Orion's blog

節錄於 http://www.eetimes.com/document.asp?doc_id=1203174

NAND Reliability Issues
One of the main considerations of working with a flash media is its reliability. This reliability is impacted by three major factors: bit-flipping, bad block handling, and Life span (number of erase cycles allowed). Let's see how these apply to NAND.

1. Bit Flipping
All flash architectures today suffer from a phenomenon known as "bit-flipping." On some occasions (usually rare, yet more common in NAND than in NOR), a bit is either reversed, or is reported reversed. This is the result of the following effects:

• Drifting Effects: A phenomena that slowly changes a cell's voltage level from its initial value.
• Program-Disturb Errors: This is sometimes referred to as "over-program" effects. A programming operation on one page induces the flip of a bit on another, unrelated page.
• Read-Disturb Errors: This effect causes a page read operation to induce a permanent change of a bit value in one of the bits read.

A flip in one bit may seem insignificant. However, this "minor" glitch may hang your system completely if it corrupts a critical file. When the problem is just of reporting, repeating the read operation may solve it. But if the bit was actually reversed, error detection/correction code (EDC/ECC) must be applied.

Since bit flipping is more common in NAND devices, all NAND vendors recommend using an EDC/ECC algorithm. When using NAND for multimedia information, this problem is not critical, but when using it as a local storage device to store the system OS, configuration files and other sensitive information, an EDC/ECC system must be implemented.

2. Bad Block Handling
Due to yield considerations, NAND devices are shipped with bad blocks randomly scattered throughout them. Shipping NAND devices free of bad blocks comes with a very high price tag caused by the low production yield rate, and is therefore not a cost-effective option.

Working with NAND devices, especially for local storage, requires initially scanning the media for bad blocks, and then mapping them all out so they are never used. Failing to do so in a reliable manner may result in a high failure rate of the final device, and even a recall.

3. Life Span/Endurance
Flash permits you to write, erase, and save information on it for at least ten years. However, like any good thing, if you use it too much, it will eventually wear out.

Each flash block can be erased some 100,000 times before you can no longer be sure if what you write is stored properly. Think of it, if you will, as a piece of paper on which you write using a pencil, then erase, then write, then erase...Eventually, you will dig a hole in the page.

Since the block size of a NAND device is usually about eight times smaller than that of a NOR device, each NOR block will be erased relatively more times over a given period of time (especially significant when working with small files) than each NAND block. This extends the gap in favor of NAND.

Endurance and reliability are closely linked. When reaching the maximum allowed erase cycles, the reliability of the flash deteriorates dramatically. Therefore, maximizing the endurance of the flash has a positive effect not only on the life of the flash but also on the quality of life of the flash.

To overcome the erase cycle limitation, a simple solution is implemented: the same file is never written twice to the same place. Instead, the file is moved around the flash media. The file's location is managed by a table that translates the virtual file/sector address used by the file system with its current physical address on the flash. In this way, the flash lifetime is prolonged without changing its physical characteristics.

To illustrate this mechanism, often called a flash translation layer (FTL), assume that the file allocation table (FAT) must be updated about 100,000 times a month, and that 1,000 free blocks are available on the media. By erasing and writing the FAT to the same block, the flash memory device will be used up after one month. By moving the FAT around the 1,000 free blocks, each capable of 100,000 erase cycles, the device life span can be extended to a whopping 1,000 months (83 years and 4 months).