USB OTG Brief Introduction

1 Introduction

With the publication of the USB2.0 version number, USB is becoming more and more popular and has become a standard interface. Today, USB supports three transmission rates: Low speed (1.5mb/s), Full speed (12mb/s) and Fast (480MB/S), four transport types: block transfer, synchronous transmission, interrupt transmission, and control transmission. USB applications are flexible and easy to meet the needs of a wide range of peripherals. With the growth of personal portable electronic products and the rapid development of embedded technology, USB host is no longer confined to simple PC, and it can be any device that contains USB controller, such as PDA, MP3 player, etc. The standard--on-the-go (OTG) for USB embedded devices is also added to the USB2.O specification, which enables peripherals to switch between the host and the device, which is 1 USB devices when connected to a PC, and it acts as a USB host when connected to other USB devices.

2 USB OTG

2.1 USB OTG Brief introduction

DSBOTG is a supplement to the USB2.O version number and is not an independent standard, it retains all the features of the USB2.0. OTG enabled 2 USB peripherals to communicate directly without having to leave the PC. To achieve such a feature, there is a new concept in OTG-dual-function devices (Drd:dual-roledevice). 1 DRD can meet the following characteristics:

Have a certain USB host capability and provide a list of 1 peripherals;

Full speed operation (or fast operation) as a peripheral

Supports full speed operation (low or fast) as a master control machine

Support for Host Negotiation Protocol (HNP) and Session Request Protocol (SRP);

Only 1 micro-AB connection port;

A current of no less than 8mA can be supplied to the power bus.

To implement host functionality, the host must store a large number of device drivers and provide a certain amount of current to the power bus. For embedded USB hosts, it is unrealistic and unnecessary to provide a large number of device drivers, and 1 embedded USB hosts only need to support some specific devices, which are the list of peripherals.

2.2 Host Negotiation Protocol (HNP)

In the USB standard, the host uses a-type interface, called a Class A device (A-device), the peripheral adopts the B-type interface, called Class B Device (B-device). The 1 DRD can be used as both a host and as a peripheral. So, when 2 DRD interconnect, which device is the host, why should it be a host? In order to solve these two problems, the new protocol-Host Negotiation Protocol (HNP) was proposed in OTG. In OTG, a new interface was defined-miniature AB sockets (Mini-abreceptacle) and miniature a plugs (Mi-ni-aplug) and mini-B plugs (mini-bplug). Added 1 pin--id pins in miniature ab receptacle, miniature a plug and mini B plug, l saw.

In the miniature a plug, the ID pin is grounded and the ID pin is floating in the mini-B plug. In OTG, if the device's ID pin is grounded (that is, the device connected by the MINI-A plug, the device on the right side), the device defaults to the host, otherwise the peripheral. At the same time, during the use of the device connection, through the host negotiation protocol, the host and peripherals are agreed to swap functions. For example, if the left b-device of Figure 1 is a 1 handheld PDA, the A-device on the right is 1 printers. Because of the connection line relationship, the printer is initialized to the host. However, the driver of the printer is in a PDA, which requires a PDA as a host, a printer as a peripheral. This feature can be conveniently achieved with the HNP, without having to unplug the connector from the wire and connect the printer and PDA again.

2.3 Session Request Protocol (SRP)

OTG transceivers are commonly used in embedded devices, which are generally battery-powered and require very tight power consumption. To conserve power, in the OTG standard, when the power bus is not in use, consent to a Class A device suspending the power bus. When 1 class B devices are to work, it must somehow notify the Class A device to supply the power bus. To achieve this, the session Request protocol (SRP) is presented in OTG. In OTG, a session was defined as the time when a class A device was effectively powering the power bus vbus. It is important to note that the power supply in OTG has always been provided by a Class A device (DRD connected to Mini-aplug). Because the host negotiation protocol, Class A devices may also be used as peripherals, at this time, the power supply must also be provided by a Class A device. When a Class A device hangs Vbus, the Class B device goes into hibernation. When a Class B device is required to work again, it can request a class A device to supply the power bus by sending 1 pulses (data-linepulsing) to the data line or sending a pulse signal (vbuspulsing) to the power bus. OTG requires that both the DRD device and the ordinary Class B device must have the ability to send a session request, and at the same time, an ordinary Class A device or DRD device must be able to respond to a session request.

USB OTG Brief Introduction