The 15k resistor between PA9 and VBUS on Quanton FC

[Ye Wang]

I got a Quanton FC from a local seller. And when I was trying to connect it to PC, Windows did not prompt new hardware at all! After a quick check with a multimeter, I found out the voltage at PA9(OTG_FS_VBUS) was about 1.3V, which might be too low to convince the MCU that it's connected to USB. According to the schematics, there's a 15k resistor(R1) between PA9 and VBUS of the USB connector, and the voltage on it was around 3.6V. After I short that resistor with my tweezer, I was able to connect it to PC. I also took a look at other STM32 boards with USB(stm32f4discovery, CC3D and PX4FMU), and only CC3D has a 10k resistor like Quanton, the other two just connect PA9 and V BUS directly. I had a CC3D before I crashed it and I was able to connect it via USB without doing anything else. So can anyone explain what had happened?

[Ye Wang]

Some updates:

I commented out #define VBUS_SENSING_ENABLED and compiled bootloader. The I flashed the new bootloader to my board. It did get detected by Windows. However when I was trying to upload firmware using rescue method, it failed to enter DFU mode.

[Lilvinz]

We have not experienced this issue a single time yet.
Please send me an email with your contact details and i will
have a free replacement board shipped to you.
Also i would like to get hands on the defective unit to further
analyze the issue and to understand how it passed our factory tests.

Cheers,

Vinz

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