Understanding AXI-Stream Interfaces for DSP Data Pipelines

As FPGA-based DSP systems grow increasingly complex, efficient communication between different processing modules becomes a key design consideration. The Advanced eXtensible Interface (AXI), part of the ARM AMBA protocol family, provides a standardized high-performance interconnect for FPGA SoC platforms.

This blog explains the types of AXI interfaces, explores AXI write and read channel architectures, and discusses how AXI-Stream supports modular and scalable DSP pipelines. It also highlights the practical applications of AXI interfaces in radar, communications, and high-speed data systems.

AXI Interface Types

The AXI protocol supports several interface types, each designed for a specific purpose within FPGA SoC systems:

1. AXI4 (Memory-Mapped)
   Designed for high-performance, burst-based data transfers between masters (e.g., processors, DMA engines) and slaves (e.g., memory, peripherals).
2. AXI4-Lite
   A lightweight, memory-mapped interface used for simple register-level communication and control of peripherals.
3. AXI4-Stream
   A unidirectional, high-speed streaming interface ideal for continuous data transfer between processing blocks such as filters, FFT engines, and data converters.

These interface types enable designers to build scalable systems that efficiently manage control, data movement, and high-speed streaming.

AXI Write Channel Architecture

The AXI4 memory-mapped interface uses five independent channels. The write channel handles data transfers from a main to a subordinate. It includes three sub-channels:

• Write Address Channel: Specifies the target address for the data to be written.
• Write Data Channel: Transfers the actual data payload.
• Write Response Channel: Provides feedback on the success or failure of the write operation.

Figure 1: AXI4 write channel architecture diagram

Each sub-channel uses a handshake protocol with VALID and READY signals to ensure reliable communication. This separation of address, data, and response enables high-throughput and efficient pipelining in data transfers.

AXI Read Channel Architecture

The read channel manages data transfers from a subordinate to a main. It includes two sub-channels:

• Read Address Channel: Sends the target address of the data to be read.
• Read Data Channel: Returns the requested data along with a response signal.
  
  

Figure 2: AXI4 read channel architecture diagram

Similar to the write channel, the handshake protocol between VALID and READY ensures proper synchronization and prevents data loss.

AXI-Stream for DSP Pipelines

AXI-Stream provides a lightweight, unidirectional data path for continuous high-speed sample transfer. Its use in DSP pipelines simplifies the connection between modular blocks, such as:

1. Data Sources (e.g., ADCs, DDS generators)
2. Pre-processing Stages (e.g., decimators, DDC)
3. DSP Cores (e.g., FIR, FFT, detection modules)
4. Data Sinks (e.g., DMA to memory, DACs)

By combining AXI-Stream with AXI4 and AXI4-Lite, developers can separate data, control, and configuration paths, creating clean and maintainable FPGA designs.

AXI Interface Applications

AXI interfaces are widely used in FPGA-based systems across various domains:

• Communications: High-speed data movement between transceivers, filters, and modulators.
• Radar Systems: Streaming radar returns from ADCs to FPGA DSP blocks for real-time signal processing.
• High-Speed Data Acquisition: Efficiently moving sampled data to memory or host processors for analysis.
• Embedded Processing: AXI4-Lite enables software to configure and monitor hardware DSP cores.
• Machine Learning on FPGA: Facilitates fast streaming between feature extraction cores and inference engines.

The AXI family of interfaces provides a flexible, scalable framework for building complex DSP systems on FPGA platforms. Memory-mapped AXI interfaces handle control and data movement, while AXI-Stream ensures efficient, high-speed streaming between modular processing cores.

A strong grasp of AXI write and read channels, together with streaming data paths, is essential for developing robust and high-performance DSP pipelines. By leveraging AXI interfaces, engineers can design systems that are not only modular but also optimized for throughput and scalability.

Next Blog Topic: Building a Complete DSP Chain: From ADC to FFT to Display 

Read all the blogs in the DSP Building Blocks series.