The NEORV32 Processor

Table of Content

• Introduction
• Features
• FPGA Implementation Results
• Performance
• Top Entity
• Getting Started
• Contact
• Legal

Introduction

The NEORV32 is a customizable mikrocontroller-like processor system based on a RISC-V rv32i or rv32e CPU with optional M, C and Zicsr extensions. The CPU was built from scratch and is compliant to the Unprivileged ISA Specification Version 2.1 and a subset of the Privileged Architecture Specification Version 1.12. The NEORV32 is intended as auxiliary processor within a larger SoC designs or as stand-alone custom microcontroller.

The processor provides common peripherals and interfaces like input and output ports, serial interfaces for UART, I²C and SPI, interrupt controller, timers and embedded memories. External memories, peripherals and custom IP can be attached via a Wishbone-based external memory interface. All optional features beyond the base CPU can be enabled configured via VHDL generics.

This project comes with a complete software ecosystem that features core libraries for high-level usage of the provided functions and peripherals, application makefiles and example programs. All software source files provide a doxygen-based documentary.

The project is intended to work "out of the box". Just synthesize the test setup from this project, upload it to your FPGA board of choice and start playing with the NEORV32. If you do not want to compile the GCC toolchains by yourself, you can also download pre-compiled toolchain for Linux.

For more information take a look a the NEORV32 datasheet.

Design Principles

• From zero to main(): Completely open source and documented.
• Plain VHDL without technology-specific parts like attributes, macros or primitives.
• Easy to use – working out of the box.
• Clean synchronous design, no wacky combinatorial interfaces.
• The processor has to fit in a Lattice iCE40 UltraPlus 5k FPGA running at 20+ MHz.

Status

The processor is synthesizable (tested with Intel Quartus Prime and Lattice Radiant/Synplify) and can successfully execute all the provided example programs.

Features

Processor Features

• RISC-V-compliant rv32i or rv32e CPU with optional C, E, M and Zicsr extensions
• GCC-based toolchain (pre-compiled rv32i and rv32 etoolchains available)
• Application compilation based on GNU makefiles
• Doxygen-based documentation of the software framework
• Completely described in behavioral, platform-independent VHDL – no primitives, macros, etc.
• Fully synchronous design, no latches, no gated clocks
• Small hardware footprint and high operating frequency
• Highly customizable processor configuration
• Optional processor-internal data and instruction memories (DMEM/IMEM)
• Optional internal bootloader with UART console and automatic SPI flash boot option
• Optional machine system timer (MTIME), RISC-V-compliant
• Optional universal asynchronous receiver and transmitter (UART)
• Optional 8/16/24/32-bit serial peripheral interface master (SPI) with 8 dedicated chip select lines
• Optional two wire serial interface master (TWI), compatible to the I²C standard
• Optional general purpose parallel IO port (GPIO), 16xOut & 16xIn, with pin-change interrupt
• Optional 32-bit external bus interface, Wishbone b4 compliant (WISHBONE)
• Optional watchdog timer (WDT)
• Optional PWM controller with 4 channels and 8-bit duty cycle resolution (PWM)
• Optional GARO-based true random number generator (TRNG)
• Optional core-local interrupt controller with 8 channels (CLIC)
• Optional dummy device (DEVNULL) (can be used for fast simulation console output)

CPU Features

The CPU is compliant to the official RISC-V specifications including a subset of the RISC-V privileged architecture specifications.

RV32I base instruction set (I extension):

• ALU instructions: LUI AUIPC ADDI SLTI SLTIU XORI ORI ANDI SLLI SRLI SRAI ADD SUB SLL SLT SLTU XOR SRL SRA OR AND
• Branches instructions: JAL JALR BEQ BNE BLT BGE BLTU BGEU
• Memory instructions: LB LH LW LBU LHU SB SH SW

Compressed instructions (C extension):

• ALU instructions: C.ADDI4SPN C.ADDI C.ADD C.ADDI16SP C.LI C.LUI C.SLLI C.SRLI C.SRAI C.ANDI C.SUB C.XOR C.OR C.AND C.MV C.NOP
• Branches instructions: C.J C.JAL C.JR C.JALR C.BEQZ C.BNEZ
• Memory instructions: C.LW C.SW C.LWSP C.SWSP
• Misc instructions: C.EBREAK (only with Zicsr extension)

