Using the USB20H04 with an On-The-Go Host
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:
V
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.
V
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.
SMSC AN 10.4
Page 4
Rev. 04-07-03
APPLICATION NOTE
SMSC [ SMSC CORPORATION ]
