How to reinstall (restore) manufacturer`s bootloader on STM32F103C8T6 development board?

[AMelk]

I have STM32F103C8T6 development board (like this: https://www.rogerclark.net/wp-content/uploads/2014/11/STM32Mini.jpg) which can be programmed by USB interface. I programmed it several time before, but now I can't do this. When I connect it to the PC, I get a warning messege (pinned) and can't load firmware to the STM.

[turboscrew]

BTW, Roger Clark is "the man" when it comes to STM32Duino.

[Omeg18650]

I have some clue here.

But the solution is temporally.

The chip and board for me is stm32f411ec6

on the Blackpill board.

I have to touch the digital pin with fingers while the USB DFU boot loaders begin. If I don't do it. The 25 Mhz clock will show up for a small while, then fall flat. And the usb enumeration will fail on my computer. The error being reading product discriptor error.

[Omeg18650]

Ok I figured my problem out. The ripple from the circuit will trigger the USART1 RX pin to make it enter serial mode. by pulling the PA10 up to prevent accidental triggering, It can finally enumerate normally.

the dmesg -w report