Embedded CPU version (E extension):

• Reduced register file (only the 16 lowest registers)
• No performance counter CSRs

Integer multiplication and division hardware (M extension):

• Multiplication instructions: MUL MULH MULHSU MULHU
• Division instructions: DIV DIVU REM REMU

Privileged architecture (Zicsr extension):

• Privilege levels: M-mode (Machine mode)
• CSR access instructions: CSRRW CSRRS CSRRC CSRRWI CSRRSI CSRRCI
• System instructions: ECALL EBREAK MRET WFI
• Counter CSRs: cycle cycleh time timeh instret instreth mcycle mcycleh minstret minstreth
• Machine CSRs: mstatus misa mie mtvec mscratch mepc mcause mtval mip mtinst mimpid mhartid
• Custom CSRs: mfeatures mclock mispacebase mdspacebase mispacesize mdspacesize
• Supported exceptions and interrupts:
  • Misaligned instruction address
  • Instruction access fault
  • Illegal instruction
  • Breakpoint (via ebreak instruction)
  • Load address misaligned
  • Load access fault
  • Store address misaligned
  • Store access fault
  • Environment call from M-mode (via ecall instruction)
  • Machine software instrrupt
  • Machine timer interrupt (via MTIME unit)
  • Machine external interrupt (via CLIC unit)

General:

• No hardware support of unaligned accesses (except for instructions in C extension that still have to be aligned on 16-bit boundaries)
• Multi-cycle in-order instruction execution

More information including a detailed list of the available CSRs can be found in the NEORV32 datasheet.

To-Do / Wish List

• Testing, testing and even more testing
• Port official RISC-V compliance test
• Port Dhrystone benchmark
• Implement atomic extensions (A extension)
• Implement co-processor for single-precision floating-point (F extension)
• Implement user mode (U extension)
• Make a 64-bit branch
• Maybe port an RTOS (like freeRTOS or RIOT)

FPGA Implementation Results

This chapter shows exemplary implementation results of the NEORV32 processor for an Intel Cyclone IV EP4CE22F17C6N FPGA on a DE0-nano board. The design was synthesized using Intel Quartus Prime Lite 19.1 ("balanced implementation"). The timing information is derived from the Timing Analyzer / Slow 1200mV 0C Model. If not otherwise specified, the default configuration of the processor's generics is assumed. No constraints were used.

Results generated for hardware version: 0.0.2.3

CPU

Peripherals / Others

Lattice iCE40 UltraPlus 5k

The following table shows the hardware utilization for a iCE40 UP5K FPGA. The setup uses all provided peripherals, all CPU extensions (except for the E extension), no external memory interface and internal instruction and data memories (each 64kB) based on SPRAM primitives. The FPGA-specific memory components can be found in the rtl/fpga_specific folder.

Place & route reports generated with Lattice Radiant 2.1 using Lattice LSE. The clock frequency is constrained and generated via the PLL from the internal HF oscillator running at 12 MHz.

Results generated for hardware version: 0.0.2.5

Performance

CoreMark Benchmark

The CoreMark CPU benchmark was executed on the NEORV32 and is available in the sw/example/coremark project folder. This benchmark tests the capabilities of a CPU itself rather than the functions provided by the whole system / SoC.

Results generated for hardware version: 0.0.2.3

**Configuration**
Hardware:         32kB IMEM, 16kb DMEM, 100MHz clock
CoreMark:         2000 iterations, MEM_METHOD is MEM_STACK
CPU extensions:   `rv32i` or `rv32im` or `rv32imc`
Used peripherals: MTIME for time measurement, UART for printing the results

Instruction Cycles

The NEORV32 CPU is based on a multi-cycle architecture. Each instruction is executed in a sequence of several consecutive micro operations. Hence, each instruction requires several clock cycles to execute. The average CPI (cycles per instruction) depends on the instruction mix of a specific applications and also on the available CPU extensions.

