AN104 datasheet(3/4 Pages) SMSC

3 / 4 page

Using the USB20H04 with an On-The-Go Host

SMSC AN 10.4

Page 3

Rev. 04-07-03

APPLICATION NOTE

Preface

This Application Note provides detailed information on how the USB20H04 can be used with an On-The-Go

(OTG) Host.

Overview

The SMSC 4-port hub controller supports the Session Request Protocol (SRP) as defined in the OTG

Supplement. This feature is used for cases that the USB20H04 has an OTG host plugged into its upstream

facing port. An example of this can be seen in Figure 1.

Figure 1 – USB20HO4 in OTG SRP Configuration

In order to conserve power, the OTG Supplement allows an A-device to leave VBUS turned off when the bus is

not being used. Then, when the B-device wants to use the bus, it follows the SRP pulsing protocol to request

that the A-device supply power to VBUS. In the configuration in Figure 1, the “OTG Host” would be considered

the A-device, and the USB20H04 Hub would be considered the B-device.

For example, if the OTG Host were to turn off VBUS, and the USB20H04 were configured in self-powered mode,

downstream ports would remain powered. The downstream devices would then go into a suspend state based

on inactivity. In the event that one of the downstream devices needed to communicate with the OTG Host, the

downstream device would signal a standard USB resume to the Hub, and the Hub would then communicate

with the OTG Host by following the SRP pulsing protocol to awaken sleeping OTG Host.

The SRP pulsing protocol is important because it allows the A-device, which may be battery powered, to

conserve power by turning VBUS off when there is no bus activity, while still providing a way for the B-device to

initiate bus activity.

OTG HOST

HUB

DOWNSTREAM

DEVICES

AN104 Datasheet(PDF) 3 Page - SMSC Corporation