USB

Here I will tell about the USB protocol..which we all use but has a very little knowledge.As the very preliminary idea is also big I have make three parts of it.The first part deals with the introduction of USB..the second part describes the data transfer protocol and the third part is the zist of the transfer steps also called enumeration-------

Initialization:-

The USB protocol has been designed in 1990 by the leading companies like, INTEL, Compaq, IBM, Microsoft and Northan Telecom.

There are mainly 4 version of USB that are available now.

1:- USB1.0 protocol. Low Speed 1.5Mbits/s

2:- USB1.1 protocol. Low Speed 1.5Mbits/s and also full speed mode of 12Mbits/s

3:- USB2.0 protocol. High Speed 480Mbits/s

4:- USB3.0 protocol. Super Speed 4.8Gbits/s..

USB3.0 is not fully functional till now, though the first demonstration has takes place in the latest version of Linux.

Data Transfer Mechanical Explanation:-

The USB connector consists of 4 pins. Gnd, D+, D-, Vcc. Each data line corresponding to D+ and D- of the device in the host consist of a pull down resistor of typical value 15kOhm while in case of the device it is 1.5kOhm. While the device is not active the host pulls both the data line in logic 0 state thus producing the Single Ended Zero (SEZ0) state in USB Terminology.

In case of active state the host pulls one of the data lines of the device high through 1.5kOhm depending on the mode it is functioning. For 1.x mode of USB transfer the D+ line is pull high for full speed data transfer while the D- line is pulled low for low speed data transfer.

We get two state called Differential0 and Differential1 state by this.

Differential0:- D+ logic low D- logic high

Differential1:- D+ logic high D- logic low.

The corresponding two states Differential0 and Differential1 are known as J and K respectively in low speed mode while they are known as K and J in full speed mode respectively. The data gets transferred by alternating J and K state between the host and the device.

In case of USB2.0 the device first gets attached to the host as the full speed device i.e., D+ line is pulled high. After receiving the USB RESET command i.e., both the D+ and D- lines are pull low, it pulls the D- line high indicating that the device is capable of high speed data transfer.