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Picturing two kinds of Flash cell

Posted: 16 Dec 2003 ?? ?Print Version ?Bookmark and Share

Keywords:nand? mlc? slc? usb? memory?

MLC NAND Flash is appropriate for applications that can tolerate slow transfer rates and lower reliability but need to pinch every penny.

Steffen Hellmold is director of Flash marketing for Samsung Semiconductor Inc.
Tell photographers that one day their digital film won't reliably store photos any longer, and the trade-offs between single and multilevel Flash memory will start to come into focus. One type of NAND Flash permits more read/write cycles than the other - just one of several differences. Whether these differences matter in your application depends on your customers' expectations, so sometimes it pays to think like an end-user.

At the same time, you have to think like a designer to understand that a single-level cell (SLC) stores two charge values, which is either 0 or 1. A multilevel cell (MLC) stores four or more charge values, so MLC can store more data per cell. MLC thus has a density advantage, which implies a lower cost. Those advantages fade, however, when you look closer, and an interesting set of trade-offs emerges.

One trade-off involves the maximum number of read/write cycles--that's where storing photos long term can get tricky. MLC typically allows 10k cycles, compared with about 100k for SLC. In some applications, 10k cycles is enough. For a digital camera or USB Flash drive, users might prefer longer endurance.

Another trade-off is that SLC-based Flash cards store data about three times faster than MLC-based ones. MLC-based Flash cards might limit you to record a snippet of video at a time on your digital video camera.

MLC also has reliability issues in conjunction with leakage of charge, so it requires more error detection and correction circuitry to ensure data reliability.

The additional circuitry needed for MLC technology takes up a lot of silicon real estate, so MLC does not really provide the 2x density advantage you would expect. Equally important, MLC's complexity causes it to lag SLC by about six to nine months, so the two types of Flash chips offer about the same capacity today.

A more complex trade-off is that SLC allows partial programming, which means that applications can write chunks of data as small as 512bytes--the typical chunk size used in most file systems. MLC requires that you write a complete page (2KB), so your application might require extra buffers and a more complex controller. Typically, the result is a significant performance penalty.

When you boil down the trade-offs, MLC NAND Flash is appropriate for applications that can tolerate slow transfer rates and lower reliability but need to pinch every penny. Low-end cameras fit this description. SLC NAND Flash suits applications that benefit from significantly higher transfer rates and longevity along with high reliability. Midrange to high-end digital still cameras, video cameras, high-end cellphones and USB Flash drives take good advantage of these characteristics.

- Steffen Hellmold

Director, Flash Marketing

Samsung Semiconductor Inc.

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