a data transfer to the device
DATA0
Data
Transfers 0 to 8 data bytes
DATA1
Data
As before (toggle Data0/1)
ACK
Handshake
Information was accepted
NAK
Handshake
Busy — send again later
STALL
Handshake
Information was incorrect
