AM79C978VCSLASHW datasheet(80/261 Pages) AMD

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Am79C978

Data Symbol RLL25 Encoding

The RLL25 code is the version of TM32 that was devel-

oped for the HomePNA PHY. It produces both the high-

est bit rate for a given value of ISBI and TIC size. In a

manner similar to run length limited disk coding, RLL25

encodes data bits in groups of varying sizes, specifi-

cally: 3, 4, 5, and 6 bits. Pulse positions are assigned

to the encoded bit groups in a manner, which causes

more data bits to be encoded in positions that are far-

ther apart. This keeps both the average and minimum

bit rates higher.

Data symbol RLL25 codes data by traversing a tree as

illustrated in Figure 42. Assuming that successive data

bits to be encoded are labeled A, B, C, D,…, etc. The

encoding process begins at the root node and pro-

ceeds as follows:

1. If the first bit (bit A) is a one, the next three bits (B,

C, and D) select which one of the eight positions 1-

8 is transmitted. The encoding process then contin-

ues at the root node.

2. If bit A is a zero and bit B is a one, the next three bits

(C, D, and E) select which one of the eight positions

9-16 is transmitted. The encoding process then

continues at the root node.

3. If bit A is a zero, bit B is a zero, and bit C is a one,

the next three bits (D, E, and F) select which one of

the eight positions 17-24 is transmitted. The encod-

ing process then continues at the root node.

4. Finally, if bits A, B, and C are all zeros, position 0 is

transmitted. The encoding process then continues

at the root node.

As a result, Symbol 0 encodes the 3-bit data pattern

000, positions 1-8 encode the 4-bit data pattern 1BCD,

positions 9-16 encode the 5-bit data pattern 01CDE,

and positions 17-24 encode the 6-bit data pattern

001DEF. If the data encoded is random, 50% of the po-

sitions used will be for 4-bit patterns, 25% will be for 5-

bit patterns, 12.5% will be for 6-bit patterns, and 12.5%

will be for 3-bit patterns.

Management Interfaces

The HomePNA PHY may be managed from either of

two interfaces (the managed parameters vary depend-

ing on the interface):

1. Remote Control-Word management commands

embedded in the HomePNA AID header on the wire

network.

2. Management messages from a local management

entity.

Figure 42.

RLL 25 Coding Tree

These select position 1 - 8

Awaiting coding and transmission

Encoded and

Start: Examine the next

bits to be encoded

These select position 9 - 16

These select position 17- 24

A = ?

B = ?

C = ?

Send symbol 1-8

Send symbol 9-16

Send symbol 17-24

Send symbol 0

Data stream from MAC controller

22206B-46

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