Technical White Paper

NAND Flash represents the lowest cost-per-bit memory for embedded electronic designs today, but it is not without its penalties. In this white paper from BPM Microsystems, you will learn how to navigate this down-side, early in your design phase and keep the cost advantages through to the manufacturing cycle.

  • Learn about:
  • NAND Flash Memory Architecture
  • NAND Flash Reliability Issues
  • Bad Block Management
  • Bad Block Management Algorithm Specification
  • Bad Block Replacement Strategy
  • NAND Flash ECC – Error Correction Codes
  • Spare Area Placement
  • Third-party and Custom BBM Schemes
  • Programming Managed NAND
  • Serializing NAND
  • Universal Factory ProgrammingBBM Scheme

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The following is an excerpt from the whitepaper- page 1 of 17:

During manufacturing of electronic systems, blank non-volatile devices must often be programmed with initial data content. This allows the target system to get up and running, and is referred to as “factory programming,” “factory pre-programming,” or “bulk programming.” Generally, this is a very straightforward process that has been in place in the industry for many years. However, with NAND flash the process is more difficult.

NAND flash is a high-density, non-volatile memory that requires increased algorithm complexity during factory programming when compared to other flash memory architectures, such as NOR. This is primarily due to the existence of bad memory blocks and other reliability issues in the device shipped from the supplier.

NAND flash manufacturers are able to achieve commercially viable yields by allowing a small portion of the memory to fail test while still classifying the device as good. This tradeoff produces the very low-cost, high-density characteristics for which NAND flash is sought. By comparison, NOR flash is shipped with no such bad blocks at a much higher cost per bit and is simply not available in the densities that NAND provides. NAND flash is also superior to NOR for erase and write performance. By contrast, NOR is the fastest when it comes to reading the memory.

NAND Flash Types

To further complicate matters, there are multiple types of NAND flash as shown in Figure 1. Since programming requirements vary amongst the different types, it is important to be capable of identifying these NAND types.

There are two main categories: Raw and Managed. Raw NAND is the most cost-effective, since it contains just the flash memory array and a Program/Erase/Read (P/E/R) controller. However, extra processing power and complexity is needed in the target embedded system to manage the raw NAND and make it reliable. Managed NAND on the other hand, contains a more sophisticated controller that internally manages the NAND. This typically increases the cost over raw NAND for the same memory density, but makes the NAND much easier to integrate.

Raw NAND is available in two types: Single-Level Cell (SLC) and Multi-Level Cell (MLC). The basic difference between the two is the number of data bits each memory cell holds. For SLC, exactly one bit of information is held in each memory cell. MLC devices store two or more bits per cell, though this is not without tradeoff.

By storing more bits per cell, the storage capacity of the NAND can be doubled, quadrupled, or even more for roughly the same cost to produce the die as compared to SLC. Looking at it another way, MLC can produce the same storage capacity as SLC but using a smaller die, and therefore at a lower cost.

The most recognized managed NAND devices are the consumer card types. Popular card types include CompactFlash (CF), Secure Digital (SD), and MultiMediaCard (MMC), just to name a few. These ubiquitous memory cards are sold direct to consumers for applications such as portable music players, digital cameras, handheld computers, and mobile phones. The card packaging is standards-based and is designed to withstand handling by consumers as they are inserted and removed many times from a variety of devices.

Embedded managed NAND types are electrically very similar to consumer cards, with the key difference being in the packaging. Embedded managed NAND devices are designed to be surface-mount (SMT) soldered into the target system. Such devices are a permanent part of the embedded system, and are not accessible to the consumer. These devices are most often in ball grid array (BGA) style packages. Embedded managed NAND devices are becoming increasingly popular due to their ease of integration.

The allure for system designers is the ability to use existing electrical interfaces designed for consumer cards. Furthermore, these devices appear the same as a consumer card to software drivers designed to interface to consumer cards. Some examples of embedded managed NAND are: Samsung’s moviNAND™, Micron’s e-MMC™, SanDisk’s iNAND™, Toshiba’s eSD™, and the MultiMediaCard Association’s eMMC™.

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