Embedded FRAM opens up MCU design
Goods and equipment are routinely transported around the world undergoing various transport mechanisms and environmental conditions between production and reaching the customer.
Medical and food is transported within a certain temperature range that should be logged and documented.
For sensitive technical equipment it is important to track transport data, including humidity, radiation and acceleration.
I will propose to usage and the main benefits of embedded FRAM for data storage in such applications.
Currently, chemical or mechanical indicators like liquid contact indicators, humidity stripes, chassis intrusion indicators or predetermined breaking points are used to detect mechanical overloads. These sensors are able to detect the incident itself even if the device is in transit and no grid power is applied. However, these methods provide no information about date, time or location where the incident happened.
In some cases, electrical sensors are already used to track certain data types, such as temperature, usually through battery-supplied stand-alone data logging systems. But only a limited amount of sensor data can be recorded when the device is on storage or transport, without access to power from the grid.
Furthermore, today’s available standard storage technologies – like flash and EEPROM – are limited in terms of write cycles due partly to the technology itself and the high power consumption for flash write operations.
A technology that enables equipment manufacturers to consolidate and store data from several sensors independent from the operating condition of a device improves visibility and tractability of usage and handling. With Ferroelectric Random Access Memory (FRAM) as a storage technology, combined in an embedded ultra low power microcontroller system, this is possible.
FRAM is a non-volatile memory technology. From a technical point of view, FRAM cells are very similar to DRAM cells and use a 70nm ferroelectric crystal layer as the dielectric material to store its state.
In comparison to DRAM no refresh cycles are required and stored information is retained when switching off the supply voltage.
The crystal lattice of the dielectric material (lead zirconate titanate) can obtain two stable states. To change the state a negative or positive voltage higher than 200kV/cm is required.
Only 1.5V programming voltage is required to perform writes to the memory since the ferroelectric crystal layer has a thickness of 70nm.
This also means that it is almost impossible to influence FRAM based devices by electrical fields applied outside of the device since the level of 200kV/cm must be exceeded. Also the crystal polarisation does not change when magnetic and nuclear radiation is applied.
FRAM has almost no limitation of write cycles, so logged data can be overwritten an almost infinite amount of times. Current datasheet values are around 1015 write cycles, compared to flash which offers up to 107 write cycles. FRAM can also be used as RAM with the advantage of being non-volatile.
The write speed of current embedded FRAM is in the range of SRAM and it does not require being programmed block wise as Flash needs to be, but can be manipulated bit wise without stopping the CPU.
This makes software development more flexible and easier since FRAM can be assigned as volatile (RAM) and non-volatile memory (Flash) dynamically from within the application.
Also the energy consumption for programming an FRAM memory cell is about 250 times smaller than of a current low power flash cell.
For instance, a Flash based TI MSP430 would take a second to write a 13 kByte data block and would draw a 2.2mA supply current. On the other hand, for an MSP430 in the Wolverine family with integrated FRAM, this operation would only require 6ms while having a current consumption of just 700μA.
The integrated crystal based real time clock consumes about 500nA while it is still possible to wake up the controller from internal or external interrupt sources as chassis intrusion indicators. Accurate time stamps can be stored in addition to every single measurement value.
The analogue infrastructure on this device also allows the connection of several different types of sensors and gives the opportunity to interpret them while still using a minimum of energy.
As an example, the internal 12-bit SAR ADC with a speed of 200ksps has a current draw of just 75μA with active internal voltage reference. It offers differential inputs so it is possible to connect Wheatstone bridges, which are required for strain gauges to detect mechanical impacts.
FRAM provides the advantage to store data in a non-volatile state while using a very small amount of energy. With FRAM built into an ultra low power microcontroller system, data can also be recorded with the data logging system only supplied by a very small battery.
If analogue infrastructure is available on that microcontroller many different sensors can be connected. A complete picture of a products life cycle can be recorded with this technology at a level of detail that was impossible to obtain in the past. This can help producers of expensive machines to argue if items arrive damaged or are stored incorrectly and this data can also be used to improve current products.
The writer is Cornelius Poth, a field application engineer for TI’s microcontrollers.