The USB packet

• The basic transfer unit on the USB data bus is a packet

• Each packet consists of the following parts:

  • An 8-bit SYNC sequence
  • An 8-bit Packet Identifier (PID): 4 bits + 4 complemented bits
  • A sequence of 0 up to N 8-bit data characters
  • A 2-bit End-Of-Packet identifier: both D+ and D- low for two bit times