VIP for AMBA CXS: Early Adopter Success

VIP Expert

Apr 20, 2020 / 2 min read

Synopsys offers a broad set of verification solutions for next-generation AMBA® protocols, including AMBA CXS. Synopsys also has verification automation solutions for Arm-based protocols including VC AutoTestbench for testbench generation and VC AutoPerformance for performance verification.

The announcement of the Verification IP for the AMBA CXS protocol underscores the collaborative nature of AMBA which enables the ecosystem to deliver AMBA-based multi-chip solutions well-positioned for next-generation applications. Synopsys Verification IP for the AMBA CXS specification enabled early customers and partners to verify their next-generation chip to chip coherency design applications for increased performance.

Let’s do a deep dive to better understand the features available in the AMBA CXS protocol.

What is CXS?

CXS is a streaming protocol, designed for transmitting packets with high bandwidth. It is typically used to transfer packets between the higher-level protocol layers (for example CCIX, CXL), and the transport layer (for example PCIe controller). The CXS interface has a Transmit interface and Receive interface. It is a full-duplex interface with independent Transmit & Receive signaling paths. The data transmitted over one clock cycle is called as ‘flit’ (flow control unit). Multiple packets can be transmitted in a single flit, or, packets can span across flits.


AMBA CXS VIP architecture diagram

Benefits of CXS

Francisco Socal, Senior Product Manager, Architecture and Technology Group at Arm says, “Emerging applications like machine learning, networking off-load and in-memory databases increasingly rely on acceleration and multi-chip solutions with high-speed and coherent chip-to-chip interconnects. They require a high-performance and low friction integration path between multiple on-chip and off-chip protocols, which CXS provides.” 

CXS defines a generic streaming interface that can be used across a wide range of applications. The CXS protocol makes efficient use of a wide interface and enables transmitting multiple packets on a single clock cycle. CXS streaming protocol is transparent to the packets being transmitted. For example, the packets being transmitted could be CCIX packets, CXL.mem/Cache transactions, etc.

Key Verification Challenges and CXS VIP Solution

The verification requirements for CXS interface:

  • The correctness of packet placement at the appropriate start and end boundaries within flit
  • The correctness of start and end pointers for the packet in the CXS control signals
  • The ability of DUT to transmit and accept uninterrupted data in continuous data mode
  • The ability of DUT to support CXS interface activation and deactivation
  • Credit grant and credit return mechanisms

CXS VIP provides the following key features to meet the DUT verification requirements:

  • Protocol features to support behavior as defined by CXS specification
  • Protocol checks for checking the protocol correctness as per the CXS specification
  • Analysis ports and Callbacks for testbench to access CXS VIP transaction objects
  • Debug port for credit information
AMBA CXS VIP integration diagram

Figure 2:Debug port (TX_AVAILABLE_CREDITS) for credit information


The rapid evolution in AI & encryption has given birth to chip protocols, which use high-speed transport interfaces (like PCIe). CXS interface supports the transmission of protocol packets at high bandwidth between the protocol layer and the transport layer of these high-speed applications.

Synopsys collaborates on the AMBA protocols, as well as the evolution of the high-speed chip–to–chip protocols, in which CXS is expected to play a significant role. Verification helps customers be confident that their product is correct and compliant with the CXS specification.

The AMBA CXS specification is an open specification available for download here:

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