Kawasaki KS152JB, 80C152 Random Backoff, Prbs Tcdcnt Load Bkoff Slot Clock Myslot, BKOFF= Myslot

KS152JB Universal Communications Controller Technical Specifications

Random Backoff

In either of the random algorithms, the first thing that happens after a collision is detected is that a I gets shifted into the TCDCNT (Transmit Collision Detect Count) register, from the right.

Thus if the software cleared TCDCNT before telling the GSC to transmit, then TCDCNT keeps track of how many times the transmission had to be aborted because of collisions:

TCDCNT = 00000000	first attempt
00000001	first collision
00000011	second collision
00000111	third collision
00001111	fourth collision
......................................
11111111	eighth collision

After TCDCNT gets a 1 shifted into it, the logical AND of TCDCNT and PRBS is loaded into a countdown timer named BKOFF. PRBS is the name of an SFR which contains the output of a pseudo-random binary sequence generator. Its function is to provide a random number for use in the backoff algorithm.

Thus on the first collision BKOFF gets loaded randomly with either 00000000 or 00000001. If there is a second collision it gets loaded with the random selection of 00000000, 00000001, 00000010, or 00000011. On the third collision there will be a random selection among 8 possible numbers. On the fourth, among 16, etc. Figure below shows the logical arrangement of PRBS, TCDCNT, and BKOFF.

PRBS TCDCNT

AND

LOAD

BKOFF

	8
SLOT	BKOFF
CLOCK
	6
	MYSLOT

BKOFF = 0

6 COMP

BKOFF= MYSLOT

Kawasaki LSI USA, Inc. Page 45 of 120 Ver. 0.9 KS152JB2