This article explains how to select memory solutions that maintain the performance of parallel SRAM while enabling PCB miniaturization and eliminating the need for battery backup. It also introduces the features of alternative technologies, including QSPI-compatible FeRAM (FRAM, ferroelectric RAM), along with key evaluation points for implementation.
Challenges and Design Constraints of Parallel SRAM
Parallel SRAM offers high-speed access and has been widely used in applications requiring real-time performance. However, despite these advantages, it comes with several constraints, including large mounting area, high power consumption, and the need for an external battery to retain data. These limitations can become significant obstacles in modern designs that demand compact size and high reliability.
Basic Structure and Operating Principle of Parallel SRAM
Parallel SRAM is a volatile memory that connects data and address buses in parallel, allowing immediate access to any address. Each bit is typically composed of around six transistors, enabling stable read/write performance without dependence on a clock signal. For this reason, it is widely used in industrial and medical equipment that require real-time processing. However, due to this structure, it has low integration density, making it difficult to scale capacity per chip, and it also cannot retain data when power is lost due to its volatile nature.
Power Consumption and Board Space Issues
Another challenge of parallel SRAM is the increase in implementation area caused by the need for wide data and address buses along with multiple control signals. This results in a large footprint on the PCB, hindering overall system miniaturization. In addition, it often operates continuously, leading to relatively high power consumption in both standby and active modes. This becomes a design bottleneck, especially in battery-powered devices, increasing the need for more power-efficient and compact alternative technologies.
Reliability Risks of Battery Backup
A common method for maintaining data in parallel SRAM is external battery backup, but this approach presents several issues. Batteries are prone to degradation over time and under high-temperature conditions, raising concerns about long-term reliability. They also require maintenance and replacement, increasing overall system costs, particularly for field-deployed equipment. Furthermore, the use of batteries can introduce restrictions in transportation and product certification. As a result, there is growing demand for battery-free data retention solutions.
Key Considerations for Memory Selection Toward Miniaturization and High Reliability
In recent embedded systems, reducing PCB size and improving reliability have become critical requirements. As a result, selected memory solutions are expected to offer additional functionality and ensure data safety during power loss. In other words, the need to meet design requirements that cannot be addressed by conventional battery-backed configurations has accelerated the adoption of alternative technologies.
Optimization Factors in Circuit Design and PCB Area
In PCB miniaturization, the mounting area of memory is a critical factor in circuit design. Parallel SRAM, with its wide bus width, imposes significant layout constraints due to the large number of pins and routing complexity. In contrast, memory with a serial interface can achieve equivalent performance while minimizing wiring. This helps reduce component count and layer count, ultimately contributing to a smaller, lighter PCB and lower manufacturing costs.
Approach to Power Loss Protection and Data Retention
Alongside miniaturization and low power design, ensuring data retention during power loss is a key requirement for high-reliability systems. Traditionally, this has been addressed using SRAM with battery backup. However, as mentioned earlier, this approach presents challenges in terms of maintenance, reliability, and safety. In recent years, non-volatile memory solutions have gained attention for their ability to retain data even after power is cut, without requiring additional hardware or power supply. Such designs allow safe data storage even during sudden power interruptions, significantly improving system robustness.
Design Approaches to Improve Maintainability and Safety
As product lifecycles become longer and maintenance frequency needs to be minimized, designs must prioritize maintainability and safety. Battery-based designs require periodic replacement, increasing maintenance burden. Furthermore, risks such as fire or leakage associated with batteries cannot be ignored. In contrast, designs using non-volatile memory can avoid these risks while enabling stable long-term operation. For these reasons, design approaches that balance reliability and maintainability are becoming increasingly mainstream.
Evaluation of Alternative Memory Technologies That Meet Requirements
When considering alternatives to parallel SRAM, it is necessary to meet not only high-speed read/write performance but also multiple requirements such as space efficiency, non-volatility, and low power consumption. Several non-volatile memory technologies are candidates to address these needs.
