STM32: no SOF (usb)

Hi,

I have some troubles using the usb interface fromt he STM32F103 processor; if the processor is connected by a usb cable to my computer x86, then I didn't receive a start-of-frame.

Using the oscilloscope, I can see that only at startup of my computer (x86), there's some communication. After that only the reset impulse will be transmitted - nothing else. Is this behaviour normal, or should usb try to send a SOF as long as the host will get the device usb datas?

How will a SOF frame look like? so that I can see if it will be transmit at startup?

I also tried to use the files (keil project) from the evaluation board STM32 - the same behaviour...

best regards
Jan

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