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STM32F405/7 DFU USB. Strange behavior at the start of the bootloader

Question asked by mennekes.rolf on Jan 25, 2016
Latest reply on May 19, 2017 by miller_smith.richard
I observe here a strange phenomenon, maybe someone can help me.

After a reset with boot 0 signal = high, the DFU mode does not start always.
After the reset, it takes sometimes several 10 seconds until the DFU Mode starts. Sometimes I have to send a second reset.
Was tested with a STM32F4 discovery and with our own board.
On our board a STM32F405VGT6 1 is installed. On the Discovery a STM32F407VGT6 1. Both have the bootloader version 0x31.
The Pins PA10, PB5, PB11 and PC11 are at a fixed level (GND or VDD)
There seems to be a Race Condition to be, because it sometimes works 20 times in succession and then every other time not.
It happens regardless of whether the reset is generated by hardware or bootloader via
NVIC_SystemReset() is called.

Has anyone ever seen this effect? And someone knows a solution for this?