I want to write code on STM32F407 board. So I wrote blink example code and complied for it. and then insert binary program in it. After do this, I disconnected usb physically and connected again. But usb doesn't responded. so I opened 'device manager' to see what's happened. STM32 usb connection revealed 'unknown device (device request descriptor failed)'. I did do my best to solve this problem. What solution exist for this problem.. ?
While your application is running there will be no USB device present unless your application itself implements a USB stack. The DFU USB device only exists while actually in the bootloader. See the Bootloader code sequence section in AN3156. To enter the DFU you must configure the BOOT0/BOOT1 pins for System Memory Boot Mode.
If you have enabled Read Protection that will disable SYSTEM and SRAM memory boot modes, and the chip will need unlocking to recover programmability via these methods.