Adoption by Industrial & Mil-Aero Promises Some Rewards & Major Issues

MLC NAND is experiencing a high rate of adoption and within the consumer electronics sector – MP3 players, digital cameras, smart phones, flash cards and USB drives – it is everywhere you look. However, other embedded segments (industrial, automotive, military, aerospace, etc), are hesitating to take advantage of MLC’s low-cost, high-density attributes. There are good reasons behind the cautious stance; these applications are often mission critical, have a low tolerance for failure, and are expected to perform consistently over a much longer lifespan than their counterparts in the nearly-disposable consumer world. These requirements are in direct conflict with some of MLC’s known shortcomings: shorter lifespan, shorter data retention times, higher error rates, more complex (and consequently slower) error detection and correction.

On the topic of lifespan, traditional single-level NOR parts are typically expected to endure up to 100,000 cycles, which could translate to 20 years of use in a typical embedded application.  Most MLC NAND is rated for 10,000 cycles, rendering these parts unusable in 2 years under the same use case.  While 2 years is a long time for many consumer grade products, it is unacceptably short for the vast majority of industrial products. Similarly, data retention requirements differ.  Traditional flash data retention rates have been 20 years, but recently some flash parts are being introduced with only a 10 or 15 year rating. Applications involving products with life times in the 10 year range need to consider such limitations.

Lower erase cycle endurance is conceptually easy to manage: track high use areas and occasionally swap the data within those areas with a low use area. However, a major difficulty is brewing that involves how errors are introduced and the performance impact of detecting and correcting them.

When writing pages within an erase block, disturb errors may be introduced, causing some number of bits to be flipped in pages other than the one being written to. The time required to read and verify the contents of the entire erase block can cause unacceptable delays, leading programmers to defer the detection until the next read operation, which may occur infrequently. Consequently, bit errors can exist in these “not written to” pages for a long time before they are detected.

And the issues with MLC error rates will worsen, as each new generation of chips pushes the cell size down even further. Future generations of MLC NAND devices beyond the 35nm range may have to distinguish between only a few hundred electrons on each cell. With so few electrons, discerning among the multiple levels of charge in a cell will be a time-consuming, error-prone process.

The somewhat obvious solution is to put in place a process to read and verify areas in the vicinity of writes in an attempt to detect disturb errors earlier.  A solution like this must be carefully balanced with the system performance requirements.

MLC NAND has many compelling reasons for adoption, but until its challenges are successfully dealt with, it will not be broadly accepted by industrial, mil-aero, and automotive device designers as a viable replacement for tried and true technologies of SLC NAND and NOR.

At Datalight, we are focused on easing many of the problems of MLC NAND. For more information on our intelligent flash management solutions, please visit our resources page.

NAND flash is everywhere these days. The iPod Nano, usb flash drives and various other flash card-based devices like GPS navigators, digital cameras and smart phones have brought high-density NAND to the mainstream. What most users don’t realize however, is that NAND storage, though “solid-state,” relies on a fragile and inherently-flawed technology to store all those bits and bytes. Enter the NAND controller, a relatively small piece of software that renders NAND errors virtually invisible to the end user, making your iPod and other devices work like a champ. To learn more about NAND controllers, read our whitepaper on the topic:

An Overview of NAND Flash Memory Controllers

The demand for NAND Flash memory is growing at a phenomenal rate. In 2005, worldwide revenue for NAND Flash rose to $10.8 billion, up 60% from 2004. The inclusion of NAND Flash in an increasing number of MP3 players, high-end cell phones, and digital cameras has fueled expectations that NAND will overtake NOR in a number of markets. Rapid increases in the capacity of NAND Flash, coupled with mobile consumer products that demand ever-increasing amounts of data storage, lay out a very bright future for this technology in the coming years.

What is NAND Flash?