Please note that the CPU-internal shifter (e.g. for the SLL instruction) as well as the multiplier and divider of the M extension use a bit-serial approach and require several cycles for completion.

The following table shows the performance results for successfully (!) running 2000 CoreMark iterations. The average CPI is computed by dividing the total number of required clock cycles (all of CoreMark – not only the timed core) by the number of executed instructions (instret[h] CSRs). The executables were generated using optimization -O2.

Evaluation

Based on the provided performance measurement and the hardware utilization for the different CPU configurations, the following configurations are suggested:

*) Including on-chip memory hardware requirements.

Top Entity

The top entity of the processor is neorv32_top.vhd (from the rtl/core folder). Just instantiate this file in your project and you are ready to go! All signals of this top entity are of type std_ulogic or std_ulogic_vector, respectively (except for the TWI signals, which are of type std_logic).

Use the generics to configure the processor according to your needs. Each generics is initilized with the default configuration. Detailed information regarding the signals and configuration generics can be found in the NEORV32 documentary.

entity neorv32_top is
  generic (
    -- General --
    CLOCK_FREQUENCY           : natural := 0;       -- clock frequency of clk_i in Hz
    HART_ID                   : std_ulogic_vector(31 downto 0) := x"00000000"; -- custom hardware thread ID
    BOOTLOADER_USE            : boolean := true;    -- implement processor-internal bootloader?
    -- RISC-V CPU Extensions --
    CPU_EXTENSION_RISCV_C     : boolean := false;   -- implement compressed extension?
    CPU_EXTENSION_RISCV_E     : boolean := false;   -- implement embedded RF extension?
    CPU_EXTENSION_RISCV_M     : boolean := false;   -- implement muld/div extension?
    CPU_EXTENSION_RISCV_Zicsr : boolean := true;    -- implement CSR system?
    -- Memory configuration: Instruction memory --
    MEM_ISPACE_BASE           : std_ulogic_vector(31 downto 0) := x"00000000"; -- base address of instruction memory space
    MEM_ISPACE_SIZE           : natural := 16*1024; -- total size of instruction memory space in byte
    MEM_INT_IMEM_USE          : boolean := true;    -- implement processor-internal instruction memory
    MEM_INT_IMEM_SIZE         : natural := 16*1024; -- size of processor-internal instruction memory in bytes
    MEM_INT_IMEM_ROM          : boolean := false;   -- implement processor-internal instruction memory as ROM
    -- Memory configuration: Data memory --
    MEM_DSPACE_BASE           : std_ulogic_vector(31 downto 0) := x"80000000"; -- base address of data memory space
    MEM_DSPACE_SIZE           : natural := 8*1024;  -- total size of data memory space in byte
    MEM_INT_DMEM_USE          : boolean := true;    -- implement processor-internal data memory
    MEM_INT_DMEM_SIZE         : natural := 8*1024;  -- size of processor-internal data memory in bytes
    -- Memory configuration: External memory interface --
    MEM_EXT_USE               : boolean := false;   -- implement external memory bus interface?
    MEM_EXT_REG_STAGES        : natural := 2;       -- number of interface register stages (0,1,2)
    MEM_EXT_TIMEOUT           : natural := 15;      -- cycles after which a valid bus access will timeout (>=1)
    -- Processor peripherals --
    IO_GPIO_USE               : boolean := true;    -- implement general purpose input/output port unit (GPIO)?
    IO_MTIME_USE              : boolean := true;    -- implement machine system timer (MTIME)?
    IO_UART_USE               : boolean := true;    -- implement universal asynchronous receiver/transmitter (UART)?
    IO_SPI_USE                : boolean := true;    -- implement serial peripheral interface (SPI)?
    IO_TWI_USE                : boolean := true;    -- implement two-wire interface (TWI)?
    IO_PWM_USE                : boolean := true;    -- implement pulse-width modulation unit (PWM)?
    IO_WDT_USE                : boolean := true;    -- implement watch dog timer (WDT)?
    IO_CLIC_USE               : boolean := true;    -- implement core local interrupt controller (CLIC)?
    IO_TRNG_USE               : boolean := false;   -- implement true random number generator (TRNG)?
    IO_DEVNULL_USE            : boolean := true     -- implement dummy device (DEVNULL)?
  );
  port (
    -- Global control --
    clk_i        : in  std_ulogic := '0'; -- global clock, rising edge
    rstn_i       : in  std_ulogic := '0'; -- global reset, low-active, async
    -- Wishbone bus interface (available if MEM_EXT_USE = true) --
    wb_adr_o     : out std_ulogic_vector(31 downto 0); -- address
    wb_dat_i     : in  std_ulogic_vector(31 downto 0) := (others => '0'); -- read data
    wb_dat_o     : out std_ulogic_vector(31 downto 0); -- write data
    wb_we_o      : out std_ulogic; -- read/write
    wb_sel_o     : out std_ulogic_vector(03 downto 0); -- byte enable
    wb_stb_o     : out std_ulogic; -- strobe
    wb_cyc_o     : out std_ulogic; -- valid cycle
    wb_ack_i     : in  std_ulogic := '0'; -- transfer acknowledge
    wb_err_i     : in  std_ulogic := '0'; -- transfer error
    -- GPIO (available if IO_GPIO_USE = true) --
    gpio_o       : out std_ulogic_vector(15 downto 0); -- parallel output
    gpio_i       : in  std_ulogic_vector(15 downto 0) := (others => '0'); -- parallel input
    -- UART (available if IO_UART_USE = true) --
    uart_txd_o   : out std_ulogic; -- UART send data
    uart_rxd_i   : in  std_ulogic := '0'; -- UART receive data
    -- SPI (available if IO_SPI_USE = true) --
    spi_sclk_o   : out std_ulogic; -- serial clock line
    spi_mosi_o   : out std_ulogic; -- serial data line out
    spi_miso_i   : in  std_ulogic := '0'; -- serial data line in
    spi_csn_o    : out std_ulogic_vector(07 downto 0); -- SPI CS
    -- TWI (available if IO_TWI_USE = true) --
    twi_sda_io   : inout std_logic := 'H'; -- twi serial data line
    twi_scl_io   : inout std_logic := 'H'; -- twi serial clock line
    -- PWM (available if IO_PWM_USE = true) --
    pwm_o        : out std_ulogic_vector(03 downto 0); -- pwm channels
    -- Interrupts (available if IO_CLIC_USE = true) --
    ext_irq_i    : in  std_ulogic_vector(01 downto 0) := (others => '0'); -- external interrupt request
    ext_ack_o    : out std_ulogic_vector(01 downto 0)  -- external interrupt request acknowledge
  );
end neorv32_top;

