Hello Maurizio,
Unfortunately I cannot to reproduce your issue. If you are using the Twr-k64f12m board please check the jumper configurations.
• For USB Host mode, use onboard J17 Micro USB connector and jumpers set to default settings. Connect the J2 (K20
DEBUG) Mini USB to the PC to enable power for the USB Host.
In addition please confirm that the ID pin is grounded.
The ID pin is for detecting if the USB port is Host or Device. When a port is Host, the ID pin should be grounded. If the port is Device, the ID pin should remain floating.
Have a great day,
Sol
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