Significant effort is involved in developing coherent NoCs from scratch.

In the selection and qualification process for semiconductor IP, design teams often consider the cost of in-house development. Network-on-Chip (NoC) IP is no different. In “When Does My SoC Design Need A NoC?” Michael Frank and I argued that most of today’s designs – even less complex ones – can benefit from NoCs. In the blog “Balancing Memory And Coherence: Navigating Modern Chip Architectures,” I discussed the complexity that coherency adds to on-chip interconnect. After I described some of the steps of NoC development based on what ChatGPT 3.5 recommended in “Shortening Network-On-Chip Development Schedules Using Physical Awareness,” it’s time to look at more detail at the development efforts that design teams would have to invest to develop coherent NoCs from scratch.

ChatGPT, here we go again!

The prompt “Tell me how to develop an optimized network-on-chip for semiconductor design, considering the aspects of cache coherency” gives an excellent starting point in ChatGPT 4.0.

Understanding Protocols: First, one needs to understand cache coherency protocols. The recommendation is to study existing protocols before selecting one. Specifically, understand existing cache coherency protocols like MESI (Modified, Exclusive, Shared, Invalid), MOESI (Modified, Owned, Exclusive, Shared, Invalid), and directory-based protocols. Analyze their strengths and weaknesses in terms of scalability, latency, and bandwidth requirements. Then, choose a protocol that aligns with your performance goals and the scale of your NoC. Directory-based protocols are often preferred for larger-scale systems due to their scalability.

ChatGPT’s recommendation for the first step is a good start. I previously discussed the complexity of specific protocols like AMBA AXI, APB, ACE, CHI, OCP, CXL, and TileLink in “Design Complexity In The Golden Age Of Semiconductors.” One must read several thousand pages of documentation to understand the options here. And – by the way – these are orthogonal to the MESI/MOESI commentary from ChatGPT above, as these are implementation choices. In a practical scenario, many of these aspects depend on the building blocks the design team wants to license, like processors from the Arm, RISC-V, Arc, Tensilica, CEVA, and other ecosystems, as well as the protocol support in design IP blocks (think PCIe, UCIe, LPDDR) and accelerators for AI/ML.

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