Making the switch from NOR to NAND flash

Third-generation networks require a new type of memory for an ever-growing range of multimedia applications, including multimedia messaging service (MMS), high-resolution cameras, photo albums, streaming video, MPEG-4 video, MP3 and streaming audio, 3-D games, Java applications and Web browsing. Increasingly, legacy NOR flash memory in feature phones is being replaced with higher-capacity NAND flash, capable of achieving the required performance and reliability to handle all this data.

The shift from NOR to NAND flash is not always smooth. Though cost-effective and much higher in performance, NAND is considered unreliable and complicated to manage. In fact, when used in cards, NAND is always accessed through a controller inside the card. The recent introduction of multilevel cell (MLC) NAND has furthered the cost advantage of NAND but has also increased the design challenges. To provide the cost, size and performance benefits of NAND flash with a legacy-compatible interface and boot support, Samsung now offers NAND, and M-Systems and Toshiba offer MLC NAND, in embedded-flash disks.

Here are guidelines for developers who are interested in migrating from NOR to NAND flash with minimum risk and fast time-to-market.

DO

• Consider how the total memory architecture will change when adding NAND support, especially the possible need to add PSRAM or SDRAM to execute code. In some cases the total memory system cost, including additional RAM to execute the code (see below), may not decrease at all, making the transition to NAND of questionable value.
• Make sure the correct hardware interface is part of the design. NAND is an I/O device and cannot be connected using standard memory signals. Make sure your chip set exports the required signals or opt for a flash disk that features a NOR-like interface.
• Plan to boot from NAND. Doing this can benefit your design both by greatly reducing memory system costs and by enabling more-efficient board usage. If you are considering booting from NAND, note that some additional RAM will be needed to store the OS kernel code, formerly run from NOR. Also, check whether your chip set includes an internal execute-in-place (XIP) boot block, or simply use a NAND-based flash disk with a built-in XIP boot block.
• Prepare your production line to work with NAND. Make sure your programming solution supports NAND or NAND flash disk, or plan for an on-board programming alternative through a fast serial port, such as a USB 2.0 port (JTAG is too slow for high-capacity programming).
• Save space by using multichip packaging (MCP). Most vendors offer packages that include a combination of NAND, NOR, PSRAM, SRAM or SDRAM, depending on customer requirements. This can help you reduce your handset size. Note, however, that most MCPs are custom-made, so you will depend on a single supply source.

DON'T

• Select a NAND solution that does not come with its own optimized software. Though some operating systems support NAND, these solutions are not mature enough and do not support the latest advances in flash technology, such as MLC NAND or large blocks.
• Choose a NAND solution without a good technical support scheme from the memory provider. You must focus your efforts on designing a better phone. NAND is complicated to work with, and you don't want to have to become a NAND expert. An experienced, on-site technical support team should be a major consideration when selecting your NAND supplier.
• Blame the NAND media for not achieving the expected performance-at least not right away. In many cases, the platform hardware must be thoroughly examined to ensure that timing is properly defined and that it supports advanced features, such as DMA, bursts and interrupts. It is also advisable to analyze the software. Check for file system efficiency. Determine whether performance is measured when the medium is full or when it's empty, whether you are using small or large files and even whether those are random.
• Reduce the bill of materials by eliminating embedded memory in favor of a card slot. Handsets do not conform to the camera business model; i.e., they can be used for voice and short message service/e-mail communication without a card. Network operators want users to download multimedia data using the network. This requires that sufficient embedded memory be available on-board, regardless of the card the user eventually may decide to purchase. The card should be used as a memory expansion.
• Take too many risks. Moving to NAND from NOR is challenging enough. Use the most mature solution and support team available. Make sure the OS and platforms support the solution you choose.

Raz Dan (raz.dan@m-systems.com), vice president of customer support for M-Systems (Kfar Saba, Israel)