Nonvolatile Memory Goes Green

Nonvolatile Memory Goes Green

By Duncan Bennett, Ramtron International

I believe that as issues of the environment move to the forefront of our collective consciousness, the electronics industry worldwide is starting to focus on low energy consumption to market its products.

Duncan Bennett, Ramtron International

By Duncan Bennett, Ramtron International

I believe that as issues of the environment move to the forefront of our collective consciousness, the electronics industry worldwide is starting to focus on low energy consumption to market its products. The energy rating of white goods has been a common marketing message for some time, focusing on how low power consumption can help reduce the power bills. Today we recognize that low energy consumption not only reduces bills, it also contributes to a better environment.

Design engineers are consistently trying to lower their products’ power consumption so that they only require one battery throughout their entire lifetime (for instance 25 years). A developing trend, ultra low power capabilities are giving way to a new breed of products that do not require a battery at all and are based on the concept of ‘energy harvesting’ (deriving energy from the environment to power the product).

The key to both such categories is the products’ ability to sleep at very low power, so low that the device is essentially shut down because leaving it in sleep mode consumes too much power. Shutting down works as long as the product can remember its state when it reboots. For this, a nonvolatile memory with low operating currents and high write endurance is required. A memory with these features enables the production of devices that never need to be pluggedin and never need new batteries.

Low Power Memory

Focusing on these memory requirements, let's examine the power consumption of nonvolatile serial memory technologies, a common memory architecture used to store configurations. The technologies compared here are FRAM, EEPROM and flash. Since flash is only available with a serial peripheral interface (SPI), we will compare it with SPI versions of EEPROM and FRAM.

For the sake of this study, we will calculate and compare the amount of energy used by each memory technology to perform writes and erases. Energy is a good way to make a comparison as it takes into account the duration of the task as well as the amount of power required to perform the task.

We have chosen to compare writing and erasing since these three memory technologies vary most during these tasks, while the reading process is roughly the same for each technology even if operating speed, voltage, and current consumption differ. One would assume that the speed of the serial interface would play an important role here, however, when the calculations are repeated for the same part at different SPI bus speeds, the total energy spent remains roughly the same (I guess Einstein’s Conservation of Energy applies even here).

To eliminate any issue associated with calculating SPI bus overheads (i.e. issuing commands and setting up the address), we will perform the comparison using a significant amount of data, 64 kilobits to be exact.

Additionally, EEPROM and flash technologies use a page buffer so that larger amounts of data can be written simultaneously, speeding up the writing process. These pages are most efficient when the amount of data is an integer number of pages. FRAM does not have a page buffer, as it is able to write data as fast as it is delivered via the SPI bus.

While FRAM and EEPROM have no erase time, flash requires a significant amount of time to erase sectors. As such, we will compare how long it takes for 64Kb to be erased and written with new data.

Finally, it should be noted that manufacturers do not always use consistent standards to quote power consumption. Some devices may be specified to operate at Vcc=1.8V, but their datasheets offer operating current figures at Vcc=2.5V.

Serial data flash uses significant amounts of energy when erasing pages/sectors due to the considerable amount of time needed for these operations. The energy requirements of both EEPROM and flash could be reduced by using polling techniques rather than waiting for the worst case time to complete the operation. It is unclear from the datasheets whether current consumed during the write/erase cycles drops off automatically when the write/erase is complete, or whether it continues.

The results above show that FRAM clearly has the advantage over the other nonvolatile memory technologies. It consumes 60 times less energy than the next best alternative and approximately 400 times less than the best performing serial data flash.

Duncan Bennett is a Strategic Marketing Manager at Ramtron International of Colorado Springs, CO. He has over 20 years experience in the semiconductor industry. He started as a design engineer in the industrial control/graphical instrumentation systems field, moving from applications to sales and, finally, marketing. Duncan is responsible at Ramtron for enabling new FRAM applications in the automotive industry and for the definition of new memory products.