# BCH

## NAND ECC - How Many Parity Bytes?

Written by Eric Deal Last Updated on Wednesday, 06 July 2011 08:35

One question that I often am asked is "How many parity bytes are consumed" for various ECC implementations. To parallel the previous post, this entry will address parity sizes for Hamming, Reed/Solomon, and BCH implementations.

## Hamming

Block Hamming codes correct only a single error and require 2*log2(n) of parity for a data block with n data bits. Block Hamming codes are capable of correcting a single-bit error and detecting most double-bit errors.

## Reed-Solomon

Parity required for an RS code depends on the symbol size, Galois field size (GF), and ECC level provided by the code. There are trade-offs in selecting the most efficient symbol size for a given application, but generally the idea is to minimize the Galois Field size for a given blocksize by selecting an appropriate symbol size. The general formula is 2*GF*ECC.

## BCH

Parity required for BCH is dependent on the Galois field size (GF) (determined by the data block size) and the ECC correction level. The number of parity bits can be computed as GF*ECC.

While the GF field for BCH is larger than that for Reed-Solomon, the factor of 2 in the RS equation makes BCH more efficient for NAND applications since errors tend to not occur in groups (RS is better at these applications).

The move to larger block sizes also makes the BCH code more efficient since higher correction levels more than compensate for the additional bytes protected (see ECC Trends whitepaper).

## Welcome to Parity Bytes

Written by Eric Deal Last Updated on Thursday, 24 February 2011 21:10

Welcome to Parity Bytes, Cyclic Design's BCH/ECC/NAND flash blog. In this blog, I will explore various aspects of NAND flash, error correction codes (ECC), and other topics related to developing hardware for NAND flash. Topics will come from questions frequently asked by my customers, things going on in the industry, and questions from readers like yourself. If you have a question, please submit it using the link to the left and I'll respond personally and possibly use it for a future blog post.

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Eric Deal

President, Cyclic Design