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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 $WORKSPACE from now on. All commands will be run from this directory unless otherwise stated.

Install FuseSoC

pip install fusesoc

Add the FuseSoC standard library

fusesoc library add fusesoc_cores

The FuseSoC standard library already contain a version of SERV, but if we want to make changes to SERV, run the bundled example or use the Zephyr support, it is better to add SERV as a separate library into the workspace

fusesoc library add serv

The SERV repo will now be available in $WORKSPACE/fusesoc_libraries/serv. To save some typing, we will refer to that directory as $SERV.

We are now ready to do our first exercises with SERV

If Verilator is installed, we can use that as a linter to check the SERV source code

fusesoc run --target=lint serv

If everything worked, the output should look like

INFO: Preparing ::serv:1.0.2
INFO: Setting up project

INFO: Building simulation model
verilator -f 
INFO: Running

Running pre-built test software

Build and run the single threaded zephyr hello world example with verilator (should be stopped with Ctrl-C):

fusesoc run --target=verilator_tb servant --uart_baudrate=57600 --firmware=$SERV/sw/zephyr_hello.hex

..or... the multithreaded version

fusesoc run --target=verilator_tb servant --uart_baudrate=57600 --firmware=$SERV/sw/zephyr_hello_mt.hex --memsize=16384

Both should yield an output ending with

***** Booting Zephyr OS zephyr-v1.14.1-4-gc7c2d62513fe *****
Hello World! service

For a more advanced example, we can also run the Dining philosophers demo

fusesoc run --target=verilator_tb servant --uart_baudrate=57600 --firmware=$SERV/sw/zephyr_phil.hex --memsize=32768

...or... the synchronization example

fusesoc run --target=verilator_tb servant --uart_baudrate=57600 --firmware=$SERV/sw/zephyr_sync.hex --memsize=16384

Other applications can be tested by compiling and converting to bin and then hex e.g. with found in $SERV/sw

Run RISC-V compliance tests

Build the verilator model (if not already done)

fusesoc run --target=verilator_tb --build servant

Download the tests repo

git clone

To run the RISC-V compliance tests, we need to supply the SERV-specific support files and point the test suite to where it can find a target to run (i.e. the previously built Verilator model)

Run the compliance tests

cd riscv-compliance && make TARGETDIR=$SERV/riscv-target RISCV_TARGET=serv RISCV_DECICE=rv32i RISCV_ISA=rv32i TARGET_SIM=$SERV/build/servant_1.0.2/verilator_tb-verilator/Vservant_sim

The above will run all tests in the rv32i test suite. Since SERV also implement the rv32Zicsr and rv32Zifencei extensions, these can also be tested by choosing any of them instead of rv32i as the RISCV_ISA variable.

Run on hardware

The servant SoC has been ported to an increasing 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

Now we're ready to build. Note, for all the cases below, it's possible to run with --memfile=$SERV/sw/blinky.hex (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.

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.

fusesoc run --target=icebreaker servant


Pin 6 is used for UART output with 115200 baud rate. Thanks to the onboard debugger, you can just connect the USB Type-C connector to the PC, and a serial console will show up.

fusesoc run --target=icesugar servant

OrangeCrab R0.2

Pin D1 is used for UART output with 115200 baud rate.

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[4]) in data/arty_a7_35t.xdc).

fusesoc run --target=arty_a7_35t servant

DE0 Nano

FPGA Pin D11 (Connector JP1, pin 38) is used for UART output with 57600 baud rate. DE0 Nano needs an external 3.3V UART to connect to this pin

fusesoc run --target=de0_nano servant

DECA development kit

FPGA Pin W18 (Pin 3 P8 connector) is used for UART output with 57600 baud rate. Key 0 is reset and Led 0 q output.

fusesoc run --target=deca servant

SoCKit development kit

FPGA Pin F14 (HSTC GPIO addon connector J2, pin 2) is used for UART output with 57600 baud rate.

fusesoc run --target=sockit 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[0]) in data/pipistrello.ucf).

fusesoc run --target=pipistrello servant

Alhambra II

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.

fusesoc run --target=alhambra servant
iceprog -d i:0x0403:0x6010:0 build/servant_1.0.1/alhambra-icestorm/servant_1.0.1.bin


Pin 95 is used as the GPIO output which is connected to the board's green LED. Due to this board's limited Embedded BRAM, programs with a maximum of 7168 bytes can be loaded. The default program for this board is blinky.hex.

fusesoc run --target=icestick servant
iceprog build/servant_1.0.2/icestick-icestorm/servant_1.0.2.bin

Other targets

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

Zephyr support

SERV, or rather the Servant SoC, can run the Zephyr RTOS. The Servant-specific drivers and BSP is located in the zephyr subdirectory of the SERV repository. In order to use Zephyr on Servant, a project directory structure must be set up that allows Zephyr to load the Servant-specific files as a module.

First, the Zephyr SDK and the "west" build too must be installed. The Zephyr getting started guide describes these steps in more detail.

Assuming that SERV was installed into $WORKSPACE/fusesoc_libraries/serv as per the prerequisites, run the following command to make the workspace also work as a Zephyr workspace.

west init

Specify the SERV repository as the manifest repository, meaning it will be the main entry point when Zephyr is looking for modules.

west config manifest.path $SERV

Get the right versions of all Zephyr submodules

west update

It should now be possible to build Zephyr applications for the Servant SoC within the workspace. This can be tested e.g. by building the Zephyr Hello world samples application

cd zephyr/samples/hello_world
west build -b service

After a successful build, Zephyr will create an elf and a bin file of the application in build/zephyr/zephyr.{elf,bin}. The bin file can be converted to a verilog hex file, which in turn can be preloaded to FPGA on-chip memories and run on a target board, or loaded into simulated RAM model when running simulations.

To convert the newly built hello world example into a Verilog hex file, run

python3 $SERV/sw/ zephyr/samples/hello_world/build/zephyr/zephyr.bin 4096 > hello.hex

4096 is the number of 32-bit words to write and must be at least the size of the application binary. hello.hex is the resulting hex file. Running a simulation can now be done as described in Running pre-built test software, e.g.

fusesoc run --target=verilator_tb servant --uart_baudrate=57600 --firmware=/path/to/hello.hex

Or to create an FPGA image with the application preloaded to on-chip RAM, e.g. for a Nexys A7 board, run

fusesoc run --target=nexys_a7 servant --memfile=/path/to/hello.hex

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
  • Make it faster and smaller