SoC NoCs: Homegrown or Commercial Off-the-Shelf?

The developers of today’s system-on-chip (SoC) devices face a myriad of decisions. Some of the early choices start when defining the overall architecture of the device. Next comes the determination of which intellectual property (IP) functional blocks to be used and their origin.

Many IPs are purchased from trusted third-party vendors, such as processors from Arm and DDR memory controllers from Cadence or Synopsys, for example. Meanwhile, the secret-sauce IPs that will differentiate this SoC from its competitors are typically developed in-house. These IPs include specialized hardware accelerators, such as artificial intelligence, machine learning, and deep learning cores.

Depiction of a system-on-chip with multiple NoCs, or network-on-chip connections.

Interconnect: The obvious choice

The next decision is how to connect all the IPs together. Paradoxically, the increasing complexity of modern SoCs simplifies this choice significantly. In the not-so-distant past, designers used a bus-based interconnect. The next step was to employ a crossbar switch. These days, the only realistic option is to use a network-on-chip (NoC) for anything but the simplest SoCs. And, even having said this, many designers employ NoCs in even the simplest SoCs because of all the advantages they convey.

Coherent, non-coherent, or both?

Processors typically come in clusters with integrated caches. A typical configuration uses four to eight cores in which each core has a dedicated L1 cache, with all cores in the cluster sharing an L2 cache.

Maintaining coherency, which means ensuring that each core works with the most current version of the data, can be implemented using hardware or software mechanisms. Hardware cache coherency provided by a coherent NoC can take almost the same die area as a CPU, but it is vastly more efficient than software cache coherency, which consumes ~25% of CPU cycles from every CPU in the cluster. Hardware-based coherency is vastly more efficient.

A processor cluster of the form described above is internally cache coherent. If only a single cluster is used, and if none of the other IPs have caches that require coherency, then a non-coherent NoC may be employed. Alternatively, if there are multiple processor clusters, which is increasingly the case, or if any of the accelerator IPs need to remain cache coherent, then a coherent NoC will be required.

These scenarios usually require an L3 cache to maintain system performance. As overheads are associated with using a coherent NoC, SoC developers opt to employ both cache-coherent and non-coherent NoCs talking to each other on the same device.

A commercial off-the-shelf NoC can transform IP blocks into optimized SoCs quickly and efficiently.

Homegrown or Commercial Off-the-Shelf?

One final decision is whether to spend time and resources developing a homegrown, also known as do-it-yourself, NoC or purchasing a commercial, off-the-shelf NoC.

Choosing between buying a commercial NoC solution vs. designing and implementing one in-house involves several considerations, which can be summarized as follows:

1. Expertise and experience: Commercial vendors specializing in NoC solutions have extensive experience designing networks optimized for various performance metrics, including power, area, and speed. This expertise can translate into a more efficient and reliable NoC than what might be achievable in-house without significant investment in research and development.

2. Reduced development time: Developing a NoC in-house requires significant time for research, design, implementation, and testing. Buying a commercial solution can significantly reduce the time to market. This is crucial in industries in which getting — and staying — ahead of competitors is key.

3. Support and documentation: Commercial solutions often come with comprehensive support and documentation, making it easier for the SoC integration team to implement the NoC into their system. This support can also be crucial for troubleshooting and future upgrades.

4. Cost efficiency: While there is an upfront cost to buying a commercial solution, the overall project cost will be significantly lower when considering the full lifecycle of the product. Designing a NoC in-house requires investment in specialized personnel, tools, and potentially prolonged development cycles, all of which add to the cost.

5. Proven reliability and performance: Commercial NoCs are tested across a wide range of applications and conditions, ensuring they meet standards of reliability and performance. Adopting a proven solution can reduce the risk of design flaws and performance issues that might not be apparent until late in the development process. 

6. Focus on core competencies: By buying a commercial NoC, a company can focus its precious resources on its core competencies and the unique aspects of its product that differentiate it in the market. This strategic allocation of resources can be much more beneficial than diverting significant effort into designing a component that doesn't offer a competitive advantage.

7. Scalability and flexibility: Commercial NoCs are engineered with scalability and flexibility to accommodate a wide range of applications, protocols, and configurations that include different topologies. This flexibility allows for easier updates and scalability of the SoC design without the need for extensive redesign.

8. Access to advanced technologies: NoC vendors often incorporate the latest technologies and methodologies into their NoC IP and development tools, offering advanced features such as higher bandwidths, lower latencies, and better energy efficiency. Access to these technologies without the need for in-house development can offer a significant advantage.

 The choice Is clear

Based on the points enumerated above, it is evident that employing a commercial, off-the-shelf NoC, whether it is coherent, non-coherent, or a combination of both, offers tremendous advantages. These benefits encompass reduced development time, expertise, support, cost efficiency, proven reliability, and the ability to focus on core competencies.

The utilization of commercial NoCs, such as the Ncore cache coherent NoC or FlexNoC non-coherent offerings from Arteris aligns seamlessly with companies' strategic goals and capabilities. This tactical choice enhances efficiency and ensures a competitive edge in today's rapidly evolving technological landscape.