AMBA C2C: Moving forward with a converged AMBA foundation for C2C connectivity

In this blog post, we introduce AMBA® C2C, a new specification being developed with the AMBA ecosystem. It provides a unified and scalable approach to connecting AMBA protocols across chiplets, including AMBA AXI and AMBA CHI.

The evolving C2C landscape

Chiplet technology and AI are evolving rapidly, bringing new benefits and protocol requirements.

AMBA CHI C2C has established strong momentum as a foundational protocol for chiplets and chip-to-chip (C2C) connectivity. It is particularly suited to fully coherent, advanced heterogeneous compute and superchips.

However, chiplets span a much wider range of use cases. These include memory expansion, IO and accelerator attach, and advanced heterogeneous systems. Each use case comes with different requirements and trade-offs in functionality, performance, bandwidth efficiency, and implementation complexity.

AMBA CHI C2C already supports scalability in implementation complexity through a property negotiation mechanism. Implementations can support only the capabilities they need while preserving interoperability across chiplet types, vendors, and generations.

Our continued collaboration with the ecosystem has highlighted an important additional requirement. Many chiplets do not need the full feature set of advanced coherent systems and require an even simpler approach for C2C connectivity. This is particularly important for the vast installed base of AXI-based designs. These designs can benefit from the greater reuse and composability that chiplets enable.

This has been an important focus area since we first introduced AMBA CHI C2C in 2023. At that time, we discussed how C2C connectivity could extend to simpler, often AXI-based use cases. Partner feedback since then has reinforced the need for a unified and converged C2C approach for AMBA protocols. This approach includes both CHI and AXI, rather than separate, protocol-specific point solutions.

Introducing AMBA C2C

To address these evolving requirements, we are developing AMBA C2C.

AMBA C2C builds on the success of AMBA CHI C2C. It provides a converged and scalable C2C foundation across AMBA protocols, including AXI and CHI.

At the heart of AMBA C2C is protocol subsetting. Defined subsets enable the protocol feature set and complexity to scale across different use cases. For example, device or accelerator attach subsets can support the simpler requirements aligned with AXI-based IO and non-coherent traffic. More advanced subsets can support the richer functionality required by coherent and heterogeneous systems.

AMBA C2C uses defined protocol subsets to scale functionality and implementation complexity across different use cases. These subsets can align with on-chip protocols such as AXI and CHI. AMBA C2C then defines how these protocols are packetized for chip-to-chip transport. The packetization is optimized for each subset while providing a common approach across AXI- and CHI-class traffic.

Interoperability remains fundamental to the architecture. AMBA C2C builds on the property negotiation mechanism introduced with AMBA CHI C2C. This mechanism enables implementations with different capabilities to interoperate across chiplet types, vendors, and generations. This includes backwards and forwards compatibility.

A layered architecture for an evolving industry

An open chiplet ecosystem requires open standards and a clear, layered standardization approach. This includes a clear separation between the protocol and the underlying transport.

This is a proven industry model that UCIe and the broader ecosystem have adopted. AMBA CHI C2C, for example, uses UCIe Streaming as discussed in the UCIe 1.1 specification announcement.

Similarly, PCIe and CXL protocols can be transported over UCIe. More recently, UALink has adopted a similar layered architecture. It defines a packetization strategy alongside its protocol functionality while separating it from the underlying data-link and physical layers.

AMBA C2C follows this same principle. AMBA defines the protocol and packetization mechanism, while transports such as UCIe provide the underlying die-to-die connectivity. This enables AMBA C2C to work with a variety of die-to-die and chip-to-chip transports. More importantly, it provides a clean architectural boundary that enables AMBA protocols and underlying transport technologies to continue evolving at pace.

Transporting AMBA protocols over UCIe with AMBA C2C

For AMBA AXI, this separation is particularly important. AXI is widely adopted across the industry and continues to evolve. However, unlike packet-based protocols, AXI defines interface behavior rather than a native packet format. AXI is also highly configurable, and capabilities vary between implementations. Transporting AXI over a C2C link therefore requires an architectural definition. This definition specifies which AXI semantics are carried and how they are packetized. That definition must evolve alongside AXI itself.

Putting it all together

AMBA C2C is the next evolution of AMBA’s C2C work. It builds on AMBA CHI C2C and extends the approach to a broader range of AMBA-based chiplet systems.

AMBA C2C provides a common foundation that can scale from simple AXI-based designs to advanced, CHI-based fully coherent heterogeneous systems. Interoperability is built into the architecture.

AMBA C2C follows the development and licensing model that has supported the AMBA ecosystem for decades. We are developing it in collaboration with industry partners. The specification will be freely available, royalty-free, and architecture-neutral. Over decades, this model has helped establish AMBA as a dependable and trusted industry foundation. It has supported widespread adoption, a thriving ecosystem, and a broad marketplace of compatible solutions.

We look forward to continuing our collaboration with the industry to enable an open, interoperable chiplet ecosystem, with AMBA C2C evolving alongside complementary industry standards.