Introduction to USB OTG

1 Introduction

With the release of USB, USB has become a standard interface. Currently, USB supports three transmission modes: low speed (1.5 Mb/s), full speed (12 Mb/s), and fast (480 Mb/s: block Transmission, synchronous transmission, interrupted transmission, and control transmission. USB is flexible and convenient to meet the needs of a variety of peripherals. With the growth of personal portable electronic products and the rapid development of embedded technology, the USB host is no longer limited to a simple PC, can be any device containing a USB master controller, such as PDAs and MP3 players. In the USB 2.o specification, add? The on-the-go (OTG) Standard for USB embedded devices enables peripherals to switch between the host and the device, that is, when it is connected to a PC, it is a USB device and serves as a USB host when connected to other USB devices.

2 USB OTG

2.1 Introduction to USB OTG

Dsbotg is a supplement to the USB 2.o version. It is not an independent standard and retains all the features of USB. OTG enables Two USB peripherals to communicate directly without the PC. To implement such a function, there is a new concept in OTG-dual-function device (DRD: Dual-roledevice ). One DRD can meet the following requirements:

● Has certain USB Host capabilities and provides a list of peripherals;

● Full-speed operations (or quick operations) can be performed as peripherals );

● As the master machine, it can support full speed operations (low speed or fast)

● Supports host negotiation protocol (HNP) and Session Request Protocol (SRP );

● There is only one micro-AB connection port;

● The power supply bus can provide a current of no less than 8mA.

To implement host functions, the host must store a large number of device drivers and provide a certain amount of current to the power bus. For an embedded USB host, it is unrealistic and unnecessary to provide a large number of device drivers. An Embedded USB host only needs to support some specific devices, which are the list of its peripherals.

2.2 host negotiation protocol (HNP)

In the USB standard, a-device is used for host compaction, and B-device is used for peripherals ). One DRD can be used as both a host and a peripheral. So, when two DRDs are connected, which device acts as the host and why is it used as the host? To solve these two problems, a new protocol-host negotiation protocol (HNP) was proposed in OTG ). The OTG also defines a new interface-mini-abreceptacle and Mi-ni-aplug) and mini-bplug ). Add? To micro AB outlet, micro a plug, and Micro B plug? One pin-ID pin, which l can see.

In the miniature a plug, the ID pin is grounded, and in the miniature B plug, the ID pin is suspended. In OTG, if the device ID pin is grounded (that is, the device connected to the mini-a plug, l the device on the right), the device will feel the host, otherwise it will be a peripherals. At the same time, the host and peripheral functions are exchanged through host negotiation agreement during device connection and use. For example, if B-device on the left of Figure 1 is a handheld PDA, A-device on the right is a printer. The printer is initialized as a host due to the connection line. However, the driver of the printer is in the PDA, and the PDA is needed as the host, and the printer is used as the peripherals. HNP can be used to conveniently implement this function, without removing the need to unplug the connection line to change the direction of the plug, and connect the printer and PDA again.

2.3 Session Request Protocol (SRP)

OTG transceiver is generally used in embedded devices. Such devices are generally powered by batteries and have strict power consumption requirements. To save power, in the OTG standard, you agree to suspend the power bus when the power bus is not in use. When a Class B device is working, it must notify Class A devices in some way to power the power supply bus. To implement this function, the Session Request Protocol (SRP) is proposed in OTG ). In OTG, a session is defined as the time when a type A device effectively powers the power bus vbus. Note that the power supply in OTG is always provided by Class A devices (DRDs connected to mini-aplug. Because of the host negotiation protocol, Class A devices may also be used as peripherals. In this case, the power supply must also be provided by Class A devices. When a type-A device suspends vbus, the type-B device enters the sleep state. When a Class B device needs to work again, it can send a pulse signal (data-linepulsing) to the data line or send a pulse signal (vbuspulsing) to the power bus) to request a Type A device to power the power supply bus. OTG requires that both DRD and B-type devices have the ability to send Session requests. At the same time, A common A or DRD device must be able to respond to one session request.

Introduction to USB OTG