ramtron

Memory for the worldwide growth of electronic metering systems
By Craig Taylor, Ramtron International Corporation

Rapid electrification worldwide has made data collection a common requirement in most electronic systems. Today’s designers of electric power meters and data collection systems face complex demands as the push toward more sophisticated utility distribution management grows. Nonvolatile ferroelectric random access memory (F-RAM) offers three distinct benefits to these designers, creating an electronic power meter with the memory technology to capture data quickly and continually, never wear out, and consume a very low level of power.

First, F-RAM is capable of collecting data for an unlimited period without wearing out. In an electric meter, F-RAM can be written every second or minute for the life of the meter. No other nonvolatile memory offers this capability. Less advanced designs for digital meters must rely on writing the data when power is failing because other memory products wear out very quickly if they are written too often.

Ramtron offers F-RAM devices with industry standard 2-wire interfaces from 4Kb to 512Kb that are ideal for electric meters. The FM24C04 4Kb and FM24C16 16Kb devices are commonly used in low-cost residential meters, while the FM24C64 64Kb, FM24C256 256Kb and FM24C512 512Kb devices are suitable for advanced three-phase commercial and industrial meters. These products are of sufficient bandwidth for metering applications and are easily integrated because of their small package outline (8-pin SOIC or DFN).

In addition to stand-alone memory, Ramtron offers integrated Processor Companion products which incorporate 4Kb to 256 Kb of F-RAM with integrated peripherals such as a real-time clock (RTC), a watchdog timer, and a reset circuit. These devices are commonly used in time-of-use meters, which require that power usage be time-stamped. In such systems, the nonvolatile F-RAM memory and the RTC record power usage and time-of-use. The watchdog timer and reset circuit enable integration with a microprocessor in the system.

In addition to their high endurance, F-RAM products operate with very low power consumption. The technology is capable of producing devices with standby current of less than 1µa and active current below 100µa. The low power allows integration without a significant increase in the power budget.

The third benefit of Ramtron’s F-RAM memories is speed. The underlying technology is based on RAM, like SRAM, rather than ROM, like EEPROM and Flash. RAM devices are much faster for reading and writing, where write speed can be less then 200 nanosecond (ns). For example, Ramtron’s FM22L16 is a 256Kx16 nonvolatile RAM that requires no battery backup. It reads and writes with a 55 ns access time. This device is ideal for many types of data collection and storage applications.

Why F-RAM?

Figure 1

F-RAM technology is a preferred memory solution for data collection. Several hundred customers are already using F-RAM in production systems today, making it the most widely used advanced memory technology available.

All current production memory technologies use stored or trapped charge to capture data. Ferroelectric memory technology stores state and is, therefore, nonvolatile. Ferroelectric materials have two unique properties:

They change state in an electric field: If an electric field is applied across a ferroelectric crystal that is properly oriented, the crystal structure changes. If the field is removed, the crystal is stable in the new orientation.
A switching crystal produces a significant charge.

Figure 2

The ferroelectric material commonly used in memories is PZT (Lead Zirconate Titanate). The figure below shows the two stable states of the crystal.

The ferroelectric material changes state in an electric field and is not affected by magnetic fields. The memory cell consists of a transistor and a capacitor. The dielectric in the capacitor is the ferroelectric material.

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