Chiselwatt

A tiny POWER Open ISA soft processor written in Chisel.

Simulation using verilator

• Chiselwatt uses verilator for simulation. Either install this from your distro or build it. Chisel uses sbt (the scala build tool), but unfortunately most of the distros package an ancient version. On Fedora you can install an upstream version using:

$ sudo dnf remove sbt
$ sudo curl https://bintray.com/sbt/rpm/rpm | sudo tee /etc/yum.repos.d/bintray-sbt-rpm.repo
$ sudo dnf --enablerepo=bintray--sbt-rpm install sbt

For verilator you can install it using:

$ sudo dnf install verilator

Next build chiselwatt:

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

• A micropython image is included in the repo. To use it, link the memory image into chiselwatt:

$ ln -s micropython/firmware.hex insns.hex

• Now run chiselwatt:

$ ./chiselwatt

Building micropython from scratch

• You can also build micropython from scratch. If you aren't building natively on a ppc64le box you will need 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:

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

Synthesis

Synthesis on FPGAs is supported with yosys/nextpnr. It 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).

Edit Makefile to configure your FPGA, JTAG device etc. You will also need to configure the amount of block RAM your FPGA supports, by editing src/main/scala/Core.scala. Here we are using 128kB of block RAM:

chisel3.Driver.execute(Array[String](), () => new Core(64, 128*1024, "insns.hex", 0x0))

For ECP5 FPGA use 8 KiB instead:

chisel3.Driver.execute(Array[String](), () => new Core(64, 8*1024, "insns.hex", 0x0))

Unfortunately due to an issue in yosys/nextpnr, dual port RAMs are not working. This means we use twice as much block RAM as you would expect. This also means Micropython likely won't fit (it needs 384 kB).

hello_world should run everywhere, so start with it. Edit src/main/scala/Core.scala and set memory to 8*1024, as mentioned above for ECP5. Then copy in the hello_world image:

$ cp hello_world/hello_world.hex insns.hex

To build:

$ make chiselwatt.bit

and to program the FPGA:

$ make prog

On an ECP5 board you can use a simple Python script to read from /dev/ttyUSB1:

#!/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))

You must see:

$ ./read_serial.py

H
e
l
l
o
 
W
o
r
l
d

Issues

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

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