Hardware Reference
In-Depth Information
Figure 2.16
Host, Hub, and device interconnections
Figure 2.17
USB buffered interface
Client software for USB functions must use USB software programming
interfaces to manipulate their functions instead of directly manipulating their
functions via memory or I/O accesses as with other buses (e.g. PCI, EISA,
PCMCIA, etc.). During operation, client software should be independent of
other devices that may be connected to the USB. This allows the programmer and
software to focus primarily on the interaction between hardware and software.
USB physical interface
The physical interface used in the Universal Serial Bus is quite straightforward.
The interface specification involves electrical characteristics (voltage levels),
cables, and connectors. We shall briefly describe each of these features:
Electrical interface
As mentioned earlier, USB uses just two differential data connections (D
+
and
D
−
) and two power connections. CMOS buffers are used to drive the relatively
low impedance of the cable, as shown in Figure 2.17. The signal voltage present