Getting Started

This overview is just a short excerpt from the Let's Get It Started section of the NEORV32 documentary:

NEORV32 datasheet

Building the Toolchain

At first you need the RISC-V GCC toolchain. You can either download the sources and build the toolchain by yourself, or you can download a prebuilt one and install it.

To build the toolchain by yourself, get the sources from the official RISCV-GNU-TOOLCHAIN github page:

$ git clone --recursive https://github.com/riscv/riscv-gnu-toolchain

Download and install the prerequisite standard packages:

$ sudo apt-get install autoconf automake autotools-dev curl python3 libmpc-dev libmpfr-dev libgmp-dev gawk build-essential bison flex texinfo gperf libtool patchutils bc zlib1g-dev libexpat-dev

To build the Linux cross-compiler, pick an install path. If you choose, say, /opt/riscv, then add /opt/riscv/bin to your PATH environment variable.

$ export PATH:$PATH:/opt/riscv/bin

Then, simply run the following commands in the RISC-V GNU toolchain source folder (for the rv32i toolchain):

riscv-gnu-toolchain$ ./configure --prefix=/opt/riscv --with-arch=rv32i –with-abi=ilp32
riscv-gnu-toolchain$ make

After a while (hours!) you will get riscv32-unknown-elf-gcc and all of its friends in your /opt/riscv/bin folder.

