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Chapter 3: Functional Description 3–3

RS Encoder

December 2014 Altera Corporation Reed-Solomon Compiler

User Guide

To use shortened codewords with the Altera RS encoder and decoder, you use IP

Toolbench to set the codeword length to the correct value, in the example, 204.

Variable Encoding and Decoding

Under normal circumstances, the encoder and decoder allow variable encoding and

decoding—you can change the number of symbols per codeword (N) using

sink_eop

but not the number of check symbols while decoding.

1 However, you cannot change the length of the codeword, if you turn on the erasure-

supporting option.

If you turn on the variable option, you can vary the number of symbols per codeword

(using the

numn

signal) and the number of check symbols (using the

numcheck

signal),

in real time, from their minimum allowable values up to their selected values, even

with the erasures-supporting option turned on. Table 3–7 on page 3–10 shows the

variable option signals.

RS Encoder

The

sink_sop

signal starts a codeword;

sink_eop

signals its termination. An asserted

sink_val

indicates valid data. The

sink_sop

is only valid when

sink_val

is asserted.

1 Only assert

sink_val

one clock cycle after the encoder asserts

sink_ena

By de-asserting

sink_ena

, the encoder signals that it cannot sink more incoming

symbols after

sink_eop

is signalled at the input. During this time it is generating the

check symbols for the current codeword. Figure 3–2 shows the operation of the RS

encoder. The example shows a codeword with eight information symbols and five

check symbols.

The

numcheck

input is latched inside the encoder when

sink_sop

is asserted.

Figure 3–2. Encoder Timing

clk

sink_ena

sink_val

sink_sop

sink_eop

rsin[8:1]

source_ena

source_val

source_sop

source_eop

rsout[8:1]

01 02 03 04 05 06 07 08 09 09 10 11 12 13 14 15 16

01 02 03 04 05 06 07 08 P1 P2 P3 P4 P5 09 10 12 11 12 13 14 15 16 P6 P7 P8 P9

1 2 ... 16 17 18 19 20 21 22 23 24 25 26 ... 35 36

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