Altera 8B10B Encoder/Decoder MegaCore Function - Manual (Page 19)

May 2011 Altera Corporation 8B10B Encoder/Decoder MegaCore Function User Guide
3. Specifications
Functional Description
The MegaCore ® function consists of an encoder (ENC8B10B) and a decoder
(DEC8B10B). See Figure 3–2 on page 3–2 . The encoder encodes one 8-bit byte of data
into a 10-bit transmission code, and the decoder decodes a 10-bit code into one 8-bit
byte of data.
Figure 3–1 illustrates the bidirectional conversion process.
The eight input bits are named A , B , C , D , E , F , G , H . Bit A is the least significant bit (LSB),
and bit H is the most significant bit (MSB). They are split into two groups: the five-bit
group A , B , C , D , E , and the three-bit group F , G , H .
The coded bits are named a , b , c , d , e , i , f , g , h , j (the order is not alphabetical). These
bits are also split into two groups: the six-bit group a , b , c , d , e , i , and the four-bit
group f , g , h , j .
In bit serial transmission, the LSB is usually transmitted first, while the MSB is usually
transmitted last.
Disparity
Disparity is the difference between the number of 1s and 0s in the encoded word.
■ Neutral disparity indicates the number of 1s and 0s are equal.
■ Positive disparity indicates more 1s than 0s.
■ Negative disparity indicates more 0s than 1s.
The MegaCore function is designed to maintain a neutral average disparity. Average
disparity determines the direct current (DC) component of a serial line. Running
disparity is a record of the cumulative disparity of every encoded word, and is
tracked by the encoder. To guarantee neutral average disparity, a positive running
disparity must be followed by neutral or negative disparity; a negative running
disparity must be followed by neutral or positive disparity.
Figure 3–1. 8b10b Conversion
9 8 7 6 5 4 3 2 1 0
7 6 5 4 3 2 1 0
H G F E D C B A
a b c d e i f g h j
Conversion 8b10b
LSB sent first MSB sent last