Electronic Design

Smaller 32-Mbyte Flash Memory System Provides More Security And Functionality For Less Cost

This single monolithic chip has an on-chip boot memory in addition to a secure flash file system.

Flash-memory storage is gaining popularity in more applications, many of which are high-volume. Such applications not only demand flash-memory chips with more storage ability, but also greater functionality, better security, smaller size, and most importantly, low cost, primarily in embedded applications. Recognizing these requirements, M-Systems Flash Disk Pioneers took advantage of Toshiba's 0.16-µm NAND flash process and created a single, monolithic chip with all control and storage circuitry as the answer. The DiskOnChip Millennium Plus chip comprises a number of important system features (Table 1).

The chip's design permits a two-chip solution when it's coupled with a processor that incorporates its own RAM. The device's security features address today's need to store private information and encryption keys for decoding downloaded data.

The DiskOnChip Millennium Plus incorporates a number of subsystems, including the flash-memory array (see the figure). This combination makes the chip unique.

The chip comes in a 48-pin JEDEC TSOP-I standard package that presents a 16-bit data bus. This is used to access both the 16-bit flash-memory array and the 16-bit programmable boot block (PBB). The chip can be accessed as an 8-bit device, too.

The flash-memory array is built from two 8-bit flash-memory blocks. Data integrity of each 512-byte sector is addressed with enhanced Reed-Solomon EDC/ECC logic. It can correct single bursts of up to 11 bits and two 10-bit blocks with two random-bit errors. Additionally, it can detect four random-bit errors, up to 31-bit single bursts, and up to 11-bit double bursts.

The PBB is an SRAM that's loaded from flash memory each time the system is reset or when changes are made to the matching flash-memory sector. The SRAM is accessed via the external CPU as normal memory and is typically used to store the operating-system boot loader.

The PBB is a significant advance because it effectively moves a sizable chunk of hardware into the DiskOnChip Millennium Plus. This gives the user an updatable boot mechanism, eliminating the need for a similar mechanism in the processor. A processor chip that incorporates its own RAM allows a two-chip solution. Plus, it eliminates the need for an external RAM that could lead to a three-chip solution. Moreover, no external circuitry is necessary for small systems.

Up to four DiskOnChip Millennium Plus chips can be connected to the same bus. The 1-kbyte PBB SRAM is additive, allowing up to a 4-kbyte PBB. No external circuitry is required for decoding because each chip is identified by two ID pins.

Two key features of the system are located in the flash-memory addressing subsystem. One handles sector remapping, and the other manages security.

Sector remapping takes into account the limited number of updates supported by flash memory. This is rarely the entire flash-memory array that's repeatedly updated. Instead, the typical mode of operation is to change one or more sectors on a regular basis.

Without sector remapping, the chip essentially becomes useless when a sector can no longer be updated. With sector remapping, a different sector can be used for each update while unused sectors are recycled for later use. This form of wear-leveling significantly increases the overall useful lifetime of the chip. The same technique is used to handle bad sectors as well.

Security Galore
The chip has a number of security and reliability features (Table 2). Two protected ranges control access to the flash-memory arrays. These are password-protected ranges that limit read or write access. Although they might overlap, the ranges can't span flash-memory arrays in a multiple-chip configuration. Each chip has its own security information.

Two regions tend to be sufficient for most applications. Normally, one region stores the operating system and application code while the second region stores licensing or customization data.

Hardware protection is supported via a LOCK# pin that's controllable by ex-ternal logic or a jumper. This bypasses the security controls so ranges can't be used even with the correct password.

By default, the chips are configured with no security settings. These can be set by the OEM in the field, or when a product is initialized.

The other security feature of the DiskOnChip Millennium Plus is the one-time programmable (OTP) memory. This 13-kbyte memory is divided into three parts that hold a serial number, customer information, and custom factory-set information.

Each chip has its own, unique serial number. This provides a way to uniquely identify a system. The customer information area is 6 kbytes and can be programmed by the OEM if necessary. This area typically contains information about system configuration, customer ID, or license data.

The custom factory data area uses the remaining OTP memory. It's programmed by M-Systems when the chip is made. The information is specified by the OEM purchasing the chips and will be unique to that OEM.

The DiskOnChip Millennium Plus doesn't incorporate any encryption hardware or store any information in an encrypted form, although the flash-memory array can obviously store encrypted information. Still, M-Systems' approach is ideal for applications such as set-top boxes where configuration information will be downloaded and needs to be saved in a secure repository. Secure storage also is very useful for a system designed to run downloaded applications that could potentially compromise the system.

The DiskOnChip Millennium Plus includes driver support for most popular operating systems, such as QNX, VxWorks, pSOS, FreeBSD, PharLap ETS, SMX, Windows CE, Windows NT, Windows 9x, Windows 2000, and DOS. Developers with more specific needs can use the OS Adaptation Kit (OSAK) for building custom OS interfaces. It provides sector-level functions that hide the more complex flash file-system operations. Developers that must customize control of the PBB can do so via the Boot Development Kit (BDK).

Price & Availability
The DiskOnChip Millennium Plus flash-memory chip is available now in sample lots for $40 each in volume quantities. Volume-production quantities will be available in the second quarter.

M-Systems Flash Disk Pioneers Ltd., 8371 Central Ave., Suite A, Newark, CA 94560; (510) 494-2090; fax (510) 494-5545; Internet: www.m-sys.com.

TAGS: Toshiba
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