I assume you mean high speed / full speed (840 Mb / 12 Mb) as high-speed devices do not support low speed, per USB 2.0 spec. High-speed hosts do support low-speed.
The i.MX25 OTG comtroller does support low-speed when it operates in host mode.
Back to speed selection:
The selection between full and high speed is an automatic process that is also described in the USB 2.0 specification.
In short, it works like this:
Initially both Host and Device are in full speed mode. The host will issue a bus reset to start the enumeration process.
A high-speed capable device will respond with a Chirp-K state on the bus to tell the host it can operate in high-speed.
If the host is also high speed capable, it sends a sequence of Chirp_K/Chirp_J transitions. The device will detect these and enter high-speed mode. The host detects the device's high-speed mode and enther highspeed mode as well.
From this point onward, the bus will be in high speed until the host resets the bus.
If anything fails in the above process, both host and device will remain in full speed mode.
The i.MX USB controllers have the option to force full speed operation. This is done by setting the bit PFSC in the PORTSC register before the controller is started (USBCMD.RS), and after the controller is reset (USBCMD.RST).
When this bit is set, the device controller will not send Chirp_K. or when in host mode, the host controller will not respond to Chirp_K. Hence the bus will remain in full speed.
Best regards,
Richard
