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Non-volatile memory is an important component in a wide range of high-performance embedded applications. Especially, many consumer, industrial, and medical applications need increased re-writability to support both more frequent code updates as well as increased data logging.

These applications require greater memory density to store either a substantially larger code footprint and/or more extensive data logs. Moreover, developers need to be able to improve power efficiency while lowering system cost.

Today, there are numerous non-volatile memory technologies available to developers, including EEPROM, NOR flash, NAND flash, MRAM, and FRAM. Each has its own distinct advantages for specific applications. However, the combination of 1) manufacturing process technologies continuing to scale smaller, 2) the need for higher densities at lower power, and 3) re-writability becoming increasing important has led to increased interest in RRAM for these applications.

This article will explore RRAM technology and how it provides developers with a new approach to meeting the changing memory requirements of high-performance embedded systems.

Memory in high-performance embedded systems

Emerging connected systems face a number of tough design challenges. For instance, medical devices—such as hearing aids, continuous glucose monitors (CGMs), and patches—must fit into a smaller form factor despite increasing data and event logging requirements necessary to enable remote monitoring and compliance with industry standards.

Next, smart equipment in Industry 4.0 systems require significantly greater code storage to facilitate functionality like remote sensing, edge processing, and firmware over-the-air (FOTA) updates for remote maintenance. Furthermore, the addition of artificial intelligence (AI) at the edge in wearables and Internet of Things (IoT) devices is driving the need for high-performance, energy-efficient non-volatile memory in smaller form factors.

The increased code size and data logging requirements of such systems exceeds the embedded non-volatile memory capabilities of microcontrollers. External memory is needed to match increasing density and performance requirements. However, code and data often need varying capabilities depending upon performance, density, endurance, and data-write size.

Thus, multiple non-volatile memories may have to be used, such as NOR flash for data logging and high-density EEPROM for code storage. This can lead to systems that use several types of external memory, increasing system cost, complexity, and energy consumption.

Ideally, systems can use a single memory type that supports both external code and data storage without compromising performance or functionality for either. An emerging non-volatile technology to fill this gap as a standalone external memory is RRAM.

Resistive RAM

Resistive RAM (RRAM) is a non-volatile random-access memory that was made available commercially in the early 2000s. It operates by changing the resistance of a switching material sandwiched between two electrodes, show on left in Figure 1.

Figure 1 Typical RRAM memory cell consists of one transistor and one resistor (left), and the memory state is altered by applying an external bias across the metal electrodes (right). Source: Infineon

The switching material can be metal oxide or a conductive bridging switching media. A typical RRAM memory cell consists of one transistor and one resistor pair (1T1R) where the resistance of the RRAM can be altered with an external bias applied across the metal electrodes, shown on the right side of Figure 1.

Initially, RRAM was developed as a potential replacement for flash memory. At the time, the cost and performance benefits of RRAM weren’t enough to supersede the advantages of other non-volatile memory technologies, especially as an external memory. However, in recent years, several factors have changed to make RRAM a compelling non-volatile alternative.

Specifically, as embedded systems become more integrated and implemented in smaller manufacturing process nodes with substantially larger code and data storage requirements, the following advantages of RRAM for external memory overtake traditional non-volatile options:

1. Scalability

Some non-volatile memory technologies are limited in their ability to scale, translating to limitations in overall memory density due to footprint, power, and cost. A major advantage of RRAM is that it can be manufactured in a compatible CMOS process, enabling it to scale to process nodes below 45 nm and even down as low as 10 nm.

For example, the memory industry has had difficulty cost-effectively scaling NOR flash memory as the technology seems to be physically limited to between 35 and 40 nm. Scalability has a direct impact on performance, density, footprint, and energy efficiency.

2. Direct write

Data storage for a NOR flash memory requires two operations: an erase operation to clear the target address followed by a write operation. The “direct write” functionality of RRAM eliminates the need to first erase memory. Thus, only a write operation is required to store data. Figure 2 shows the operations required for writing to both NOR flash and RRAM.

Figure 2 NOR flash requires an erase operation before every write operation, increasing write time, energy consumption, and wear on memory cells. RRAM’s ability to direct write speeds write operations, conserves energy, and extends cell endurance. Source: Infineon

This leads to much faster large-scale write operations for RRAM, such as during FOTA updates.

3. Byte re-writeable

Some non-volatile memories perform writes based on page size. For example, NOR flash page size is typically either 256 or 512 bytes. This means every write impacts the entire page. To change one byte, the page must be read and stored in a temporary buffer; the change is made to the temporary duplicate.

The flash must then erase the page and write the entire page back in from the buffer. This process is time-consuming and wears the flash (typically 100k+ writes). In addition, data cells that are not changed are worn unnecessarily. Consequently, data logging with NOR flash requires that data is cached and then written in page-sized chunks, adding complexity and potential data loss during a power event.

In contrast, RRAM write size is much smaller (few bytes) with higher endurance than NOR flash. This is more manageable and accommodates data logging requirements well since cells are worn only when written to. Thus, RRAM is robust and efficient for both code storage and data logging in the same memory device.

4. Energy efficiency

Through optimizations such as byte re-writability and eliminating erase operations during data writes, RRAM achieves better energy efficiency, up to 5x lower write energy and up to 8x lower read energy compared to traditional NOR flash.

5. Radiation tolerance and electromagnetic immunity

RRAM technology is inherently tolerant to radiation and electromagnetic interference (EMI). This makes RRAM an excellent choice for those applications where environmental robustness is essential.

Consolidate code storage and data logging

RRAM is a proven technology whose time has come. It’s an established technology that has been in embedded form in chips for over a decade as an internal non-volatile memory. With its ability to scale to smaller process nodes, provide higher endurance and re-writability at low power, and minimize write time and power consumption through direct write functionality, RRAM delivers high performance without compromising robustness or efficiency (Table 1).

Table 1 The above data shows a comparison between RRAM and other non-volatile memory technologies. Source: Infineon

RRAM is an ideal memory for consolidating both code storage and data logging in a single external memory to simplify design and reduce system complexity, making RRAM a compelling alternative to traditional non-volatile memories for many consumer, industrial, and medical applications.

Bobby John is senior product marketing manager for memory solutions at Infineon Technologies.

Related Content

• Resistive RAM Memory is Finally Here
• RRAM set to follow 3-D flash, says IMEC
• RRAM: A New Approach to Embedded Memory
• RRAM Startup Raises £7M to Support Data-Hungry Applications
• Monolithic embedded RRAM presents challenges, opportunities

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