NAND flash data retention

In my previous post, I raised the issue of NAND flash data retention. Even though vendors describe NAND flash memory as “non-volatile,” data can and will eventually be lost. The issue is “When?”

If this question peeks your interest — and it should — please read the Spansion Practical Guide to Endurance and Data Retention . This is a very well-written, easy-to-read white paper.

Basically, reprogrammable non-volatile memory is rated for endurance and data retention. Endurance is the number of sector erase operations that may be performed before failure. Erase operations are performed on a sector basis (i.e., not per-byte or per-bit.) Software can alleviate this problem by distributing erasures evenly across sectors, marking out failed sectors, data refresh, etc.

The Spansion 8Gbit NAND flash device (Spansion S34ML08G1) is rated for 100,000 erasures. Fortunately, we don’t write (and thereby erase) locations in wave memory very often. Further, we don’t write (erase) locations in voice, performance, pattern, etc. memory very often either. (Please don’t forget that user data is often stored in reprogrammable non-volatile memory, too!) As given in the table in the Spansion article, you could write (erase) a sector every 53 minutes (average erase frequency) or 27 times per day and still attain a 10 year device lifetime.

Data retention is the period of time for which data are reliably retained and retrieved. Data retention is affected by “temperature and voltage, electrostatic environment,
exposure to radiation, cumulative erase cycles, etc.” That’s right, data retention decreases with the number of erase cycles, too. Erasure introduces minute defects into memory cell structure and these defects accumulate. Fortunately, again, we’re looking at relatively low erasure frequency for wave and user data memory.

Quoting the article, “Spansion single-bit-per-cell floating-gate flash devices are designed to provide 20 years of data retention after initial programming when exposed to a 55°C environment.” Data retention time is 10 years when the cumulative erase cycles per sector is 1,000 erasures or less.

So, should you worry that your synth or arranger workstation will lose its contents? Probably not. I would worry more about pressing the wrong button at the wrong time and accidentally losing my work! (You are backed up? Right?) However, as a manufacturer, I would definitely anticipate some data failures in the long run and have a means to restore original factory programming through a field service program. The PSR/Tyros products, for example, do a quick wave memory self-check at start-up. One or two such failures have been reported on the PSR Forum. Presumably, the memory devices can be replaced and reflashed by a qualified service technician.

Sleep tight, but don’t forget to back up your data. Overall, you are your own worst enemy when it comes to data loss!