AM79C901AVCT datasheet(36/90 Pages) AMD

36 / 90 page

Am79C901A

PR ELI M IN AR Y

Data Symbol RLL25 Encoding

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

oped for the HomePNA PHY. It produces the highest bit

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

similar to run length limited disk coding, RLL25 en-

codes data bits in groups of varying sizes, specifically:

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 farther

apart. This keeps both the average and minimum bit

rates higher.

Data symbol RLL25 codes data by traversing a tree as

illustrated in Figure 21. Assuming that successive data

bits are encoded and labeled A, B, C, D,…, etc., the en-

coding process begins at the root node and proceeds

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 continues 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 encoding

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.

Mode Interface

The HomePNA PHY may be managed from either of two

interfaces (the managed parameters vary depending 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 21.

RLL 25 Coding Tree

These select position 1 - 8

Awaiting coding and transmission

Encoded and sent

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

22304B-23

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