Hardware Reference
In-Depth Information
Figure 2.12
USB topology showing levels or tiers
Figure 2.13
USB cable (high-speed)
As mentioned earlier, USB provides two basic data transfer rates; the full-
speed bit rate of 12 MB/s and the reduced, low-speed, bit rate of 1.5 MB/s.
Dynamic mode switching between transfers allows both modes to be sup-
ported in the same bus. The low-speed mode is defined to support a number
of low-bandwidth devices, such as keyboards and pointing devices. The low-
speed mode is also somewhat less demanding in terms of screening and EMI
protection.
The USB clock signal is effectively encoded along with the differential
data. The clock encoding scheme is NRZI with
bit stuffing
to ensure adequate
transitions. In order to allow a receiver to synchronize its bit recovery clock, a
SYNC field precedes each packet.
Note also that USB is a
polled bus
and that the
host controller
(i.e. the PC)
initiates all of the data transfers. All bus transactions involve the transmission
of up to three packets. Each transaction begins when the host controller, on
a scheduled basis, sends a USB packet describing the type and direction of
