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Basics of solid-state memory technologies in consumer electronics (Part 3)

08 Oct 2012  | Thomas Coughlin

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Let's take an example of an MP3 player. We will assume that a player has 8 GB of storage and can store 2,000 songs. The most ambitious user might download an entire new collection of 2,000 songs once a day.

If we choose a two level MLC NAND (typical) with a 104 endurance, and assume that no bit errors can be tolerated by the controller (unlikely) the simplest equation says that the MP3 player can be reprogrammed 104 times before a bit error is likely to be encountered. This means that it could fail after 104 days, or about 27 years. If a smaller number of songs is stored, and the controller performs wear leveling, those 27 years would be extended by the inverse of the fraction of the memory that is used (i.e., if 3/4 of the memory is used, then the endurance would extend to 4/3 times 27 years or about 36 years). Of course, this is all academic because a consumer electronics device is unlikely to be used for even five years before being replaced.

Let's evaluate another scenario. Although it is uncommon for an amateur photographer to spend the entire day shooting photos every day of the year, it is conceivable that a professional photographer would take one photo every five seconds for an 8-hour workday every workday of the year. This would amount to about 1.4 million photos per year. At this rate, if the photographer constantly used the same 512 MB card (one based on the 4 Gb chip used in the preceding analysis) to shoot 1 MB photos, then that card would have a bit error about once every two years.

We should note that a 512 MB card is pretty small for today's professionals, especially for the photographer in this example. At the five-second rate, the photographer would be shooting 720 photos per hour, filling the card relatively frequently, so time would have to be taken simply to read the photos off the card, or to erase them in the camera. It is most likely that this photographer would opt for a significantly larger card. Purists shoot in raw format, which might expand that 1 MB photo size used in the preceding analysis to about 45 MB. This would get the photographer only eleven photos on that card, or about 1 min of shooting.

As pointed out before, the frequency of failures declines as the card size increases, so this particular usage pattern would most likely require the photographer to suffer from a bit failure every several years.

Both of these relatively extreme users see a far longer time between failures than the 8-1/2 months outlined in the previous worst-case scenarios. This illustrates the need to understand the usage expected in the system to understand expected failure rates.

Adapted from "Digital Storage in Consumer Electronics: The Essential Guide," by Thomas Coughlin (Newnes Press). Printed with permission from Newnes Press, a division of Elsevier.

To download the PDF version of this article, click here.

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