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| 型號(hào)： | AN104 |
| 廠商： | SMSC Corporation |
| 英文描述： | AN104 |
| 中文描述： | AN104 |
| 文件頁數(shù)： | 3/4頁 |
| 文件大?。?/td> | 481K |
| 代理商： | AN104 |
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
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