Intel SA-1100 manual Bit Encoding, Bit Value Digital Data Nrzi Data, 11-58

Peripheral Control Module

11.8.1.2Bit Encoding

USB uses nonreturn to zero inverted (NRZI) to encode individual bits. Both the clock and the data are encoded and transmitted within the same signal. Instead of representing data by controlling the state of the signal, transitions are used. A zero is represented by a transition, and a one is represented by no transition (this produces the data). Each time a zero occurs, the receiver logic synchronized the baud clock to the incoming data (this produces the clock). To ensure the receiver is periodically synchronized, any time six consecutive ones are detected in the serial bit stream, a zero is automatically inserted by the transmitter. This procedure is known as “bit stuffing”. The receiver logic, in turn, automatically detects stuffed bits and removes them from the incoming data. Bit stuffing causes a transition on the incoming signal at least once every seven bit-times to guarantee baud clock lock. Bit stuffing is enabled for an entire packet beginning when the start of packet is detected until the end of packet is detected (enabled during the sync field all the way through the CRC field). Figure 11-15shows the NRZI encoding of the data byte 0b1101 0010.

Figure 11-15. NRZI Bit Encoding Example

Bit Value

Digital Data

NRZI Data

1

1

0

1

0

0

1

0

A4795-01

11-58 SA-1100 Developer’s Manual