NUF2221W1T2

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

enable 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 megabytes−without

shutting down and without having to open your computer.

Typical USB System

Computer PC

CPU

KeyBoard

Mouse

Monitor

Telephone

Plotter

HUB

Scanner

Printer

Figure 4. Typical USB System

Typically the USB system consists of one host, hubs and

devices.

The Host

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

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 hubs’ 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 seed device and vice versa

– prevent from low speed traffic to reach full speed device.

The 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 for

USB 1.1:

Full−speed devices − operates in 12 Mb/s

Low−speed devices that work in 1.5 Mb/s.

NUF2221W1T2 Device Information

The Universal Serial Bus (USB) specification revision 1.1

requires the USB drivers to have certain impedance

characteristics. The series resistance plus the USB driver

output resistance must be close to the USB cable’s

impedance of 45

W unbalanced (90 W balance) to minimize

transmission line reflections. In addition USB 1.1 also

requires upstream ports to be terminated with a pull−up

resistor (usually 1.5 K

regulator). In case of EMI, the radiated and conducted EMI

should be kept within the required levels as specified by the

FCC regulations. Actual FCC regulations requires that class

B computing devices meet the specifications of maximum

levels for both radiated and conducted EMI (Radiated EMI

covers the frequency range from 30 MHz to 1 GHz and

Conducted EMI covers the 450 kHz to 30MHz range).

The NUF2221W1T2 device from On Semiconductor

provides series termination, EMI filtering and ESD

protection for the two data lines (D+, D−) as well as for the

VBUS.

The

ESD

device’s

ratings

comply

with

IEC61000−4−2 (Level 4), HBM class 3B and MM class C.

All these futures are offered in an integrated solution placed