Duncan Bennett, Product Marketing Manager at Everspin, examines how MRAM (Magnetic RAM) has significant advantages over alternative memory technologies like Flash/EEPROM or SRAM, when operating in HiRel (high reliability) environments such as high altitude, space or military applications.
Outside the earth’s protective atmosphere, a semiconductor device may be exposed to temperatures and radiation at higher levels than seen with earth-based applications. Cooling airflow may be non-existent or limited, driving operating temperatures potentially even higher. Failures cannot be repaired, so operating reliability is critical.
Equipment in military applications, although less at risk from radiation, is also at risk from extreme temperatures, vibration and shock. These risks are mitigated slightly by the availability of periodic maintenance and replacement units, but failures can be life threatening.
MRAM memory technology has some unique advantages for the HiRel industry.
High temperature performance: MRAM outperforms other non-volatile memory technology when it comes to data retention at high temperatures.
Radiation tolerance: MRAM memory bits are immune to the effects of alpha particles that disrupt SRAM/EEPROM/Flash bits.
Data security: The combination of fast write and symmetrical read/write operations enables MRAM to be a better storage option for sensitive data.
A major expense item in any high reliability product is the time it takes for development and qualification, including validation of system requirements to handle power failure. Often, the most valuable item on the PCB of any low volume product is not the semiconductors or even the passive components, it’s the development time of the engineers.
Ease of use
MRAM helps with time to market because its easy to use. Parallel MRAM has an SRAM-like interface (address, data and control). Serial MRAM uses a standard SPI bus. Both of these interfaces are readily available on microcontrollers so are easy and familiar to implement. Development time is reduced when writing the firmware. Unlike Flash, MRAM does not need to be erased before writing. MRAM has no blocks or pages. There are no special operations required to make the data non-volatile. You can read or write any individual byte just like SRAM. All this means that development and verification takes far less time.
Critical data can be stored in a protected memory like battery-backed SRAM that preserves data when the power is lost. HiRel applications often cannot use batteries. Batteries are either made from flammable materials (Lithium) or they are too heavy for equipment used in flight.
Another approach is to use capacitors to keep the system alive long enough after power failure is detected for data to be written to Flash or EEPROM. A difficulty with this approach is the performance of capacitors. Typically, capacitance varies significantly with temperature. It may be as much as ±50% over the -40°C to 125°C operating temperature range. The leakage current of capacitors at 125°C can be significant. Larger value capacitors are physically large (a problem in space limited HiRel systems). Capacitors that operate at 125°C are relatively expensive.
MRAM offers two significant advantages when protecting data from power failure:
1. MRAM is the world’s fastest non-volatile write. The time between detecting a power failure and the power failing is often expensive, so anything that shortens this time is beneficial. Data can be transferred from volatile memory to MRAM faster than to any other type of non-volatile memory.
2. MRAM endurance is unlimited. This opens up an approach where the system uses MRAM as its working memory instead of SRAM. All critical data is always in the MRAM so when a power failure is detected the system just stops. No last gasp, no struggle to write critical data, it just stop. That sounds easy.
In short, MRAM offers perhaps the easiest way of protecting non-volatile data from a power failure and consequently gets the product to market faster.
A number of HiRel applications are targeted for flight. SEUs (Single Event Upsets) are caused by heavy particles hitting the semiconductor device and upsetting its performance. They are blocked by the atmosphere so they are more common at altitude. For analogue systems such as a voltage regulator, the SEU may cause the output to briefly stray out of regulation. For flash, EEPROM and SRAM memories, the SEU results in data corruption. Although SEUs are infrequent, there are many millions of bits so if you leave your program code in flash memory at high altitude for long enough it will eventually be corrupted.
Daring to be different
Once again, MRAM offers something entirely different. MRAM bits are immune to SEUs. There is no need for expensive overheads such as EDAC (Error Detection And Correction) or scrubbing of memory (cleaning the data to ensure that it does not contain any corrupted bits).
MRAM has become the memory of choice for systems that need to be SEU tolerant. A number of companies take MRAMs or MRAM technology from Everspin Technologies and produce ‘hardened’ MRAMs for HiRel applications. These include Cobham Semiconductor Solutions, Honeywell, 3DPlus and OCE Technology.
Data security has its own unique set of requirements. Like all HiRel applications, data reliability is important but so is the method and result of erasing data. Erasing MRAM is helpful to secure applications by:
Fast erase: If the system detects that it is being attacked (maybe the unit has been stolen and it is being opened) the system needs to erase any secure data as quickly as possible. MRAM offers the fastest non-volatile memory write, destroying data faster than data stored in any other non-volatile memory.
Erase without remanence: An erase operation with flash and EEPROM doesn’t always remove all the charge from the memory cell. Enough charge is removed so that the read circuit sees it as empty of charge but it still has data remanence. Hackers can exploit these effects of both SRAM and Flash/EEPROM to recover the secure data that the system designer thought had been destroyed.
MRAM does not behave the same way and erases without any remanence.
Power sniffing: A useful technique when trying to hack into a system is to prevent the destruction of data in the flash/EEPROM by detecting a write operation and switching the power off before the write starts. This is possible because the write current of a flash or EEPROM is a lot higher than the read current and a write takes around 10ms.
MRAM writes are so fast that by the time the hacker has detected a write it’s already too late. The on-chip capacitance is enough for the write operation to complete.
MRAM writes and reads consume similar amounts of power so that detecting a write from a read is not as easy as it is with flash/EEPROM.
Because MRAM stores information as magnetic spin, it is far less susceptible to data loss at higher temperatures than charge based memory technologies. MRAM is available with operating temperatures up to 125°C. MRAM retains data for 20 years and ten percent of that time is at 125°C. Almost all HiRel applications can work within this impressive specification.
Some of the HiRel applications for MRAM are described here.
The oil industry requires certain equipment to withstand high temperatures for a long time. It is one of the very few applications that requires equipment to operate above 125°C for a long time. There are applications that need to operate at 200°C. MRAM is probably the best non-volatile memory for these HiRel applications.
MRAM uses error detection and correction. If you re-write a byte with a bit in error, the ECC will ensure that the error is corrected. This can help with extreme data retention. Certain military applications have data that is sensitive. An FPGA might be programmed with an algorithm that performs security encryption/decryption or a memory might contain target location. This sensitive information should not fall into enemy hands.
An interesting problem is what to do to prevent an enemy from gaining valuable intelligence from an aircraft that has been shot down. Can you be certain that an aircraft crash destroys sensitive data?
MRAM can use its fast erase to an advantage here. It is possible for the aircraft to know that it is about to be hit and take appropriate action. The system could detect that the end is very close and its final act is to erase the MRAM. No other non-volatile memory would be quick enough in these circumstances.
MRAM’s tolerance to SEU radiation makes it ideal for any electronics that flies. If an engineer needs SRAM they either have to protect it with EDAC/memory scrubbing etc or they could use MRAM. MRAM bits are SEU immune and MRAM can replace SRAM because MRAM has unlimited endurance and is written exactly the same way as SRAM.
It is perhaps not a surprise to find that Airbus use MRAM in the avionics of the A350 wide body aircraft.