learn-fpga (Amaranth HDL version)

This repository contains code to follow the excellent learn-fpga tutorial by Bruno Levy from blinker to RISC-V using Amaranth HDL instead of Verilog.

The tutorial starts from a very simple 'blink' example and will end with a fully functional RISC-V CPU core.

This code repository is only meant as a supplement to Bruno Levy's main repository. There are very few explanations in this repository that go beyond explaining certain aspects of Amaranth HDL. Please refer to the main tutorial for detailed information of every step of the implementation.

Using Amaranth HDL the code can certainly be simplified / restructured, but the aim of this project is to keep close to the original.

Note: The repository is currently undergoing a transition to incorporate language changes from Amaranth HDL 0.5. If you want to use older Amaranth versions, check out the tag 'pre-amaranth-0.5'.

Board support

Support for the following boards is included:

• Digilent Arty A7
• Digilent CMOD A7
• Digilent CMOD S7 (untested)
• Sipeed tang nano 9k

If you don't have a board, you can still run the code in the Amaranth Python simulator.

Toolchain

Amaranth HDL supports the following toolchains for the boards:

• AMD/Xilinx Vivado, proprietary
• F4PGA (former Symbiflow), using FLOSS tools (Yosys, Nextpnr)
• Gowin, proprietary
• Project Apicula (for Gowin FPGAs)

Running the simulation

Each directory contains a bench.py file. This contains a test bench for the simulator. Run it like so:

source env.sh        # Add directories to Python library path
cd 01_blink
python bench.py

Building a firmware bitfile

The platform specific code is in the boards directory. To build e.g. step 5 for the Arty A7 board:

source env.sh        # Add directories to Python library path
python boards/digilent_arty_a7.py 5

RISC-V assembler

This repository also contains a (minimal) RISC-V assembler written in Python in the tools directory.

UART connection

Due to deviation of the internal oscillator frequency from the nominal value it is possible that the UART baudrate is not exactly the same as what is set in the code (by default 1 MBaud). In this case it helps to vary the receiver baudrate by +- 20% and check if reception works. If an oscilloscope is available, you can also measure the clock frequency by patching out the clock signal to one of the pins and measuring the signal period.

Sipeed Tang Nano 9k

The built-in UART-USB converter does not work very well (at least not on Linux). For this reason, it is better to connect an external UART-USB converter to the Pins 53 (rx) and 54 (tx). When testing the receiver had to be tuned to between 900 kBaud and 960 kBaud.

Licensing

The files in this repository are licensed under the BSD-3-Clause license. Exceptions are marked in the respective files. See the files in the LICENSES directory for details.

Author

• Bastian Löher (bl0x)