USB

Universal Serial Bus (USB) is a standard interface for connecting peripheral devices to a host computer.

The USB system was originally devised by a group of companies including Compaq, Digital Equipment,

IBM, Intel, Microsoft, and Northern Telecom to replace the existing mixed connector system with a simpler

architecture.

USB was designed to replace the multitude of cables and connectors required to connect peripheral

devices to a host computer. The main goal of USB was to make the addition of peripheral devices quick

and easy

USB (Universal Serial Bus) is as its name suggests, based on serial type architecture. However, it is an

input-output interface much quicker than standard serial ports. Serial architecture was used for this type

of port for two main reasons:

Serial architecture gives the user a much higher clock rate than a parallel interface because a parallel

interface does not support too high frequencies (in a high speed architecture, bits circulating on each

wire arrive with lag, causing errors);

Serial cables are much cheaper than parallel cables.

USB 1.0 was first introduced in 1996, but was not adopted widely until 1998 with USB 1.1

In 2000, USB 2.0 was released and has since become the de facto standard for connecting devices to

computers and beyond.

In 2008, the USB specification was expanded with USB 3.0, also known as Super Speed USB

USB 3.0 specification include a number of significant changes including:

Higher data transfer rate (up to 5 Gbps)

Increased bus power and current draw

Improved power management

Full duplex data communications

Link Training and Status State Machine (LTSSM)

Interrupt driven, instead of polling

Streaming interface for more efficient data transfers

Types of connectors

There are two types of USB connectors:

Connectors known as type A, where the shape is rectangular and generally used for less bandwidth

intensive devices (keyboard, mouse, webcam, etc.);

Connectors known as type B, where the shape is square and mainly used for high speed devices

(external hard disks, etc.);

1. Power supply +5V (VBUS) 100mA maximum

2. Data (D-)

3. Data (D+)

4. Mass (GND)

Architectural OverviewUSB is a host-scheduled, token-based serial bus protocol.

USB allows for the connection of up to 127 devices on a

single USB host controller. A host PC can have multiple host

controllers which increases the maximum number of USB

devices that can be connected to a single computer.

Operation of the USB

One characteristic of USB architecture is that it can supply electricity to devices to which it connects, with

a limit of 15 W maximum per device. To do so, it

uses a cable made up of four wires (the GND mass,

the BUS supply and two data wires

called D- and D+).

The USB standard allows devices to be chained by using a bus or star topology. So, devices can either

be connected one to another or branched. 

Branching is done using boxes called "hubs" comprising of a single input and several outputs. Some are

active (supplying electric energy), others passive (power supplied by the computer).

D+ D-

Single-ended zero (SE0) 0 0

Single-ended one (SE1) 1 1

Low-speed J 0 1

Low-speed K 1 0

High-/Full-speed J 1 0

High-/Full-speed K 0 1

Communication between the host (computer) and devices is carried out based on the token ring principle.

This means that bandwidth is temporarily shared between all connected devices. The host (computer)

issues a signal to begin the sequence every millisecond (ms), the time interval during which it will

simultaneously give each device the opportunity to "speak". When the host wants to communicate with a

device, it transmits a token (a data packet, containing the address of the device coded over 7 bits)

designating a device, so it is the host that decides to "talk" with the devices. If the device recognises its

address in the token, it sends a data packet (between 8 and 255 bytes) in response, if not it passes the

packet to the other connected devices. Data is exchanged in this way is coded according to NRZI coding

In NRZI encoding, a digital 1 is represented by no change in the line state and a digital 0 is represented as

a change of the line state. Thus, every time a 0 is transmitted the line state will change from J to K, or vice

versa. However, if a 1 is being sent the line state will remain the same.

Since the address is coded over 7 bits, 128 devices (2^7) can simultaneously be connected to a port of

this type. USB has no synchronizing clock line between the host and device. However, the receiver can

resynchronize whenever a valid transition is seen on the bus.

There are currently four speeds at which wired USB operates: low-speed (1.5 Mbps), full-speed

(12 Mbps), high-speed (480 Mbps), and SuperSpeed (5 Gbps). In order to determine the bus speed of a

full-speed or low-speed device, the host must simply look at the idle state of the bus. Full-speed devices

have a pull-up resistor on the D+ line, whereas low-speed devices have a pull-up resistor on the D- line.

Therefore, if the D+ line is high when idle, then full-speed connectivity is established. If the D- line is high

when idle, then low-speed connectivity is in effect.