SERV - the SErial RISC-V CPU by olofk
SERV is an award-winning bit-serial RISC-V core
If you want to know more about SERV, what a bit-serial CPU is and what it's good for, I recommend starting out by watching the movies introduction to SERV and the presentation from the Zürich 2019 RISC-V workshop
There's also an official SERV user manual with fancy block diagrams and an in-depth description of how some things work.
Create a directory to keep all the different parts of the project together. We
will refer to this directory as
$SERV from now on.
Download the main serv repo
cd $SERV && git clone https://github.com/olofk/serv
pip install fusesoc
Initialize the FuseSoC standard libraries
Create a workspace directory for FuseSoC
Register the serv repo as a core library
cd $SERV/workspace && fusesoc library add serv $SERV
Check that the CPU passes the linter
cd $SERV/workspace && fusesoc run --target=lint serv
Running test software
Build and run the single threaded zephyr hello world example with verilator (should be stopped with Ctrl-C):
cd $SERV/workspace fusesoc run --target=verilator_tb servant --uart_baudrate=57600 --firmware=$SERV/serv/sw/zephyr_hello.hex
..or... the multithreaded version
fusesoc run --target=verilator_tb servant --uart_baudrate=57600 --firmware=$SERV/serv/sw/zephyr_hello_mt.hex --memsize=16384
...or... the philosophers example
fusesoc run --target=verilator_tb servant --uart_baudrate=57600 --firmware=$SERV/serv/sw/zephyr_phil.hex --memsize=32768
...or... the synchronization example
fusesoc run --target=verilator_tb servant --uart_baudrate=57600 --firmware=$SERV/serv/sw/zephyr_sync.hex --memsize=16384
Other applications can be tested by compiling and converting to bin and then hex e.g. with makehex.py found in
Run the compliance tests
Build the verilator model (if not already done)
cd $SERV/workspace && fusesoc run --target=verilator_tb --setup --build servant
Download the tests repo
cd $SERV && git clone https://github.com/riscv/riscv-compliance
Run the compliance tests
cd $SERV/riscv-compliance && make TARGETDIR=$SERV/serv/riscv-target RISCV_TARGET=serv RISCV_DECICE=rv32i RISCV_ISA=rv32i TARGET_SIM=$SERV/workspace/build/servant_1.0.1/verilator_tb-verilator/Vservant_sim
Run on hardware
The servant SoC has been ported to a number of different FPGA boards. To see all currently supported targets run
fusesoc core show servant
By default, these targets have the program memory preloaded with a small Zephyr hello world example that writes its output on a UART pin. Don't forget to install the appropriate toolchain (e.g. icestorm, Vivado, Quartus...) and add to your PATH
Some targets also depend on functionality in the FuseSoC base library (fusesoc-cores). Running
fusesoc library list should tell you if fusesoc-cores is already available. If not, add it to your workspace with
fusesoc library add fusesoc-cores https://github.com/fusesoc/fusesoc-cores
Now we're ready to build. Note, for all the cases below, it's possible to run with
(or any other suitable program) as the last argument to preload the LED blink example
instead of hello world.
Pin A6 is used for UART output with 115200 baud rate.
cd $SERV/workspace fusesoc run --target=tinyfpga_bx servant tinyprog --program build/servant_1.0.1/tinyfpga_bx-icestorm/servant_1.0.1.bin
Pin 9 is used for UART output with 57600 baud rate.
cd $SERV/workspace fusesoc run --target=icebreaker servant
Pin D1 is used for UART output with 115200 baud rate.
cd $SERV/workspace fusesoc run --target=orangecrab_r0.2 servant dfu-util -d 1209:5af0 -D build/servant_1.0.2/orangecrab_r0.2-trellis/servant_1.0.2.bit
Arty A7 35T
Pin D10 (uart_rxd_out) is used for UART output with 57600 baud rate (to use blinky.hex change D10 to H5 (led) in data/arty_a7_35t.xdc).
cd $SERV/workspace fusesoc run --target=arty_a7_35t servant
Saanlima Pipistrello (Spartan6 LX45)
Pin A10 (usb_data<1>) is used for UART output with 57600 baud rate (to use blinky.hex change A10 to V16 (led) in data/pipistrello.ucf).
cd $SERV/workspace fusesoc run --target=pipistrello servant
Pin 61 is used for UART output with 115200 baud rate. This pin is connected to a FT2232H chip in board, that manages the communications between the FPGA and the computer.
cd $SERV/workspace fusesoc run --target=alhambra servant iceprog -d i:0x0403:0x6010:0 build/servant_1.0.1/alhambra-icestorm/servant_1.0.1.bin
The above targets are run on the servant SoC, but there are some targets defined for the CPU itself. Verilator can be run in lint mode to check for design problems by running
fusesoc run --target=lint serv
It's also possible to just synthesise for different targets to check resource usage and such. To do that for the iCE40 devices, run
fusesoc run --tool=icestorm serv --pnr=none
...or to synthesize with vivado for Xilinx targets, run
fusesoc run --tool=vivado serv --pnr=none
This will synthesize for the default Vivado part. To synthesise for a specific device, run e.g.
fusesoc run --tool=vivado serv --pnr=none --part=xc7a100tcsg324-1
Good to know
Don't feed serv any illegal instructions after midnight. Many logic expressions are hand-optimized using the old-fashioned method with Karnaugh maps on paper, and shamelessly take advantage of the fact that some opcodes aren't supposed to appear. As serv was written with 4-input LUT FPGAs as target, and opcodes are 5 bits, this can save quite a bit of resources in the decoder.
The bus interface is kind of Wishbone, but with most signals removed. There's an important difference though. Don't send acks on the instruction or data buses unless serv explicitly asks for something by raising its cyc signal. Otherwise serv becomes very confused.
Don't go changing the clock frequency on a whim when running Zephyr. Or well, it's ok I guess, but since the UART is bitbanged, this will change the baud rate as well. As of writing, the UART is running at 115200 baud rate when the CPU is 32 MHz. There are two NOPs in the driver to slow it down a bit, so if those are removed I think it could achieve baud rate 115200 on a 24MHz clock.. in case someone wants to try
- Applications have to be preloaded to RAM at compile-time
- Store bootloader and register file together in a RAM
- Make it faster and smaller
ISC License Copyright 2019, Olof Kindgren Permission to use, copy, modify, and/or distribute this software for any purpose with or without fee is hereby granted, provided that the above copyright notice and this permission notice appear in all copies. THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.