The use of MRAM to improve efficiency in edge computing

This article originally appeared in the March '22 magazine issue of Electronic Specifier Design – see ES's Magazine Archives for more featured publications.

While there are many alternatives to MRAM, including SRAM (static random-access memory), DRAM (dynamic RAM), flash, and EEPROM (electrically erasable programmable read-only memory), each of these widely-used technologies require trade-offs in one or more areas (such as efficiency) that make them less than ideal for edge computing. Accordingly, designers can look to MRAMs, or magnetoresistive random access memories.

MRAM devices store data in magnetic storage elements and offer true random access, allowing both reads and writes to occur randomly in memory. Their structure and operation are such that they feature low latency, low leakage, high write cycle count, and high retention, all of which are highly desirable for edge computing. MRAM technology is analogous to flash technology with SRAM compatible read/write timings (MRAM is sometimes referred to as persistent SRAM, or P-SRAM).

Due to its characteristics, MRAM is particularly suited to applications that must store and retrieve data with minimal latency. It combines this low latency with low power, infinite endurance, scalability, and non-volatility. MRAM’s inherent immunity to alpha particles also makes it suitable for devices that are regularly exposed to radiation.

This article, while touching on the performance capabilities of common memory technologies, will review the benefits of using MRAM in several edge computing applications, and then introduce specific MRAM devices from Renesas, some MRAM usage tips, and an evaluation platform to help designers get started on their design.

How MRAM works

As the name implies, data in MRAM is stored by magnetic storage elements.

The elements are formed from two ferromagnetic plates, each of which can hold a magnetisation, separated by a thin insulating layer. This structure is called a magnetic tunnel junction (MTJ). One of the two plates is a permanent magnet set to a specific polarity during manufacture; the other plate’s magnetisation can be changed to store data. Renesas uses MRAM devices to use a proprietary spin transfer torque MRAM (STT-MRAM) that is based on a perpendicular magnetic tunnel junction (p-MTJ).

The p-MTJ includes a fixed and unchangeable magnetic layer, a dielectric barrier layer, and a changeable ferromagnetic storage layer (see Figure 1). During a programming operation, the magnetic orientation of the storage layer is electrically switched from a parallel state (low resistance state ‘0’) to an antiparallel state (high resistance state ‘1’), or vice versa, depending on the current direction through the p-MTJ element.

These two distinct resistance states are used for data storage and sensing. MRAM use cases Data logging, memories in IoT nodes, machine learning/artificial intelligence in edge computing devices, and RFID tags in hospitals are examples of MRAM use cases. Data loggers require multiple megabits of non-volatile memory to accommodate their long-term accumulation of data.

Although they are typically battery-powered, they can also rely on energy harvesting for power, and therefore require low-power memory. In the event of power loss, the logged data must be retained indefinitely. MRAM meets the performance demands of data loggers. MRAM persistence, combined with an extremely low energy mode, is able to enable a unified memory solution for code and data in IoT nodes that operate from energy harvesters or battery sources in extremely small form factors (Figure 2).

Startup time is often an important consideration in IoT nodes. Implementing a code-in-place structure that uses MRAM can reduce the time required to boot up, as well as overall bill of materials cost since there is less need for DRAM or SRAM. The persistence offered by MRAM is also enabling a new generation of IoT nodes that are capable of machine learning where the inference algorithms do not have to be reloaded every time after device wakeup.

The local processing encompasses the analysis of sensor data, the ability to make decisions – and in some cases, even reconfigure the node. This localised intelligence demands persistent and low-power memory. These devices can implement local coarse inference in real time and can use the cloud for enhanced analysis.

The speed of MRAM is beneficial for implementing machine learning in edge devices such as enterprise resource planning (or ERP) systems, manufacturing execution systems (MES), and supervisory control and data acquisition (SCADA) systems. In these systems, data is analysed and intermediate patterns identified and shared with adjacent domains. The edge architecture requires the right processing speed and persistent memory.

