Our team has noticed that with a stock STM32F722RE, and 0.4V applied to the VDD pins (which is occuring in our case due to a 3.3v CAN transceiver), when 3.3V is finally applied to the chip (ie voltage goes from 0.4->3.3V), bootloader entry via USB connection will fail (on ubuntu using the dmesg command we get persistent 'device descriptor read/64, error -32'). This condition persists even if we hardware reset the chip by toggling the reset pin.
If the 0.4V is removed (by disabling the transceiver or momentarily shorting the pins together) and then 3.3V reapplied (0.4->0.0->3.3V) ie a complete power cycle, the stm32 bootloader starts up fine.
Has anyone had a similar problem? On reading the datasheet we cant find any indication that the USB functionality of the chip will fail if VDD is less than the brown-out threshold (~2.5V in our case).
