Simple 6502 system on a ULX3S FPGA board

To build this you will need the following FPGA tools

• Trellis - Lattice ECP5 FPGA project https://github.com/SymbiFlow/prjtrellis
• Yosys - Synthesis https://github.com/YosysHQ/yosys
• Nextpnr - Place and Route https://github.com/YosysHQ/nextpnr

Additionally, you'll need the ujprog JTAG programming tool for the ULX3S board. Note that getting this tool running properly can be tricky - I had to build it from source, making changes to the Makefile and the source code in order for it to build, run and recognize my ULX3S board.

You will also need the following 6502 tools:

• cc65 6502 C compiler (for default option) https://github.com/cc65/cc65

git clone https://github.com/emeb/ulx3s_6502
cd ulx3s_6502
git submodule update --init
cd trellis
make

Make sure you've properly set up udev rules for the ULX3S, then plug it in. Then load the previously built bitstream.

make prog

The 8 ULX3S LEDs should begin flashing a binary sequence. If you'd like to try the serial port then that can be done with the command

make prog_term

This will load the bitstream and immediately start a 9600 bps terminal session where you can type and see characters echoed by the 6502. To exit the terminal session just type "~."

By default the build system fills the ROM with code based on the C and assembly source in the cc65 directory. If you are modifying only the C or assembly code then you can do a partial rebuild that changes only the ROM contents which will complete somewhat more quickly using the following command:

make recode

within the icestorm directory.

Simulation is supported and requires the following prerequisites:

• Icarus Verilog simulator https://iverilog.icarus.com/
• GTKWave waveform viewer https://gtkwave.sourceforge.net/

To simulate, use the following commands

cd icarus
make
make wave

This will build the simulation executable, run it and then view the output.

Thanks to the developers of all the tools used for this, as well as the authors of the IP core I snagged for the 6502. I've added that as a submodule so you'll know where to get it and who to give credit to.