I found the issue...It was BOOT1 pin left unconnected. I connected it directly to ground. DFU mode is stable now..I hope topic author has same, easy issue like me and not all this calibration nightmare :beaming_face_with_smiling_eyes:

That's a good point. I am not very advanced with a hardware. I have a hobby oscilloscope 200 Mhz. It leaves me with 5 ns, so I cannot measure anything reliable here.Now I have brute forced all possible HSTRIM values from 12 to 32. DFU have not starte...

I did like you said. Routed MCO to pin. Measured 4 working and 2 not working boards:Working:16.04 Mhz15.98 Mhz15.96 Mhz16.00 MhzNot working:15.92 Mhz16.02 Mhz I have played around with HSITRIM: uint16_t CalVal = 16; // middle RCC->CR |= (uint32_t)...

Thank you for the time providing all the information.I noticed that all the documentation is provided for different family STMs. Is procedure the same for F4 family? Can the same timer (TIM14) be used to connect directly to the HSI?

Sorry, I forgot to mention that I am using 8 Mhz crystal with internal load capacitors (P/N: CSTCE8M00G52-R0 ). I tried to re-solder crystal from working boards and switch them hoping that it will help, but the result is the same.Is there any more wa...