STF202–22

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 IEC6100–4–2 (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

Figure 6.