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AMBA AXI: Advantages and Disadvantages

amba axi bus architecture soc interconnect

This page covers the advantages and disadvantages of the AMBA AXI bus. It highlights the benefits and drawbacks of AMBA AXI.

Introduction to AMBA

AMBA stands for Advanced Microcontroller Bus Architecture. It’s an open standard defining interconnection specifications for various modules on a System on a Chip (SoC). AMBA helps in designing SoCs correctly on the first attempt. It’s especially useful for board development that includes more than one microcontroller or microprocessor (like DSPs).

AMBA standards cover various bus specifications across different versions, including:

  • Advanced High-Performance Bus (AHB)
  • Advanced System Bus (ASB)
  • Advanced Peripheral Bus (APB)
  • Advanced Trace Bus (ATB)
  • AMBA Extensible Interface (AXI)

What is AMBA AXI?

AMBA AXI is designed for high performance, high frequency, and high-speed submicron interconnects. It’s suitable for designs that demand high bandwidth and low latency. It’s also backward compatible with previous interfaces like AHB and APB. AXI enables high-frequency operation without requiring complex bridges.

Here are the key features of the AMBA AXI protocol:

  • Separate address, control, and data phases
  • Supports unaligned data transfers
  • Uses only start-address-based burst transactions
  • Separate read and write data channels to enable low-cost DMA
  • Ability to issue multiple outstanding addresses
  • Out-of-order transaction completion
  • Easy addition of register stages to provide timing closure

AMBA AXI architecture

There are different versions of AXI interfaces, including AXI3, AXI4, and AXI-Lite, as defined in various standard specifications. As shown in Figure 1 (above), an AXI system consists of several master and slave devices connected using some form of interconnect.

Figure 2 (below) shows the timeline of AMBA standard evolution.

AMBA standard evolution timeline

AXI protocol defines interfaces between the following:

  • Between a master and interconnect
  • Between a slave and interconnect
  • Between a master and a slave

Most systems use one of these three interconnect methods:

  • Shared address and data buses
  • Shared address buses and multiple data buses
  • Multi-layer with multiple address and data buses

AXI specifications detail the channel architecture for read address, read data, write address, write data, and write response channels. Refer to the AMBA AXI protocol specifications for more in-depth information.

Benefits (Advantages) of AMBA AXI

Here are the benefits or advantages of AMBA AXI:

  • Offers higher throughput compared to AMBA AHB.
  • Provides higher performance than AMBA AHB and APB protocols.
  • Easier to integrate IP from different domains and develop proprietary IPs.
  • Widely adopted interface standard with support from many partners.
  • The AXI protocol supports three different configurations: “memory map/full” for OPBv46 replacement, “streaming” for data streaming, and “Lite” for simple systems.

Drawbacks (Disadvantages) of AMBA AXI

Here are the drawbacks or disadvantages of AMBA AXI:

  • Offers higher latency than AMBA AHB.
  • Consumes more power compared to AMBA AHB.
  • AXI is not a shared bus standard.
  • AMBA AXI4 has limitations regarding burst data and beats of information to be transferred.
  • Bursts must not cross a 4K boundary. Bursts longer than 16 beats are only supported for the INCR burst type.
  • Both WRAP and FIXED burst types remain constrained to a maximum burst length of 16 beats.

Also, refer to the advantages and disadvantages of the AMBA AHB bus and the difference between AMBA AXI and AHB bus types.