weizx208 / fakepga

Simulating Verilog designs on a microcontroller

FakePGA

Simulating Verilog designs on a microcontroller

What does it do?

This project aims to simulate Verilog HDL designs on a Raspberry Pi Pico (or any other RP2040-based board). It achieves this by using Verilator to compile the RTL into a cycle-accurate C++ model of your design which can then be executed on the microcontroller. This allows you to access the signals of the simulated design through the GPIO pins of the board, which gives a more hands-on experience than simulating on a computer. It is many orders of magnitude slower than a real FPGA (max clock speed is 5kHz, and it goes down as the complexity of your design increases), but it can still be used as an educational tool. In terms of user experience, it feels like using a slower FPGA (at least for simpler designs), hence the name of the project: FakePGA.

What do I need in order to use it?

Hardware-wise, you're going to need an RP2040-based dev board. The included Verilog example makes use of the on-board LED and two pushbuttons. This example blinks the LED, stopping and starting the blinking when the buttons are pressed.

Software-wise, you're going to need a Linux system to compile on. This could work on Windows, but I only tested it with Ubuntu (running in WSL). You need to have the pico-sdk installed and working, as well as Verilator.

After cloning this repository, you'll need to set the path of the Verilator library sources in the CMakeLists.txt file, on the 12th line. Out of the box, it is set to the default path for the Ubuntu install of Verilator.

How do I use it?

The Verilog design files reside in the verilog folder. The top module resides in the top.v file and it must be named top. It must have an input port named i_clk, which coresponds to the clock source for the design, no matter if it's used or not.

The simulation is configured in the config.txt file. Just like when using an actual FPGA, you'll need to map the ports of your design to real pins on the device. You can also set the clock freqency here. The config file used for the included example is presented below:

// Clock frequency in Hz
CLOCK_FREQ 5000;

// Input mapping
INPUTS
{
    ADD_IN{TOP i_s, 14, PULLUP}; // Maps port i_s to GPIO14 and pulls it up
    ADD_IN{TOP i_r, 15, PULLUP}; // Maps port i_r to GPIO15 and pulls it up
}

// Output mapping
OUTPUTS
{
    ADD_OUT{TOP o_led, PICO_DEFAULT_LED_PIN}; // Maps port o_led to the default LED pin
}

Before building the project for the first time, or after deleting the build folder, you must run the preBuild.sh script, in order to create some dummy files that the build system expects to exist. Afterwards, you can simply build it with CMake:

cd build
cmake ..
make