AM437x USBx_VBUS Input Impedance

This parameter is not specified, but per USB OTG specification VBUS is required to have an impedance to ground in the range of 40k ohms to 100k ohms.

Also note that the USB PHY supports VBUS pulsing which requires it to be able to source current from the USB 3.3 volt supply and sink current to ground, so care should be taken about USB configuration in your use case.

Hello Maeda-san,

Daisuke Maeda said:

Hi Biser-san,

Thank you for your reply.

Does AM437x drive the USBx_VBUS for the VBUS pulsing?

No.

When AM437x is used as Dual-Role Device or connects to Dual-Role Device, should a 5V VBUS charge pump be controlled by the USBx_DRVVBUS?

Yes. For AM437x, it is permissible to hard-wire USBn_VBUS to system 5V and leave USBn_DRVVBUS floating, but only in dedicated Host applications. In all other use-modes (DRD, Device-only), the controller must directly control USBn_VBUS via USBn_DRVVBUS.

Maeda-san,

In answer to your questions:

1)  Is it permissible that 5V is applied to USBx_DRVVBUS before AM437x power terminals are supplied? USBx_DRVVBUS is a 3.3V output. I think you meant USBn_VBUS, in which case yes, it is permissible for 5V to be present on this pin prior to AM437x power-up as the pin is designed for this purpose.

2) I understand that an external controller, e.g. USB HUB IC, must monitor VBUS line and control a 5V VBUS charge pump in DRD applications when AM437x directly controls USBn_VBUS via USBn_DRVVBUS. A USB hub will control power to it's own downstream ports. The reason that it must also monitor the upstream (Host) VBUS is to know when to activate/deactivate it's pull-ups and to enter/exit suspend mode (if enabled). Just to be sure this is understood: DRD functionality is not supported through a USB hub...a Hub will always be connected upstream to a dedicated USB Host and downstream to dedicated USB Device(s).