Hardware Reference
In-Depth Information
host enables the port and addresses the USB device through the device's control
pipe at the default address.
The host assigns a unique USB address to the device and then determines if
the newly attached USB device is a
hub
or a
function
. The host then establishes
its end of the control pipe for the USB device using the assigned USB address
and endpoint number zero.
If the attached USB device is a
hub
and USB devices are attached to its ports,
then the above procedure is followed for each of the attached USB devices.
Alternatively, if the attached USB device is a
function
, then attachment notifi-
cations will be handled by host software that is appropriate for the particular
function in question.
When a USB device has been removed from one of a hub's ports, the hub
disables the port and provides an indication of device removal to the host. The
removal indication is then handled by the appropriate USB system software. If
the removed USB device is a hub, the USB system software must handle the
removal of both the hub and all the USB devices that were previously attached
to the system through the hub in question.
Finally,
enumeration
is the name given to process of allocating unique
addresses to devices attached to a USB bus. Because the USB allows USB
devices to attach to or detach from the USB at any time, bus enumeration has
to be an on-going activity for the USB system software.
USB bus topology and physical connections
The USB connects USB devices with the USB host. The USB physical intercon-
nect is a star topology that operates at a number of levels, extending downwards
from the host. Hubs or nodes (i.e. peripheral USB devices) may be present at
different levels but note that the nodes connected to any particular hub appear
in the
next level down
. Each physical connection is a point-to-point connection
between the relevant hub and node or between the relevant hub and another hub.
Figure 2.12 illustrates the topology of the USB.
The USB transfers signal and power over a four-wire cable, shown in
Figure 2.13. The signalling occurs over two wires on each point-to-point
segment. In order to deliver power to devices, the cable also carries V
BUS
(nominally
+
5 V) and GND wires on each segment. Cable segments may be of
variable lengths (up to several metres) and the terminations allow rapid connec-
tion or disconnection at each port with differentiation between full-speed and
low-speed devices.
It is important to note that each USB segment can provide only a
limited
amount of power. Furthermore, whilst the host can supply power for use by
USB devices that are directly connected, any USB device may have its own
power supply. USB devices that rely totally on power from the cable are called
bus-powered devices
. In contrast, those that have an alternate source of power
are called
self-powered devices
. USB hubs supply power for any connected
USB devices and this power may be derived from the host controller or may
be externally derived. It is also worth noting that the mechanical specification
for USB cables and connectors ensures that upstream and downstream connec-
tors are not mechanically interchangeable, thus eliminating the possibility of
loopback connections
at hubs.
