The USB protocol basics


The protocol is completely master-driven
The slave must feature a good-enough clock
"Internal RC" oscillators are not enough

Both master and slave devices are implemented in hardware
Software can't deal with speed and determinism of the bus

The master port is depicted as a "Root Hub"
A PC usually has several root hubs
Root hubs can be single-port or multi-port

Bandwidth is effectively shared between devices

When a device appears, it must be enumerated
Hubs are enumerated too (including the root hub)