My question is about USB 2.0.
I found this information:
Low-speed, 10-1500 Kbps (keyboards, mice, joysticks, gamepads);
Full-speed, 0.5-12 Mbps (audio, video devices);
High-speed, 25-480 Mbit / s (video devices, storage devices).
And how does it work? Is it made with some software like USB knows that you use a keyboard and works in a low-speed mode?
USB 2.0 makes signaling over two wires, D+ and D-. When a USB device is plugged in, it pulls up one of the wires with 1.5 k pull-up.
If a device pulls up D-, the host identifies it as Low-speed device (port status register shows the connection as LS). Then the host runs "port reset" and follows with enumeration at LS signaling rate.
If a device pulls up D+, the host drives "port reset" as a FS mode: both D+ and D- are driven to LOW. Then two (actually, three) scenarios would occur.
2.1. There is no activity on any line. The host identifies this as FS device, and proceeds with enumeration at FS data rate.
2.2. If the device is HS-capable, it drives D- line up with 18 mA source. It is called "Chirp-K" signaling. Once the USB host detects "Chirp-K, it starts "chirping sequence", which signals to the device that the host accepts this "HS negotiation. Both host and device assert proper bus termination, and the host proceeds at HS signaling rate and signal amplitudes.
Third scenario happens when a device assert the chirp, but the host doesn't respond with chirp toggling. This means that the USB host is not HS, and the link proceeds as a FS link.
More details of this process are covered in this SU post.
