How to limit USB 2.0 channels to Full-speed

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Topic: USB

Question. How to limit USB 2.0 channels to Full-speed (12Mbit)?

Answer. Software should set the PFSC bit (bit 24) in the PORTSC register during USB controller init and prior to setting the RS bit in the USBCMD register (start controller). The port will than work in full speed only. However, the ULPI interface speed will remain 60 MHz so the CLK will still be at 60 MHz.

To avoid issues one may try to use non-high speed (if present) PC USB port. Also please check (USB) device descriptors, what options are supported. Particularly, pay attention on bcdUSB field (USB spec Release Number - if set for 1.1 the host will not try high speed).