am79c901a Advanced Micro Devices, am79c901a Datasheet - Page 36

• Manufacturer Part Number: am79c901a
• Description: Homephy Single-chip 1/10 Mbps Home Networking Phy
• Manufacturer: Advanced Micro Devices

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,
2. If bit A is a zero and bit B is a one, the next three bits
36
and D) select which one of the eight positions 1-8 is
transmitted. The encoding process then continues at
the root node.
(C, D, and E) select which one of the eight positions
9-16 is transmitted. The encoding process then
continues at the root node.
Start: Examine the next
bits to be encoded
A = ?
B = ?
C = ?
0
0
0
1
1
1
1
Send symbol 1-8
Send symbol 9-16
Send symbol 17-24
Send symbol 0
Encoded and sent
Figure 21. RLL 25 Coding Tree
P R E L I M I N A R Y
Data stream from MAC controller
1
0
0
0
These select position 1 - 8
Am79C901A
B
1
0
0
These select position 9 - 16
A
C
C
1
These select position 17- 24
0
Awaiting coding and transmission
B
D
D
D
3. If bit A is a zero, bit B is a zero, and bit C is a one, the
4. Finally, if bits A, B, and C are all zeros, position 0 is
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
2. Management messages from a local management
C
E
E
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.
transmitted. The encoding process then continues
at the root node.
embedded in the HomePNA AID header on the
wire network.
entity.
D
F
E
F
22304B-23