From SPI to xSPI: How NOR Flash Interfaces Are Evolving for Modern Embedded Systems

As embedded systems continue to evolve, the performance focus is no longer limited to processors alone. Memory access, once a secondary consideration, is increasingly becoming a critical factor in overall system responsiveness. In applications ranging from automotive electronics to AI and industrial control, the way code and data are accessed can directly shape real-time behavior and user experience.

At the center of this shift is NOR Flash, and the steady evolution of its interface from traditional SPI to today's high-bandwidth xSPI.

When Memory Access Becomes a System Bottleneck

In a human brain, the hippocampus is responsible for memory. In an electronic system, that role is played by memory devices. Flash memory stores firmware, boot code, and increasingly complex algorithms that must be accessed repeatedly during operation.

As embedded workloads grow more demanding, especially in AI-enabled and real-time systems, the latency of memory access begins to matter as much as raw compute capability. Algorithms need to be fetched quickly, control loops must respond instantly, and delays that were once acceptable can now limit system performance.

This shift has pushed memory interfaces into the spotlight.

Why SPI NOR Flash Became the Industry Default

Flash memory generally falls into two main categories based on internal architecture: NOR Flash and NAND Flash. While NAND Flash excels in density and cost efficiency for large data storage, NOR Flash has long been favored for code storage due to its fast random read performance, reliability, and deterministic behavior.

Early NOR Flash devices used parallel interfaces, but these designs use larger packages with 32 to 50+ pins dependent on supported address/data bus and made it difficult to maintain pinout/package compatibility across different capacities. As systems became more compact and flexible, serial interfaces with its low pin count, smallest package, and lowest overall system cost advantages emerged as a better solution.

SPI (Serial Peripheral Interface) gradually replaced parallel connections, becoming the mainstream interface for NOR Flash. Today, when engineers refer to NOR Flash in most embedded designs, they are typically referring to SPI NOR Flash.

The Limits of Traditional SPI Performance

The legacy SPI NOR solution uses 6-active pins and fits on an 8-pin package. That is a lot of pin and space savings on the PCB design compared to the 32 to 50+ parallel flash packages. Single SPI stores boot code and parameter data. From system power-up, the SPI NOR is used as a code shadowing tool, a technique where the System-on-Chip (SoC) fetch the boot code from the SPI NOR and transfer that code to RAM and performs the Execute-In-Place (XIP) on RAM.

For many years, single-channel SPI interfaces were sufficient. As performance requirements increased, dual-channel, quad-channel and Double Transfer Rate (DTR) SPI NOR memories were introduced to improve data throughput and come close to the XIP performance of parallel interfaces. These added features enabled XIP code execution directly in the SPI NOR flash.

These steps further accelerated the transition from parallel NOR to SPI NOR-based designs, but they did not eliminate a fundamental limitation: interface bandwidth was becoming a bottleneck. Driving higher clock frequencies beyond 166MHz STR and 80-90MHz DTR became a challenge on traditional SPI architectures and became a struggle to keep up with modern system demands.

In applications requiring instant-on, frequent code execution or rapid algorithm access, memory read latency increasingly constrained overall system responsiveness.

xSPI and the Next Step in Interface Evolution

To address these challenges, the SPI interface continued to evolve. Building on the SPI NOR foundation, a new innovative faster interface must be developed. The new interface adds 4 more Input/Output lines totaling the Quad IO lines to a byte-wide Octal bus (x8) and overcome the SPI NOR frequency barriers by adding a support Data Strobe (DQS) pin in aid of valid data transfers per clock edges. This interface emerges and it was named xSPI.  At the maximum 200MHz DTR frequency, it significantly increases bandwidth support up to 400Mbytes/s data throughput transfer which is 4-5 times faster than existing SPI NOR solutions.

This was not a minor incremental update. xSPI represents a structural shift in how SPI NOR Flash interfaces deliver performance, enabling data throughput levels that were previously unattainable with conventional SPI designs.

What Higher-Bandwidth Interfaces Change at the System Level

The impact of xSPI is best understood at the system level rather than through raw interface specifications.

Consider an MPU-based AI application. During operation, the processor frequently retrieves code and algorithms from external Flash memory while comparing incoming data to stored models. With older Flash interfaces, memory access latency could introduce delays that affected responsiveness.

With high-performance xSPI interfaces, code and data can be accessed more quickly and more consistently, helping systems maintain real-time behavior even as workloads grow more complex. In this sense, the difference between traditional quad-SPI and xSPI can be compared to the shift from 3G to 5G networks: both work, but only one enables seamless, low-latency performance at scale.

As embedded systems continue to scale in performance and complexity, interface bandwidth and latency are no longer optional optimizations. They are foundational requirements.

By delivering higher throughput, improved efficiency, and better support for real-time operation, xSPI is increasingly becoming the baseline interface for high-performance SPI NOR Flash. Across automotive, AI, IoT, and industrial applications, this evolution reflects a broader industry trend toward faster, more responsive memory architectures.

FAQ

What is xSPI?

SPI NOR Flash is a type of non-volatile memory commonly used for code and firmware storage, accessed via the Serial Peripheral Interface (SPI) in embedded systems.

xSPI is a high-performance evolution of the SPI interface that increases data bandwidth by using more data lines and supporting double data rate operation. It is designed to meet the growing performance demands of modern SPI NOR Flash applications.

How did SPI NOR Flash interface performance evolve over time?

SPI NOR Flash interfaces have evolved through several stages. Early single-channel SPI interfaces around 2000 delivered about 2.5 MB/s. Dual-channel SPI introduced around 2004 increased throughput to approximately 12.5 MB/s, followed by Quad SPI around 2009 at roughly 52 MB/s. Further efficiency improvements came with DTR operation on slower frequencies around 2010, matching the throughput of fastest STR Quad SPI NOR products.  

The next generations of products improved the STR frequency from 104MHz to 166MHz and DTR frequency from 54MHz to ~90MHz, enabling Quad SPI throughput of about 83MB/s to 90 MB/s. By around 2015, xSPI combined wider data paths with DTR to reach throughput levels of approximately 400 MB/s in high-performance configurations.

How is xSPI different from traditional Quad SPI?

Compared with Quad SPI, xSPI offers higher throughput by expanding parallel data transfer and improving interface efficiency, allowing significantly faster memory access without relying solely on higher clock speeds.

Why does xSPI matter for real-time and AI systems?

In real-time and AI-driven applications, code and algorithm access latency directly affects responsiveness. xSPI reduces read latency and helps systems maintain deterministic behavior as workloads increase.Is xSPI becoming the mainstream interface for SPI NOR Flash?
As performance requirements rise across embedded systems, xSPI is increasingly adopted as the preferred interface for high-performance SPI NOR Flash designs.