Table of Contents Introduction 3 History of USB 3 Main Content 3 References 6

Introduction

This report well detail and describe the functionalities and the various protocols in which the Universal Serial Bus (USB) uses. The USB is now regarded as the initial tool for any academic, business means or to simply store photos or home videos etc. It is the ultimate portable storage device currently on sale in the market. How did the USB attain this high importance?

History of USB

The concept of a portable storage device was the idea of an Israeli computer hardware company in the 1990’s. The company created, developed and patented the first portable storage device which was named ‘DiskOnChip’ in 1995. However the development process was helped by a collection of seven companies: Compaq, DEC, IBM, Intel, Microsoft, NEC and Nortel.

The USB standard was pre-released in 1994 under version USB 0.7 and officially released in January 1996 under version USB 1.0. The data rate was between 1.5mb/s and 12mb/s.

The development of the USB has been constant over the past decade from version USB 1.0 to the recently released version of USB 3.0 of speeds up to 5gb/s. USB version 2.0 was the longest in operation with many devices now still using version 2.0 from the release in the year 2000. The importance and use of USB version 2.0 has been set at speeds 35mb/s to 280mb/s.

Many if not all computer devices on the market have USB 2.0 incorporated for USB devices such as plug and play devices i.e. mouse, keyboard, flash drive etc.

Main Content

Signalling and Communication in data communication is of key importance as it prepares the receiver and the sender for the initiation of data flow. This approach maintains that the data flow is sent at ease and with no difficulties. To do this the data flow is placed into a transaction. This transaction contains three packet types in which to send data. These are: * Token * Data * Handshake
Initially the token packet prepares and sets out the type of transaction being sent to the receiver. It contains the transaction information. Each packet contains an integer value of bytes at the current rate of transfer Followed by this is the Data packet which carries the data being sent by the user. Finally the handshake packet contains the overall status of the transaction. The above set up described is for the typical data transaction transfer used to communicate data across USB.

The first byte in every packet is a PID (Packet Identifier). The byte is recognised by the SIE (Serial Interface Engine) so it is not used and included in any error/CRC checks. The PID described is four bits in length.

Token Packet

There are three types within the token packet: * In – Notifies the USB device that the host wishes to read data. * Out - Notifies the USB device that the host wishes to send data. * Setup – Included to begin a control transfer.

Data Packets

There are two types of data packets within the data packet: * Data0 * Data1

Handshake Packets

There are three types of handshake packets within the handshake packet: * ACK - Acknowledgment that the packet has been received without error. * NAK - Reports that the device temporary cannot send or received data. * STALL - The device finds in a state that it requires assistance from the host.

There are four transfer types in total which communicate data and these are: * Control – Main transfer. Uses endpoints. * Bulk – Beneficial if time is no concern. * Interrupt – Periodic. * Isochronous – Guaranteed delivery time.

These packets described have key identifiers encoded to carry out specific pieces of information when called upon.

Speed

Speed is a big concern in data communication as faster is key. The development of USB has seen the increase of data transfer rates to speed up the transfer of data.

In USB 1.0 the low speed rate is 1.5mb/s. This was almost the equivalent to full bandwidth with the exception the bit takes eight times longer to transfer. The full speed of the standard USB 1.0 can operate up to 12mb/s.

With the introduction of USB version 2.0 in 2001 the data transfer rate was increased to 480mb/s. The newly updated USB devices were able to operate on the full bandwidth if necessary. These devices were also able to run USB 1.1 operations.

The most recent development of USB to version 3.0 sees the increase of speed up to 5gb/s.
At the end of the device the link of a 1.5 kohm resistor pulls one of the lines up to a 3.3V. This is on ‘D-‘ for a low speed device, and on ‘D+’ for a high speed device. A host device can determine the required velocity by observing which line is put high.
When the host device is communicating a packet, it is sent to every device connected to an enabled port. It travels downwards towards each hub on the chain which resynchronises the data. Only one device actually accepts the data which is the one addressed. The others receive the packet but do no decode it.

Only one USB device at a time is able to transmit to the host. Each hub repeats any data it receives from a lower level device in an upward direction. Downstream direction ports are enabled once the device connected to them is addressed.

References

Websites

http://www.beyondlogic.org/usbnutshell/ http://usb.org http://www.hardwarebook.info/ http://www.totalphase.com/ http://computer.howstuffworks.com/ http://www.everythingusb.com Books

Serial Port Complete: COM Ports, USB Virtual COM Ports, and Ports for Embedded Systems, Jan Axelson, 2007