STF202−22T1G
http://onsemi.com
3
APPLICATIONS BACKGROUND
What Is USB?
The USB is not a serial port, it is a serial bus, a fact that
enables a single port on the computer to be a link for a myriad
of devices, (up to 127 devices in a USB system). We can
easily chain one device to another and use one port as a
connecting point of many devices by using a hub. All these
enables us to look at the USB system as a small network of
devices.
The Universal Serial Bus (USB) makes connecting
devices to your computer faster, easier and virtually
limitless. High−Speed USB devices are capable of
communicating at speeds up to 12 megabits without shutting
down and without having to open your computer.
Figure 5. Typical USB System
SCANNER
PRINTER
HUB
MONITOR
KEYBOARD
COMPUTER PC
CPU
TELEPHONE
MOUSE
PLOTTER
Typically the USB system consists of one host, hubs and
devices.
The Host in the USB system, is responsible to the whole
complexity of the protocol (simplifies the designing of USB
devices). The host controls the media access, therefore, no
one can access the bus unless it got an approval required
from the host.
The Hub provides an interconnect point, which enables
many devices to connect to a single USB port. The logical
topology of the USB is a star structure, all the devices are
connected (logically) directly to the host. It is totally
transparent to the device what is its hub tier (the number of
hubs the data has to flow through). The hub is connected to
the USB host in the upstream direction (data flows “up” to
the host) and is connected to the USB device in the
downstream direction (data flows “down” from the host to
the device). The hub’s main functionality is the
responsibility of detecting an attachment and detachment of
devices, handling the power management for devices that
are bus−powered (get power from the bus), and
responsibility for bus error detection and recovery. Another
important role of the hub is to manage both full and low
speed devices. When a device is attached to the system the
hub detects the speed, which the device operates in, and
through the whole communication on the bus prevents from
full speed traffic to reach low speed device and vice versa −
prevent from low speed traffic to reach full speed device.
The Device is defined as everything in the USB system,
which is not a host (including hubs). A device provides one
or more USB functions. Most of the devices provide only
one function but there may be some, which provides more
than one and called compound devices. We refer to two
kinds of devices − self powered or bus powered devices. A
device that gets its power from the bus is called bus powered
and on the other hand a device which supplies its own power
is called self powered. There are two kinds of devices:
Full−speed devices − operates in 12 Mb/s
Low−speed devices that work in 1.5 Mb/s
STF202 Device Information
The Universal Serial Bus (USB) specification revision 1.1
requires EMI Filtering and line termination for the USB I/O
lines. The STF202 device from ON Semiconductor
provides upstream termination, EMI Filtering and ESD
Protection to IEC61000−4−2 (Level 4) in an integrated
solution placed in a small and single package (TSOP−6,
Case 318G). The equivalent circuit of this device is shown
in the Figure 6.
Rpu
C
C
GROUND
D
D
R
S1
R
S2
Figure 6.
