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STM32F205 exiting DFU mode almost immediately.

TSmit.1
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We have an STM32F205RFT6 device that we are trying to get into DFU mode using an HID message over USB.

We can get it into DFU mode, but it exits almost immediately.

We changed the loading caps on the 8MHz external crystal from 16pF to 10pF and this seems to mostly correct the issue, but I'd like to know if there's any way of determining what condition was causing the exit.

In normal operation, I've checked the oscillator startup to be about 1.2 msec and when the DFU fails, the oscillator seems to only be active for about 800usec.

Reducing the capacitor values for the crystal has likely reduced the startup time, but I'd like to know what the startup time should be for DFU mode?

i.e. do I need to further reduce the loading caps to account for production tolerances?

Thanks in advance.

Tim

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