RM0090: Reference manual - STMicroelectronics

RM0090 USB on-the-go high-speed (OTG_HS)
B-device session request protocol
The application must set the SRP-capable bit in the Core USB configuration register. This
enables the OTG_HS controller to initiate SRP as a B-device. SRP is a means by which the
OTG_HS controller can request a new session from the host.
Figure 399. B-device SRP
VBUS_VALID
B_VALID
DISCHRG_VBUS
SESS_END
OTG_HS_FS_DP
OTG_HS_FS_DM
CHRG_VBUS
Suspend
Low
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Data line pulsing Connect
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VBUS pulsing
ai1568b2
1. VBUS_VALID = VBUS valid signal from PHY
B_VALID = B-device valid session to PHY
DISCHRG_VBUS = discharge signal to PHY
SESS_END = session end signal to PHY
CHRG_VBUS = charge VBUS signal to PHY
DP = Data plus line
DM = Data minus line
1. To save power, the host suspends and turns off port power when the bus is idle.
The OTG_HS controller sets the early suspend bit in the Core interrupt register after 3
ms of bus idleness. Following this, the OTG_HS controller sets the USB suspend bit in
the Core interrupt register.
The OTG_HS controller informs the PHY to discharge VBUS. 2. The PHY indicates the session’s end to the device. This is the initial condition for SRP.
The OTG_HS controller requires 2 ms of SE0 before initiating SRP.
For a USB 1.1 full-speed serial transceiver, the application must wait until VBUS discharges to 0.2 V after BSVLD (in OTG_HS_GOTGCTL) is deasserted. This
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Doc ID 018909 Rev 3 1308/1416