Designer Reference Manual USB08 Evaluation Board
62 Universal Serial Bus (USB) Interface MOTOROLA
Universal Serial Bu s (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 Table 4-1 are relevant for low-speed USB.
In addition to the packets shown in Table 4-1, 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 Tok en Starts a control transfer
IN Toke n Starts a data transfer to the host
OUT Toke n 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