STM32 USB Device Not Recognized - Sometime recognized but not working

[tteksoy]

I have a simple STM32H753VIT6 board with simple power circuitry and USB connection also 25Mhz xtal.

I can program the board in DFU mode and verifies fine. I do not have the LSE xtal. not sure if it is needed for the USB operation. Complete circuit diagram attached. Can any one help me why am I failing with such a simple board.

Thanks

[Pavel A.]

Get a USB bus analyzer (a real one, not software). Look for anomalies.

[tteksoy]

Thanks for quick reply, I am a retired hobiest. Based on rule of thumb experience is the circuit ok, do I need LSE xtal for usb functionaliy. I know the debug port pins are wrong. But power circuit and all seems to work but not USB except during DFU mode.

[AScha.3]

Just - whats on pins VCAP , Vref+  ?

[tteksoy]

Like in the circuit diagram they are floating and not connected. Thanks for reply. This is the complete circuit diagram.

[AScha.3]

Thats wrong.

Need caps on VCAP . (core supply !)

see ds ...

Without caps...any random behavior possible... 🙂

+

Vref+ -> see manual, if you want any analog function on yor chip, like ADC ...

usually vref+ on Vdda = Vdd :

[tteksoy]

Thank you very much, I guess I have over optimized. Do I also need the second LSE xtal for USB operation. May be I should just included it any way, better safe than sorry. Just curious though. I really appreciate your answer.

[AScha.3 · Accepted solution]

No, the LSE is for time .. rtc ; if you have HSE , just need to set clock tree to have 48M at the usb-clock.

[tteksoy]

Thank you very much all the help,  much appreciated. Thanks