The AMBA CHI (Coherent Hub Interface) specification defines the interfaces for the connection of fully coherent processors. For example, the Cortex-A76 and Cortex-A65, and dynamic memory controllers, such as the CoreLink DMC-620, to high performance, non-blocking interconnects, such as the CoreLink CMN-600. It is appropriate for a wide range of applications that require coherency, including mobile, networking, automotive, and data centers.
The AMBA CHI specification separates the protocol and transport layers to allow differing implementations to provide the optimal trade-off between performance, power and area. This separation allows interconnect designs ranging from an efficient, small cross-bar to high performance, large scale mesh network.
AMBA CHI has been architected to maintain performance, as the number of components and quantity of traffic rises. This includes placing additional requirements on masters to respond to coherent snoop transactions, which means forward progress for particular masters can be more easily guaranteed in a congested system. The separation of the identification mechanism into master identifiers and transaction identifiers allows the interconnect to be constructed in a more efficient manner.
The protocol also provides a Quality of Service (QoS) mechanism to control how resources in the system, shared by many processors, are allocated without needing a detailed understanding of every component and how they might interact.
Some of the key features include:
- Support for high frequency, non-blocking coherent data transfer between many processors.
- A layered model to allow separation of communication and transport protocols for flexible topologies, such as a cross-bar, ring, mesh or ad hoc.
- Cache stashing to allow accelerators or IO devices to stash critical data within a CPU cache for low latency access.
- Far atomic operations enable the interconnect to perform high-frequency updates to shared data.
- End-to-end data protection and poisoning signaling.
Issue D of the AMBA CHI specification introduces the AMBA interface parity extension for use in applications such as automotive, which have resilience or functional safety requirements. CPUs, such as the Cortex-A78AE, include this feature and are designed to undertake complex and demanding safety-critical tasks with varying workloads.
Issue E of the AMBA CHI specification adds new optional features, such as Memory Tagging (MTE), multiple interfaces, replicated channels, and a series of new transactions types and optimizations.
AMBA CHI Specification