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FRAM: Alternative to flash memory in embedded designs

Posted: 31 Jul 2012 ?? ?Print Version ?Bookmark and Share

Keywords:Ferroelectric random access memory? non-volatile? microcontrollers?

Ferroelectric random access memory (FRAM) is generally known as a non-volatile, standalone memory technology that has been a part of the semiconductor industry for more than a decade.

In recent years, integrated circuit manufacturers have been considering FRAM as a strong contender for embedded, non-volatile memory, as an alternative to flash technology. This article discusses key technology attributes of FRAM while exploring specific use cases that demonstrate FRAM's advantages.

Today there are multiple memory technologies that have the potential to change the landscape of embedded processing. However, none so far have surfaced as a strong contender for replacing flash technology in microcontrollers (MCUs) until FRAM.

What is FRAM?
FRAM is non-volatile memory that has power, endurance and read/write speeds similar to commonly used static RAM (SRAM). Information stored in an FRAM cell corresponds to the state of polarization of a ferroelectric crystal that can hold its contents even after the power source is removed. This is what makes FRAM truly non-volatile. Also, since the energy required to polarize a crystal is relatively low when compared to programming a flash cell, FRAM writes are inherently lower power than flash.

Here are a few typical applications that use microcontrollers with flash technology today. Let's look at how leveraging FRAM-based MCUs, rather than flash-based MCUs, bring cost, energy and efficiency optimization.

Data logging
A typical data logger application such as a temperature data logger can sample at rates anywhere between 1-1,000Hz. Now consider the write time of a single byte in flash memory is approximately 75?s.

In comparison, FRAM technology can be written to at a rate of about one byte every 125ns. This is close to 1000 times faster than flash! Now consider the application reaches the end of a flash segment and needs to move to the next one, suddenly there is a 20ms latency while waiting for a segment erase to complete.

The erase latency does not apply to FRAM as it is not required to pre-erase FRAM bytes between writes. A 20ms latency every segment does not seem prohibitive until we calculate how significantly it impacts the maximum write speed. For the purpose of this discussion, consider that the block of memory being written to is 512B in length. A flash memory block can be written 26 times per second including the time taken to complete an erase cycle every time 512B are written. This brings us to a total speed of 13kBps [1].

In comparison, a 512B FRAM block can be written to at speeds greater than 8MB/s [2]. Not every application requires such high write speeds, but consider if your target application was required to write only 1kB every second, the MCU with flash technology would spend 7% of the time staying active to perform the write. However an FRAM MCU would complete that task in 0.01% of the time allowing the MCU to remain in standby 99.9% of the time, providing significant power savings.

Energy harvesting
Many applications today are focused on using cleaner and greener energy, energy that is derived from natural sources such as sunlight, vibrations, heat or mechanical change. Such applications rely on small bursts of energy that provide power in short time intervals and the MCU is usually down to the wire in terms of how many lines of code can be executed before power is lost. Flash-based applications pay a premium in power, not only because of higher average power while accessing flash, but also because of higher peak power during flash write events.

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