Information Technology Reference
In-Depth Information
USB is a balanced bus architecture which hides the complexity of the operation from the
devices connected to the bus. The USB host controller controls system bandwidth. Each de-
vice is assigned a default address when the USB device is first powered or reset. Hubs and
functions are assigned a unique device address by USB software.
Typical examples of USB connected devices are:
•
Digital speakers/ microphones.
•
Joysticks.
•
Scanners/ modems/ printers/ monitors.
•
Game controllers/ graphics tablets.
•
Video conferencing cameras.
•
Musical interfaces, such as MIDI.
9.2 USB
9.2.1 Physical USB connection
USB uses a four-wire cable to connect to devices. One pair of the twisted-pair lines gives the
differential data lines (D+ and D-), while the other two gives a 5 V and a GND supply rail, as
given in Table 9.1.
Data transfer rate is up to 12 Mbps, with a 1.5 Mbps subchannel for low-data-rate devices
(such as a mouse). A single unit can connect directly to the PC, but a hub is required when
more than one device is connected. Each peripheral can extend up to 5 m from each hub con-
nection, with a maximum of 127 different devices to a single PC.
Table 9.1
USB connections
Pin
Name
Description
1
V
CC
+5 V
DC
2
D-
Data-
3
D+
Data+
4
GND
Ground
9.2.2 Bus protocol
Each bus transaction involves the transmission of up to three packets. These are
•
Token packet transmission - on a scheduled basis, the host controller sends a USB
packet which describes the type and direction of a transaction, the USB device address
and endpoint number. The addressed USB device selects itself by decoding the appropri-
ate address fields.
•
Data packet transmission - the source of the transaction then sends a data packet, or in-
dicates it has no data to transfer.
•
Handshake packet transmission - destination device responds with a handshake packet to
indicate whether the transfer was successful.
