USB byte order, encoding format and bit stuffing

byte order

• LSB
  • When sending a byte, send the low-order data first, and then send the high-order data
  • When sending a word, send the low byte data first, and then send the high byte data

For example:

• Send 0x2D, ​​the order of sending is: 10110100 (low bit first, high bit last)
• Send 0x0110, then send 0x10 first, then send 0x01 (low byte first, high byte last)

Encoding format

• The encoding format adopts NRZI encoding
  • Level flip when data is 0
  • The level does not flip when the data is 1

bit stuffing

• Bit stuffing: In order to prevent the level from changing for a long time (which is not conducive to the synchronization of the signal), bit stuffing is introduced
  • Sending end: encounter 6 consecutive 1s and insert a 0
  • Receiver: Encounter six consecutive 1s, decode the 0s inserted later, remove the bit stuffing, and restore the original data

SETUP coding example

ACK encoding example

bit stuffing example