USB protocols followed in USB design

A USB design must follow the rules of the USB specification at the electrical, protocol and software levels.

1. Physical and electrical: four-wire cable (VBUS, D+, D-, GND); differential signalling with 90 ohm impedance; 1.5 kohm pull-up on D+ (full speed) or D- (low speed); 15 kohm pull-downs at the host; 5 V on VBUS, 100 mA default and up to 500 mA after configuration (900 mA in USB 3.0).
2. Bus states and encoding: J, K and SE0 states; NRZI coding with bit stuffing; LSB first.
3. Packet protocol: every transaction uses token, data and handshake packets with SYNC, PID, address, endpoint, CRC5/CRC16 and EOP; data toggle (DATA0/DATA1) for error recovery.
4. Transfer protocols: control, bulk, interrupt and isochronous, selected according to the device needs and bandwidth.
5. Enumeration protocol: on attach and reset, the host reads the descriptors (device, configuration, interface, endpoint, string), assigns a unique 7-bit address and selects a configuration. The device must respond at address 0 until it is addressed and must support endpoint 0.
6. Device class protocols: follow standard classes (HID, mass storage, CDC, audio) so that generic drivers can be used.
7. Power management: suspend (draw < 2.5 mA after 3 ms idle), resume/remote wake-up, bus-powered or self-powered declaration.
8. Hub protocol: hubs report port status, handle reset and low-speed isolation (PRE).
9. Compliance: pass the USB-IF compliance tests; a vendor ID (VID) and product ID (PID) must be assigned.