NAND Flash is a form of non-volatile memory introduced by Toshiba and Samsung in 1989. Its benefits include high storage density, fast access times, low power requirements in operation, and excellent shock resistance. These benefits are tempered somewhat by the inherent limitations of the technology:

• Due to production yield constraints, NAND Flash ships from the factory with a number of bad blocks that cannot be used.
• NAND Flash has a serialized data interface similar to a hard disk and so cannot be used for execute in place functionality, such as running code directly from the chip.
• Over time and after multiple erase cycles the memory will tend to “wear” and become less reliable.
• The bits stored in the chip can sometimes flip – or reverse – from 0 to 1 or vice versa.

Over time, NAND Flash technology and software has evolved to the point where these limitations are invisible to the end user. Owners of iPod Nanos do not overly concern themselves with bad blocks or bit-flipping.

What does a NAND controller do?

Increased Performance

Approximately 3% of the overall flash array is reserved as a “spare area” in order to cope with flash vulnerabilities, like bit-flipping and bad blocks. Approximately 3 to 6 bytes in the spare area are reserved for error detection and correction algorithms, while the remainder of the spare area is used for remapping bad blocks. Without a NAND controller the algorithms that handle these functions would be executed by the general purpose CPU. However, NAND controllers are available that will perform these functions in hardware. This reduces the CPU load on the device which has the effect of increasing battery life as well as increasing performance – both important factors in mobile technology. In addition, the advent of Multi-Level Cell (MLC) NAND Flash Technology requires even more rigorous error detection algorithms due to the increased chance of an error occurring. A NAND controller is perhaps the only reasonable way of performing this task.

NAND controllers can implement read and write caching and transfer data to and from the NAND Flash chip independently of the general purpose CPU. This increases the overall throughput and can again reduce the load on the system.

Faster Integration

The serialized data interface to NAND presents a difficult scenario for transferring data in and out of the chip. Unlike other memory technologies, like NOR or DRAM, an address must be fed in a bit at a time, at just the right time, and then read or write the actual data in a similar fashion. A NAND controller encapsulates the interface with the NAND chip and handles this communication, thereby presenting an effortless interface to the user.

The lack of standardization among NAND Flash manufacturers has been problematic throughout its brief history. The use of NAND controllers allows some flexibility in the choice of a NAND Flash chip as any given controller will support a range of NAND chips. Changes in NAND Flash, therefore, will not necessarily require a change in NAND controller or software.

Integrated NAND Controllers

Integrated NAND controllers are growing in popularity as mobile processor vendors, such as Freescale and Texas Instruments, build NAND controllers directly into their processors. The advantages of this approach are that the design can be kept small and the costs can be minimized. The disadvantage is the inevitable delay between a new NAND Flash technology appearing and the release of an integrated processor that supports it.

A dedicated, or external, NAND controller is one in which the NAND controller is a separate chip that can be sourced independently of other parts. The advantage of choosing a dedicated NAND controller is the wide range of latest and greatest NAND Flash chips available. This is due to the relatively short time to market for a chip with this level of complexity. The disadvantage of using a dedicated NAND controller is that chip uses additional PCB space and will cost more money than the integrated approach.

Specialized NAND Controller Software

The diagram below shows an overview of the interaction between the embedded application, file system, Flash management software, NAND controller, NAND controller driver, and the NAND Flash memory chip.

NAND Controllers require specialized software to hide the unpleasant details of the NAND controller and NAND Flash itself and presents the user with a clean and highly usable interface to the memory.

At a higher level, the NAND controller driver is often utilized by an intelligent block device driver, such as FlashFX Pro from Datalight, or a flash file system. Some important functions of this software are:

• Reducing the integration time for developers of embedded applications.
• Handling bad blocks present on the NAND Flash chip.
• Providing small block emulation, which is required due to the relatively large “erase blocks” present on NAND Flash.
• Mitigating the effects of wear on the NAND flash through the application of wear leveling algorithms that spread writes and erases over a wider area.
• Providing a file system interface that allows you to interact with the Flash chip at the file level.

Summary

The strong desire for more storage and better performance in consumer devices has elevated the pressure on NAND Flash and introduced new technologies like NAND Controllers to device engineers.  In order for new flash technologies to be adopted, it is critical that the enabling software also continues to evolve.