lpcware

USB0_VBUS connection for bus-powered USB device

Discussion created by lpcware Employee on Jun 15, 2016
Latest reply on Jun 15, 2016 by lpcware
Content originally posted in LPCWare by zgora on Thu May 01 07:37:30 MST 2014
What is the best way to connect USB0_VBUS signal on bus-powered USB device (without OTG support)?

After looking through the datasheet, user manual and several kit schematics I'm quite confused. All 4 kit schematics I saw have USB0_VBUS directly connected to USB VBUS. Which is VERY BAD! There are posts on this forum there people confirm the MCU can latch up if 5V VBUS is already applied while VDDIO is not yet above 2.2V.

The user manual says: "For self-powered USB products that only use USB0 and that do not run in OTG mode, USB0_VBUS can be left disconnected as long as the VC bit in the OTGSC register is set". However, will it be possible to boot from USB0 if USB0_VBUS is left disconnected? Does ROM bootloader firmware set VC bit? Also the user manual says that it should be at least 1.8V on USB0_VBUS to enable pull-ups on USB0_DP and USB0_DM pins. Without this pull-ups USB host will not be able to detect the connected USB device.

So my question is maybe the best way is to connect USB0_VBUS to 3.3V VDDIO?

My concern is that I found a Vth (threshold voltage for VBUS valid) parameter in the datasheet, it is 4.4 V minimum. But I haven't found any information is this parameter related to OTG mode or for device only mode. What is the minimum USB0_VBUS signal value for USB0 to operate properly in device only mode (without OTG support) and to support USB0 boot?

Thank you,
Sergey.

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