Representative Non-Volatile Memories and Comparison: FeRAM, MRAM, nvSRAM
There are various types of non-volatile memory, each with different characteristics. For example, FeRAM (FRAM, ferroelectric RAM) offers high-speed writing, excellent endurance, and low power consumption. MRAM uses magnetic properties and provides high speed and durability, but it is more complex to manufacture and tends to be costly. nvSRAM combines SRAM with non-volatile memory internally, enabling data retention during power loss while maintaining SRAM-level speed; however, its structure is more complex and power consumption tends to be higher. It is important to compare these options based on specific design requirements.
Potential of High-Speed, Low-Wiring Access with Quad SPI FeRAM
One technology gaining attention is non-volatile memory equipped with a Quad SPI (QSPI) interface. Compared to conventional SPI, QSPI enables the transfer of 4 bits of data simultaneously, achieving high-speed read/write performance while reducing pin count. In particular, QSPI-compatible FeRAM can deliver data transfer speeds comparable to parallel SRAM while also providing non-volatile data retention during power loss. This configuration contributes to simplified wiring, reduced component count, and smaller PCB size, making it a highly effective option for compact embedded systems and IoT devices.
Examples of FeRAM Adoption in Compact and Low-Power Designs
FeRAM is increasingly being adopted in real-world embedded system designs. For example, in battery-powered industrial sensor nodes, minimizing power consumption is critical. Traditionally, low-power SRAM combined with external batteries was used, but replacing it with QSPI-compatible FeRAM eliminates the need for batteries while improving reliability and maintainability. In safety-critical fields such as medical devices, FeRAM is also being adopted due to its ability to retain data even during unexpected power loss. These examples demonstrate that FeRAM is the ideal solution for achieving both miniaturization and high reliability.
Toward Optimal Memory Selection
Choosing a memory is not simply a matter of replacing components; it is a critical decision that directly impacts overall system performance, reliability, and lifecycle. In order to meet modern requirements such as space reduction and battery-free operation, it is necessary to evaluate options from multiple perspectives, including performance, implementation, and cost.
Building a Selection Process to Meet Technical Requirements
Memory selection in design begins with clearly defining the required specifications of the target system, such as read/write speed, capacity, power consumption, and reliability. Then, candidate memories that meet these requirements are compared in terms of their functional and physical characteristics. When aiming to maintain performance equivalent to parallel SRAM while achieving miniaturization and battery-free operation, introducing non-volatile memory with high-speed access becomes a strong option. It is also important to evaluate system-level factors, such as interface choices like Quad SPI, as they directly impact overall system architecture and reliability.
Evaluation Points and Considerations for FeRAM and Other Options
When selecting non-volatile memory including FeRAM, it is important to consider not only basic performance metrics such as read/write speed, endurance, power consumption, and mounting area, but also compatibility with the operating environment and signal interfaces. QSPI-compatible FeRAM, in particular, enables high-speed access despite using a serial interface, achieving performance comparable to parallel SRAM. This allows for simplified board design through reduced wiring and pin count, while also providing data retention without battery backup. However, compared to conventional SRAM, there may be limitations in capacity and cost, so a comprehensive evaluation is required, including compatibility with existing design assets and system-wide impact. Rather than relying solely on specifications, it is essential to consider total cost, development effort, and maintainability from a practical engineering perspective.
Practical Approach to Implementation in Development Environments
To introduce new memory technologies, it is important to consider not only performance but also compatibility with existing design environments and internal evaluation processes. For relatively new technologies such as FeRAM, practical factors such as availability of evaluation samples, documentation readiness, and compatibility with development tools play a key role in successful adoption. In addition, stable supply and vendor support are essential for long-term production planning. In practice, it is recommended to begin with small-scale prototyping to verify feasibility, followed by gradual adoption. Ultimately, success depends not only on theoretical optimal solutions but also on practical feasibility in implementation and operation.
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