Chiselwatt

A tiny POWER Open ISA soft processor written in Chisel.

Getting chiselwatt

• Grab the latest version of chiselwatt from github:

git clone https://github.com/antonblanchard/chiselwatt
cd chiselwatt

Simulation using verilator

• Chiselwatt uses verilator for simulation. It is built by default and run in a Docker container. To build with local verilator install, edit Makefile.

• The micropython and hello_world sample images are included in the repo. To use either, uncomment the relevant MEMORY_SIZE and RAM_INIT_FILE lines in the Makefile.

• Then build and simulate chiselwatt with:

make
./chiselwatt

• If your operating system is not Linux, run chiselwatt inside a container:

make dockerlator
# Inside the container prompt, run:
./chiselwatt

# type "exit" to exit the container
exit

Synthesizing for FPGAs using Open Source tools (yosys/nextpnr)

Synthesis on FPGAs is supported with yosys/nextpnr. At the moment the tools support Lattice ECP5 FPGAs. The build process uses Docker images, so no software other than Docker needs to be installed. If you prefer podman you can use that too, just adjust it in Makefile, DOCKER=podman.

hello_world

The hello_world example should run everywhere, so start with it. Uncomment the relevant MEMORY_SIZE and RAM_INIT_FILE lines in the Makefile.

Building and programming the FPGA

The Makefile currently supports the following FPGA boards by defining the ECP5_BOARD parameter on make:

• Lattice ECP5 Evaluation Board - evn
• Radiona ULX3S - ulx3s
• Greg Davill Orangecrab - orangecrab
• Q3k Colorlight - colorlight

For example, to build for the Evaluation Board, run:

make clean
make ECP5_BOARD=evn synth

and to program the FPGA:

make clean
make ECP5_BOARD=evn prog

# or if your USB device has a different path, pass it on USBDEVICE, like:

make clean
make ECP5_BOARD=evn USBDEVICE=/dev/tty.usbserial-120001 prog

Programming using OpenOCD on Docker does not work on Docker Desktop for Mac since the container is run in a Linux VM and can not see the physical devices connected to the MacOS.

For the ULX3S board, the current OpenOCD does not support ft232 protocol so to program it, download ujprog for your platform and program using ./ujprog chiselwatt.bit or to persist in the flash, ./ujprog -j FLASH chiselwatt.bit.

After programming, if you connect to the serial port of the FPGA at 115200 8n1, you should see "Hello World" and after that all input will be echoed to the output. On Linux, picocom can be used. Another option below is a simple python script.

Micropython

Unfortunately due to an issue in yosys/nextpnr, dual port RAMs are not working. More details can be found in https://github.com/YosysHQ/yosys/issues/1101.

This means we use twice as much block RAM as you would expect. This also means Micropython won't fit on an ECP5 85F, because the ~400kB of available BRAM is halved to ~200k. Micropython requires 384 kB.

Once this is fixed, uncomment the relevant MEMORY_SIZE and RAM_INIT_FILE lines in the Makefile

For example, to build for the ULX3S, run:

make clean
make ECP5_BOARD=ulx3s synth`

and to program the FPGA:

make clean
make ECP5_BOARD=ulx3s prog

If you connect to the serial port of the FPGA at 115200 8n1, you should see "Hello World" and after that all input will be echoed to the output. On Linux, picocom can be used. Another option below is a simple python script.

#!/usr/bin/python

import serial

# configure the serial connections
ser = serial.Serial(
    port='/dev/ttyUSB1',
    baudrate=115200,
    parity=serial.PARITY_NONE,
    stopbits=serial.STOPBITS_ONE,
    bytesize=serial.EIGHTBITS
)

# read from serial
while 1:
    while ser.inWaiting() > 0:
        byte = ser.read(1);
        print("%s" %(byte))

Building from scratch

Hello World

The hello world sample simply prints "Hello from Chiselwatt, an OpenPower processor!" and echoes the input back to the terminal as a serial console. The source is in ./samples/hello_world and it can be built by make hello_world. The Makefile generates the insns.hex file that will be used on the synthesized core to be run on Verilator or loaded into FPGA.

Micropython

You can also build micropython from scratch. As a convenience, there is a Makefile target to cross-build on a Docker container and generate the firmware. Just run make micropython and it will generate the insns.hex file that will be used on the synthesized core to be run on Verilator or loaded into FPGA.

If running on a container is not an option, you need a ppc64le box or a cross compiler. This may be available on your distro, otherwise grab the the powerpc64le-power8 toolchain from bootlin.

If you are cross compiling, point CROSS_COMPILE at the toolchain. In the example below I installed it in usr/local/powerpc64le-power8--glibc--bleeding-edge-2018.11-1/bin/ and the tools begin with powerpc64-linux-*:

git clone https://github.com/micropython/micropython.git
cd micropython
cd ports/powerpc
make CROSS_COMPILE=/usr/local/powerpc64le-power8--glibc--bleeding-edge-2018.11-1/bin/powerpc64le-linux- -j$(nproc)
cd ../../../

• Build chiselwatt, import the the micropython image and run it. We use bin2hex.py to convert a binary file into a series of 64 bit hex values expected by the tools:

cd chiselwatt
make
scripts/bin2hex.py ../micropython/ports/powerpc/build/firmware.bin > insns.hex
./chiselwatt

Issues

Now that it is functional, we have a number of things to add:

• A few instructions
• Wishbone interconnect
• Caches
• Pipelining and bypassing