December 05, 2016, anysilicon
The Advanced Micro controller Bus Architecture (AMBA) bus protocols is a set of interconnect specifications from ARM that standardizes on chip communication mechanisms between various functional blocks (or IP) for building high performance SOC designs. These designs typically have one or more micro controllers or microprocessors along with several other components — internal memory or external memory bridge, DSP, DMA, accelerators and various other peripherals like USB, UART, PCIE, I2C etc — all integrated on a single chip. The primary motivation of AMBA protocols is to have a standard and efficient way to interconnecting these blocks with re-use across multiple designs.
The first step in learning AMBA protocols is to understand where exactly these different protocols are used , how these evolved and how all of them fit into a SOC design.Following diagram (reference from the AMBA 2.0 spec) illustrates a traditional AMBA based SOC design that uses the AHB (Advanced High performance) or ASB (Advanced System Bus) protocols for high bandwidth interconnect and an APB (Advanced Peripheral Bus) protocol for low bandwidth peripheral interconnects.
With increasing number of functional blocks (IP) integrating into SOC designs, the shared bus protocols (AHB/ASB) started hitting limitations sooner and in 2003 , the new revision of AMBA 3 introduced a point to point connectivity protocol — AXI (Advanced Extensible Interface). Further in 2010, an enhanced version was introduced — AXI 4. Following diagram illustrates this evolution of protocols along with the SOC design trends in industry.
Following diagram illustrates how an AXI interconnect can be used to build an SOC with various functional blocks talking through a master-slave protocol. The interconnect could be a custom crossbar or a switch design or even an off the shelve NOC (Network on Chip) IP that supports multiple AXI masters and slaves. The AXI interconnect helps in scaling up connectivity for number of agents compared to previous AHB/ASB bus. An AXI to APB bridge on one of the slave port is normally used to bridge communications to a set of peripherals shared on an APB bus.
Further evolution happened in the era of mobile and smartphones with SOCs having dual/quad/octa core processors with shared caches integrated and the need for hardware managed coherency across the memory subsystem. This lead to the introduction of ACE (AXI Coherency Protocol Extension) in AMBA revision 4.
Lastly, in the current era of heterogeneous computing for HPC and data center markets, the integration trend continues with increasing number of processor cores along with several heterogeneous computing elements like GPU, DSP, FPGAs, memory controllers and IO sub systems. In 2013, AMBA 5 introduced the CHI (Coherent Hub Interconnect) protocol as a re-design of the AXI/ACE protocol. The signal based AXI/ACE protocol was replaced with the new packet based CHI layered protocol that can scale very well for near term future.
Now that hopefully you understand how the protocols evolved and how each of them fit in to an SOC design— here are few basics and references to resources that you can use to learn more in depth about each of the protocol.
ARM has open sourced all of the protocols and all the specifications can be downloaded from the ARM website free by signing up.
Hope this gives an overview and helps getting started to learn more in depth into these protocols. The best way to learn further is to read the specifications to understand details of each protocol. The APB and AHB are relatively easy and can be learned easily. AXI and ACE/CHI are relatively complex and will need detailed reading along with understanding of basics of cache coherency and general communication protocols.
If you are not actively working on any of these on any projects, but do have time, you can also develop a IP/VIP for a master or slave for any of these protocols, simulate and understand the details in depth.
This is a guest post by Ramdas Mozhikunnath. For more related topics and questions on Verification/VLSI, do refer to Ramsas Quora profile – https://www.quora.com/profile/Ramdas-Mozhikunnath