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A survey of non-volatile memories: Ideal workloads for NAND, NOR, and F-RAM

June 20, 2019

mnshgrg-June 20, 2019

In the past, memory advancements were primarily measured by improvements in density, or cost-per-bit. But, as performance requirements for nonvolatile memories evolved, the criteria used to select them also changed. Applications such as factory automation, autonomous vehicles, portable medical devices, edge computing, and IoT sensors are demanding memories purpose-built for their unique requirements. Mass storage in data centers, computers, and consumer devices require the highest densities and lowest cost-per-bit, typically supported with NAND Flash. Embedded computers store boot code and transaction data, usually in NOR Flash. Data recorders, sensors and edge computers capturing rapidly changing data over long periods of time use specialized technologies, such as ferroelectric RAM (F-RAM). Matching memory to workload delivers optimum performance and reliability, and each of these solutions trades off certain characteristics to best fit the target application.

For decades, NAND Flash has served consumer markets with ever-improving cost/gigabyte and adequate read/write performance. NAND-based solid state drives (SSDs) are also widely deployed in data centers and cloud servers. Higher densities have been achieved through process improvements and geometry shrinks, but this comes with trade-offs. In the very small NAND Flash memory cells, each program/erase (P/E) cycle stresses the cell, which translates to lower endurance, even with overprovisioning and wear-leveling. Data retention is also relatively low—approximately 10 years—owing to the smaller geometries.

In contrast to NAND, NOR Flash has been optimized for reliability and ease-of-use. NOR Flash uses a relatively large memory cell which provides high endurance and long data retention. Combined with a byte-addressable architecture, NOR Flash is ideal for boot code—including execute-in-place systems—and transaction data. In response to the increased focus on fail-safe operation, the latest products offer functional safety and secure boot mechanisms as well. NOR Flash also supports very fast read performance which enables instant-on displays for automotive instrument clusters and other applications. However, this high reliability and improved performance comes with a trade-off, as NOR Flash densities are lower than NAND Flash (see NOR Flash for Automotive and Functional Safety).

F-RAM is ideal for devices that continuously capture data over long periods of time (see Ultra Low Energy F-RAM). Unlike NAND and NOR memories which rely on stored charge to represent binary data, the F-RAM memory cell switches polarity within a crystal lattice made of a thin ferroelectric film of lead zirconate titanate, commonly referred to as PZT. The switching requires extremely low energy, enabling very long battery life, and it doesn’t wear out, delivering virtually infinite endurance (see Designing with Low Power F-RAM). There is also no charge leakage, so data retention is very high—approximately 100 years. A common misconception is that F-RAM can be disturbed by magnetic field. This is not true. While excellent for data logging, the F-RAM densities generally do not meet the needs of code and mass storage.

Table 1: Performance characteristics, ideal workloads, and target applications (Source: Cypress)

 

Performance

Characteristics

Ideal
Workload

Target

Applications

NAND Flash

 High density / low cost

✓ Low endurance

✓ Medium data retention (~10 yrs)

Generic mass storage

✓ SSDs for data centers and consumer devices

NOR Flash

✓ Medium density

✓ High endurance

✓ High data retention (~20 yrs)

Boot code and transaction data

✓ Automotive systems

✓ Industry 4.0

✓ Industrial IoT

F-RAM

✓ Medium density

✓ Virtually infinite endurance

✓ Very high data retention (~100 yrs)

✓ Very low energy

High-frequency data logging

✓ Industrial sensing

✓ Portable medical

It is worth noting that engineers are migrating from parallel to serial interfaces for new system designs. Serial interfaces reduce pin count for both the memory and the SoC, shrinking the PCB for reduced cost and smaller form factors. Octal SPI, Quad SPI, SPI, and HyperBus now offer performance that rivals parallel interfaces, including up to 400 MBytes/sec for Octal SPI and HyperBus.

Choosing the right memory ultimately helps ensure the success of your product. NAND Flash is ideal for mass storage workloads typically found in cloud computing and personal electronics. In contrast, NOR Flash and F-RAM are suitable for embedded systems and edge devices, which are often located in extreme and remote environments. In tandem with edge computing growth, advances in artificial intelligence are driving new capabilities which raise the bar on fail-safe operation, pushing manufacturers to deliver NOR Flash and F-RAM with safety and security features.


Manish Garg is Senior Product Marketing Manager in the Memory Products Division at Cypress Semiconductor. Previously he has worked in different roles in Product Planning and Marketing at Altera (now Intel PSG) and Lattice Semiconductor. He holds a MS in Electrical Engineering from the University of Wisconsin and an MBA from Santa Clara University.

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