Designers can also apply MRAM in healthcare devices where radio frequency identification can be beneficial. Its low power consumption, combined with its immunity to radiation, make it suitable for hospital environments. RFID tags in hospitals are used for a variety of reasons including inventory management, patient care and safety, medical equipment identification, and the identification and monitoring of consumables.

High-performance serial MRAM memory Designers of edge computing systems (including industrial controls and automation, medical devices, wearables, network systems, storage/RAID, automotive, and robotics) can use Renesas’s M30082040054X0IWAY (Figure 3). It is available in densities ranging from 4Mbits to 16Mbits. Renesas’s MRAM technology is analogous to flash technology with SRAM-compatible read/write timings.

Data is always non-volatile with 1,016 write cycles endurance and greater than 20-year retention at 85°C. The M30082040054X0IWAY has an SPI (serial peripheral interface), eliminating the need for software device drivers. SPI is a synchronous serial interface which uses separate lines for data and clock to help keep the host and slave in perfect synchronisation. The clock tells the receiver exactly when to sample the bits on the data line. The M30082040054X0IWAY supports eXecute-In-Place (XIP) which allows the completion of a series of read and write instructions without having to individually load the read or write command for each instruction.

Accordingly, this XIP mode saves command overhead and reduces random read and write access time. The M30082040054X0IWAY offers both hardware and software-based data protection schemes. Hardware protection is through the WP# pin. Software protection is controlled by configuration bits in the Status register.

Both schemes inhibit writing to the registers and memory array. It has a 256-byte augmented storage array which is independent from the main memory array. It is user-programmable and can be write-protected against inadvertent writes. To further cater to low-power applications, the M30082040054X0IWAY has two low-power states: Deep Power Down and Hibernate. Data is not lost while the device is in either of these two low-power states.

Moreover, the device maintains all its configurations. The device is available in small-footprint, 8-pad DFN (WSON) and 8-pin SOIC packages. These packages are compatible with similar low-power volatile and non-volatile products. It is offered with industrial (−40°C to 85°C) and industrial plus (−40°C to 105°C) operating temperature ranges.

Using MRAM

MRAM can significantly reduce overall energy consumption compared with other memory technologies.

But the amount of energy savings can vary depending on the usage patterns of the specific application design. Like other non-volatile memories, the write current is much higher than the read or standby current. As a result of the above, write times need to be minimised in power-critical applications, especially in designs which require frequent writes to memory. MRAM’s shorter write times can mitigate this consideration and reduce energy consumption compared with other non-volatile memory choices, such as EEPROM or flash.

Additional energy savings are possible with MRAM owing to the use of a power gating system architecture and by placing the memory into standby as often as possible. MRAM’s faster power-up-to-write time makes it possible to put MRAM into standby more frequently than other non-volatile memories. MRAM’s zero leakage when in standby is also beneficial in this case.

Note that a larger decoupling capacitor is often needed to support power up energy needs when power gating is employed.

MRAM eval board

To help designers get started with the M30082040054X0IWAY, Renesas also provides the M3016-EVK evaluation kit. This contains the 16-Mbit MRAM and enables users to develop interactive hardware solutions using the popular Arduino board (Figure 4).

The plug-n-play kit features an Arduino host board and terminal emulator software that communicates with the PC computer USB interface. The evaluation board mounts on top of the Arduino UNO host board via the UNO R3 headers. The provided test programs allow users to quickly evaluate the functionality of the MRAM device.

Designers can look to MRAM for edge computing Designing edge computing devices using conventional memory technologies such as DRAM, SRAM, flash, and EEPROM requires a variety of trade-offs that can limit performance. For edge computing, designers can look to MRAMs that offer true random access, allowing both reads and writes to occur randomly in memory.

As shown in this article, MRAM supports the memory needs of edge computing designers including: a device that must store and retrieve data without incurring large latencies; low power consumption due to zero leakage when in standby; and the ability to endure 1,016 write cycles with a data retention capability of greater than 20 years at 85°C.

Further reading

Intelligent Security Systems using Edge-Computing