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FEMTORV32 / FEMTOSOC: a minimalistic RISC-V CPU

(Everything can fit on the IceStick < 1280 LUTs)

Quick links:

External links:

Features

  • Comes in different variants: quark (RV32I), electron (RV32IM), intermissum (RV32IM + irq), gracilis (RV32IMC + irq), petitbateau (RV32IMFC + irq).
  • Synthesis using the freeware tools (Yosys and nextpnr).
  • Main goal: to be used for teaching, easy to read, fitting on the ICEstick, fun demos (graphics), equip students for approx. $40.
  • Disclaimer: I'm no FPGA expert, please feel free to comment, to give me some advice !
  • Runs at 80MHz on the ICEStick and on the ULX3S.
  • 6kb RAM (ICEStick) or 256kb (ULX3S)
  • Firmware can be generated with gnu RISC-V toolsuite (script included), in C or in assembly.
  • SOC memory-mapped device drivers and hardware for UART, built-in LEDs, OLed display, led matrix.
  • femtolibC, femtoGL (everything fits in 6kb).
  • includes @ultraembedded's fat_io_lib (access to FAT filesystem on SDCards).
  • "femtOS" virtual output support: redirects printf() to UART, OLED screen (or led matrix, WIP).
  • many RISC-V assembly and C demo programs, including graphics for the OLED display.
  • now directly available in LiteX, see tutorial

LUT count (FemtoRV32 + FemtoSOC)

Statistics measured with ICE40/IceStick target.

Parameter value1 value2 value3
NRV_TWO_STAGE_SHIFTER ON OFF OFF
NRV_NEGATIVE_RESET OFF OFF OFF
NRV_IO_LEDS ON ON ON
NRV_IO_UART ON ON OFF
NRV_IO_SSD1351 OFF OFF OFF
NRV_IO_MAX7219 OFF OFF OFF
NRV_IO_SPI_FLASH OFF OFF OFF
NRV_FREQ 50 50 50
NRV_RAM 6144 6144 1024
NRV_COUNTERS OFF OFF OFF
NRV_COUNTERS_64 OFF OFF OFF
NRV_RV32M OFF OFF OFF
LUT count 1180 1140 980

First column is a standard configuration for IceStick (UART and LED configured, 6Kb RAM, two-level shifter). This corresponds to the second line of the Dhrystones test. Second column measures the impact of the two-level shifter (eats up 40 LUTs). Third column is a minimalistic configuration, with no peripheral (just LEDs) and minimal RAM, to have an idea of how many LUTs the processor alone uses (less than 1000, achievement unlocked !).

FemtoRV32 makes a compromise between complexity (the sources fit in 1000 lines of Verilog and - I think - are easy to read), LUT count (1000 is also the magic number here) and performance (around 1000 Dhrystones/s/MHz, most instructions take between 2 and 3 cycles).

FemtoRV32 License

FemtoRV is licensed under the Three-clause BSD license:

Copyright (c) 2020-2021, Bruno Levy
All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are
met:
* Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
* Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the distribution.
* Neither the name of the <copyright holder> nor the names of its
contributors may be used to endorse or promote products derived from
this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL <COPYRIGHT
HOLDER> BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

References to other RISC-V cores

  • The reference: Claire Wolf's picorv32 (borrowed many ideas from there).
  • The smallest RISC-V: SERV, you can fit 5 instances (!) in 1000 LUTs.
  • Faster cores, Linux capable cores: biriscv, VexRiscv
  • FemtoRV32's best friend: ICE-V, written in Silice (a higher-level HDL)
  • DarkRiscv
  • Domipheus RPU
  • LamndaConcept Minerva
  • Glacial
  • neorv32
  • j-core
  • Syntacore scr1