Project Deep Dive: Running Linux on an FPGA for RF Comms

The “Linux on FPGA” project listed on my homepage is one of the most challenging and rewarding projects I’ve worked on. It goes beyond simple programming and into the realm of custom hardware acceleration and operating system integration. Here’s a closer look at how it works.

The Goal: High-Performance, Flexible Signal Processing

The primary objective was to upgrade a digital signal processor (DSP). Traditional DSPs are fast but rigid. By using a Field-Programmable Gate Array (FPGA), we gain the ability to create custom, parallel processing hardware in code (VHDL), while an embedded ARM processor runs a full Linux OS for high-level control and networking. This gives us the best of both worlds: the raw speed of custom hardware and the flexibility of software.

Key Architectural Components

The system architecture was built around three main pillars:

• The Hardware (FPGA Logic): Written in VHDL, this is the custom logic that performs the heavy lifting of signal processing—tasks like filtering, modulation, and demodulation. Because this logic runs in parallel, it can process data significantly faster than a traditional CPU.
• The Processor (Embedded ARM Core): We used a Zynq series FPGA, which contains a hardened ARM Cortex-A9 dual-core processor on the same chip as the programmable logic. This isn’t a “soft” processor built from logic gates; it’s a real, physical CPU.
• The Operating System (Linux): We ported a lightweight, Debian-based Linux distribution (similar to Raspberry Pi OS) to run on the ARM core. This provides a familiar environment with drivers, a file system, networking stacks, and the ability to run complex applications written in C++ or Python.

The Biggest Challenge: The Hardware/Software Interface

The most complex part of the project was creating a seamless communication bridge between the FPGA logic and the Linux OS. This was achieved using the AXI (Advanced eXtensible Interface) bus, a standard high-performance bus that connects the processor to the programmable logic.

We had to design and implement custom AXI-compliant hardware peripherals in VHDL. Then, in Linux, we wrote custom kernel drivers to recognize these hardware blocks as devices. This allowed a C++ application running in Linux to simply read from or write to a file (like /dev/my_dsp_core) to send commands and data directly to the high-speed VHDL processing chain.

This project was a fantastic exercise in system-level design, blending the worlds of hardware description, kernel-level software, and application development.