Flashing code using USB

[SAbir.1]

​Hi, i have developed 60 electronic card using an STM32F205RCTx.On 40 card flashing the code using USB was possible(STM32 cube programmer detect USB) but on the 20 card STM32 cube programer do not detect USB and the PC show me the msg"unknown peripheral"

​Can anyone help me please.

[Tesla DeLorean]

Check: Power, Connectivity, Clocking

Assume no one is familiar with the specifics of your design

[SAbir.1]

Hello,

I have checked the connectivity from multiple PC, i have changed the cable. Power and Clocking are checked too. But always the same problem?

[waclawek.jan]

Are you taking about the default DFU bootloader?

Tell us more about your hardware -as Clive said, tell us how do you power the board, check physically if all power and ground pins (including VDDA) are connected. Tell us how USB is connected, check for bad solder joints. Tell us how do you manage BOOTx pins. How are other boot-related pins connected, can't the boulder go falsely to other protocols. What crystal are you using. If you can program the boards in other way, check if HSE and USB are working.

JW