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5.3
Connection ex. Host SoC (on the separate board)/EXT port, Portable
Device/INT port
5.3.1
ADJ settings
The S2R72A21 would be connected to the Host SoC via USB cables only, so the HS transmit current control
function via the I2C register cannot be used. In this case please set SEL_I2C=0 and follow, section 4.2 control
pin of the S2R72A21 data sheet and set up the HS transmit current control function by ADJ2-0.
In addition, EXT port would only be able to control the transmit current (Host SoC side).
5.3.2
BC control
The Bus Switch of S2R72A21 makes a connection between INT and EXT port when reset is released
(XRESET=1). After Portable Device is connected to the INT port, the Portable Device would communicate with
the BC regulator (mounted on the separate board) and determine the SDP/CDP/DCP by BC protocol. After the
BC protocol, the Portable Device would issue FS_J and the connection with Host SoC would start.
5.3.3
Role Switch control
If the Portable Device is a Device which does not execute the Role Switch, the Host SoC connected to the EXT
side would consistently function as a Host, and the Portable Device connected to the INT would act as a Device.
If the Portable Device is a Device which executes the Role Switch, the Portable Device which is connected to the
INT port would become the Host via some designated process after HS connection. At the same time, the Host
SoC which is connected to the EXT side would switch as a Device (In this case, the Portable Device and the Host
SoC would change its role nevertheless it's naming. However for the convenience, within this documents would
keep calling the name as Portable Device and Host SoC). If the Bus becomes Floating during the Role Switch
period, we recommend to pull down the Bus by 1MΩ and etc.
5.3.4
Method to detect disconnection of Portable Device
When the Portable Device does not execute the Role Switch, it requires the following 3 steps to make the USB
connection disconnected via S2R72A21.
1: Detection of HS disconnection by S2R72A21
S2R72A21 would detect the HS disconnection by detecting the SOF amplitude level changes which is sent
out by its INT port. After that, in case EN_DETACH=0, the HS Synchronizer gets "Off", Bus Switch
would be connected in between INT-EXT port and the transition to FS_LS state is done. In case
EN_DETACH=1, the S2R72A21 transiently gets into DETACH state for 3ms where both the HS
Synchronizer and the Bus Switch would get "Off", and then the transition to FS_LS state is done where
the Bus Switch connects INT-EXT port. In case the total cable length of INT port and EXT port is long,
the EN_DETACH pin can be set to 1.
2: Detection of HS disconnection by Host SoC
Host SoC would also monitor the SOF amplitude level and detect the Portable Device disconnection via its
built in HS disconnection detector.
3: S2R72A21 is stopped by Host SoC (Optional)
Notification from the Host SoC to S2R72A21 of the disconnection detection can be done by shut-off
VBUS supply as appropriate.
When the Portable Device executed the Role Switch, it requires the following 3steps to make the Host SoC detect
HS disconnection of the Portable Device.
S2R72A21 Application Note
(Rev.1.00)
Seiko Epson Corporation
19
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