Using a Prebuilt Toolchain

Alternatively, you can download a prebuilt toolchain. I have uploaded the toolchain I am using to GitHub. This toolchain has been compiled on a 64-bit x86 Ubuntu (Ubuntu on Windows). Download the toolchain of choice:

https://github.com/stnolting/riscv_gcc_prebuilt

Dowload the Project and Create a Hardware Project

Now its time to get the most recent version the NEORV32 Processor project from GitHub. Clone the NEORV32 repository using git from the command line (suggested for easy project updates via git pull):

$ git clone https://github.com/stnolting/neorv32.git

Create a new HW project with your FPGA synthesis tool of choice. Add all files from the rtl/core folder to this project and add them to a new library called neorv32.

You can either instantiate the processor's top entity in you own project, or you can use a simple test setup as top entity. This test setup instantiates the processor, implements most of the peripherals and the basic ISA. Only the UART, clock, reset and some GPIO output sginals are propagated:

entity neorv32_test_setup is
  port (
    -- Global control --
    clk_i      : in  std_ulogic := '0'; -- global clock, rising edge
    rstn_i     : in  std_ulogic := '0'; -- global reset, low-active, async
    -- GPIO --
    gpio_o     : out std_ulogic_vector(7 downto 0); -- parallel output
    -- UART --
    uart_txd_o : out std_ulogic; -- UART send data
    uart_rxd_i : in  std_ulogic := '0' -- UART receive data
  );
end neorv32_test_setup;

This test setup is intended as quick and easy "hello world" test setup to get into the NEORV32.

Compiling and Uploading One of the Example Projects

Make sure GNU Make and a native GCC compiler are installed. To test the installation of the RISC-V toolchain, navigate to an example project like sw/example/blink_led and run:

neorv32/sw/example/blink_led$ make check

The NEORV32 project includes some example programs from which you can start your own application: SW example projects

Simply compile one of these projects. This will create a NEORV32 executable neorv32_exe.bin in the same folder.

neorv32/sw/example/blink_led$ make clean_all compile

Connect your FPGA board via UART to you computer and open the according port to interface with the NEORV32 bootloader. The bootloader uses the following default UART configuration:

• 19200 Baud
• 8 data bits
• 1 stop bit
• No parity bits
• No transmission / flow control protocol (raw bytes only)
• Newline on \r\n (carriage return & newline)

Use the bootloader console to upload and execute your application image.

  << NEORV32 Bootloader >>
  
  BLDV: Jun 22 2020
  HWV:  0.0.2.3
  CLK:  0x0134FD90 Hz
  MISA: 0x42801104
  CONF: 0x01FF0015
  IMEM: 0x00010000 bytes @ 0x00000000
  DMEM: 0x00010000 bytes @ 0x80000000
  
  Autoboot in 8s. Press key to abort.
  Aborted.
  
  Available commands:
  h: Help
  r: Restart
  u: Upload
  s: Store to flash
  l: Load from flash
  e: Execute
  CMD:> 

Going further: Take a look at the NEORV32 datasheet.

Contact

If you have any questions, bug reports, ideas or if you are facing problems with the NEORV32 or want to give some kind of feedback, open a new issue or directly drop me a line:

stnolting@gmail.com

Citation

If you are using the NEORV32 Processor in some kind of publication, please cite it as follows:

S. Nolting, "The NEORV32 Processor", github.com/stnolting/neorv32

Legal

This is a hobby project released under the BSD 3-Clause license. No copyright infringement intended.

BSD 3-Clause License

Copyright (c) 2020, Stephan Nolting. All rights reserved.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
2. 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.
3. 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 THE COPYRIGHT HOLDER OR CONTRIBUTORS 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.

"Windows" is a trademark of Microsoft Corporation.

"Artix" and "Vivado" are trademarks of Xilinx Inc.

"Cyclone", "Quartus Prime" and "Avalon Bus" are trademarks of Intel Corporation.

"iCE40", "UltraPlus" and "Lattice Radiant" are trademarks of Lattice Semiconductor Corporation.

"AXI4" and "AXI4-Lite" are trademarks of Arm Holdings plc.

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This project is not affiliated with or endorsed by the Open Source Initiative (https://www.oshwa.org / https://opensource.org).

Made with :heart: in Hannover, Germany.