1 THE UNIVERSAL SERIAL BUS 4
1 The Universal Serial Bus
In 1994 an alliance of four industrial partners (Compaq, Intel, Microsoft and
NEC) started to specify the Universal Serial Bus (USB). The bus was originally
designed with these intentions:
• Connection of the PC to the telephone
• Ease-of-use
• Port expansion
The specification (version 1.0) was first released in january 1996 and the
latest official version 1.1 was released in september 1998 [4]. The document
is still under development and a version 2.0 was announced in 1999. More
information and all specification papers can be found in [1]. The USB is strictly
hierarchical and it is controlled by one host. The host uses a master / slave
protocol to communicate with attached USB devices. This means that every
kind of communication is initiated by the host and devices cannot establish any
direct connection to other devices. This seems to be a drawback in comparison
to other bus architectures but it is not because the USB was designed as a
compromise of costs and performance. The master / slave protocol solves
implicitly problems like collision avoidance or distributed bus arbitration. The
current implementation of the USB allows 127 devices to be connected at the
same time and the communication bandwidth is limited to 12Mbit/s.
1.1 Host Controllers
Today the USB host controller is integrated on most motherboard chipsets.
Older boards which are not equipped with such a controller can be upgraded
by PCI cards with such host controllers. All these controllers are compatible
with either the Open Host Controller Interface (OHCI by Compaq, Microsoft
and National Semiconductor) or the Universal Host Controller Interface (UHCI
by Intel [7]) standard. Both types have the same capabilities and USB devices
do not have to care about the host controller. Basically the hardware of UHCI
is simpler and therefore it needs a more complex device driver, which could
cause slightly more CPU load.
1.2 USB Devices and Transfer Characteristics
There are different types of USB devices as they can be used for different
purposes. First a device can be self powered, bus powered or both. The
USB can provide a power supply up to 500mA for its devices. If there are
only bus powered devices on the bus the maximum power dissipation could be
exceeded and therefore self powered devices exist. They need to have their
own power supply. Devices that support both power types can switch to self
powered mode when attaching an external power supply.
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