Using the USB20H04 with an On-The-Go Host

SMSC AN 10.4

Page 2

Rev. 04-07-03

APPLICATION NOTE

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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 V

BUS

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 V

BUS

. 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 V

BUS

, 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 V

BUS

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

Using the USB20H04 with an On-The-Go Host

SMSC AN 10.4

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Rev. 04-07-03

APPLICATION NOTE

Protocol Definition

The SRP pulsing protocol works by using two signaling methods: Data-Line Pulsing, and V

BUS

Pulsing.

Before attempting either of these two methods, the USB20H04 must determine that the following two initial
conditions have been met:

BUS

< 800mV

DP/DM lines on OTG Host idle (SE0) for 2ms

Once the initial conditions have been satisfied, both pulsing methods will be tried by the hub. First, the data-line
pulsing will be issued, if there is no response, the V

BUS

pulsing method will be issued. The A-device is only

required to support one method, while the B-device is required to support both methods.

Data Line Pulsing: USB20H04 turns on its data line pull-up resistor (either DP or DM) for a period of 5 ms
to 10 ms. If the OTG-Host is designed to support the data-line pulsing method of SRP, it will detect that a
data line (either DP or DM) has gone high and generate an indication that SRP has been detected. This
indication will be the assertion of V

BUS

, followed by the initiation of a standard USB reset.

BUS

Pulsing: USB20H04 will drive V

BUS

long enough to cause the capacitance on VBUS to be charged to

2.1V when connected to an OTG Host. Because the maximum allowed capacitance on a dual-role OTG
device is 6.5uF, the line will be driven to at least 2.1V. This pulse will wake up the sleeping OTG Host.
However, if the USB20H04 is connected to a standard host, the capacitance will be greater than 96uF;
therefore the line will never exceed 2.0V and will not be detected by standard host. This technique has
been designed to avoid damage to a standard USB host.

After the OTG Host detects one of the above pulsing methods, it will reset the entire USB tree, and will re-
enumerate each device. After completion of enumeration, the downstream device will be capable of
communicating through Hub, to the Host.

Электронный компонент: AN104