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5V in 3.3V logic level when powering F205 through USB OTG HS

Question asked by Tuomas on Dec 21, 2011
Latest reply on Dec 22, 2011 by Tuomas
Hi

We have an odd problem.

We have a custom F205RBT6 based board. It can be powered through separate 5V pins or USB. The 5V goes to a 800mA linear regulator which then provides 3.3V to the F205. We have an extra led there controlled via a GPIO but otherwise it's a really simple board containing no extra components.

The problem is the board logic levels when powered through the USB. For some reason all our lines are 5V until the F205 firmware sets the pin mux properly. This means that all the time the F205 is in the bootloader mode waiting for user actions, all our 3.3V voltages are actually 5V.

This has something to do with the USB OTG HS VBUS sensing line going to pin PB13. That line comes directly from the USB VBUS and is therefore 5V, this should be OK. But somehow that 5V gets through the F205 to all our 3.3V lines (including all the F205 VDD pins) until we set the pin mux. If we cut the line from the board, all 3.3V lines are properly 3.3V.

We are using the USB OTG HS without any external PHYs so the pins are directly connected to the F205.

Any hints what might be causing this kind of behaviour? Is it possible that the 5V gets through the F205 to 3.3V lines if the pin mux is not set?

Thanks.

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