Kawasaki 80C51, KS152JB CSMA/CD Data Encoding, Manchester Encoding BIT Time, Collision Detection

KS152JB Universal Communications Controller Technical Specifications

3.2.4 CSMA/CD DATA ENCODING

Manchester encoding/decoding is automatically selected when the user software selects CSMA/ CD transmission mode (See Figure below). In Manchester encoding the value of the bit is deter- mined by the transition in the middle of the bit time, a positive transition is decoded as a 1 and a negative transition is decoded as a 0. The Address and Info bytes are transmitted LSB first. The CRC is transmitted MSB first.

MANCHESTER ENCODING

0 1 1

BIT

TIME

0 0 1

If the external 1X clock feature is chosen the transmission ode is always NRZ (see Section 3.5.11). Using CSMA/CD with the external clock option is not supported because the data needs reformatting from NRZ to Manchester for the receiver to be able to detect code violations and col- lisions.

3.2.5 COLLISION DETECTION

The GSC hardware detects collisions by detecting Manchester waveform violations at its GRXD pin. Three kinds of waveform violations are detected: a missing 0-to-1 transition where one was expected, a 1-to-0 transition where none was expected, and a waveform that stays low (or high) for too short a time.

Jitter Tolerance

A valid Manchester waveform must have a transition at the midpoint of any bit cell, and may have a transition at the edge of any bit cell. Therefore, transitions will nominally be separated by either 1/2 bit-time or 1 bit-time.

The GSC samples the GRXD pin at the rate of 8 x the bit rate. The sequence of samples for the received bit sequence 001 would nominally be:

samples: 11110000		: 11110000		: 00001111		:
bit value:	0	:	0	:	1	:
	:<-bit cell->: <-bit cell->: <-bit cell->:

The sampling system allows a jitter tolerance of + 1 sample for transitions that are 1/2 bit-time apart, and +2 samples for transitions that are 1 bit-time apart.

Kawasaki LSI USA, Inc. Page 41 of 120 Ver. 0.9 KS152JB2