usb: general ideas


USB is a high speed serial bus
Four wires:: D+, D-, 5V, GND
Differential signalling, NRZI, with bit stuffing
Current limit: 500mA
OTG connectors have a fifth pin: "ID"
Packet-based communication
Point-to-point physical connection
One of the parties is the controlling one
Called "usb host" or "usb master"
The other party is only replying to queries
Called "usb device" or "usb slave"
You can use hubs to extend the bus
A hub device can be externally powered or not
Each bus can enumerate no more than 127 devices
The specs include a special protocol for current management