Let’s Get It Started!

This user guide uses the NEORV32 project as is from the official `neorv32` repository. To make your first NEORV32 project run, follow the guides from the upcoming sections. It is recommended to follow these guides step by step and eventually in the presented order.

 This guide uses the minimalistic and platform/toolchain agnostic SoC test setups from `rtl/test_setups` for illustration. You can use one of the provided test setups for your first FPGA tests. Alternatively, have a look at the `setups` folder, which provides more sophisticated example setups for various FPGAs/FPGA boards and toolchains.

## 1. Software Toolchain Setup

To compile (and debug) executables for the NEORV32 a RISC-V toolchain is required. There are two possibilities to get this:

2. Download and install a prebuilt version of the toolchain; this might also done via the package manager / app store of your OS

 The default toolchain prefix (`RISCV_PREFIX` variable) for this project is `riscv32-unknown-elf-`. Of course you can use any other RISC-V toolchain (like `riscv64-unknown-elf-`) that is capable to emit code for a `rv32` architecture. Just change `RISCV_PREFIX` according to your needs.

### 1.1. Building the Toolchain from Scratch

To build the toolchain by yourself you can follow the guide from the official https://github.com/riscv/riscv-gnu-toolchain GitHub page. You need to make sure the generated toolchain fits the architecture of the NEORV32 core. To get a toolchain that even supports minimal ISA extension configurations, it is recommend to compile for `rv32i` only. Please note that this minimal ISA also provides further ISA extensions like `m` or `c`. Of course you can use a `multilib` approach to generate toolchains for several target ISAs.

Listing 1. Configuring GCC build for `rv32i` (minimal ISA)
``````riscv-gnu-toolchain$./configure --prefix=/opt/riscv --with-arch=rv32i –-with-abi=ilp32 riscv-gnu-toolchain$ make``````
 Keep in mind that – for instance – a toolchain build with `--with-arch=rv32imc` only provides library code compiled with compressed (`C`) and `mul`/`div` instructions (`M`)! Hence, this code cannot be executed (without emulation) on an architecture without these extensions!

#### 1.2.1. Use The Toolchain I have Build

I have compiled a GCC toolchain on a 64-bit x86 Ubuntu (Ubuntu on Windows, actually) and uploaded it to GitHub. You can directly download the according toolchain archive as single zip-file within a packed release from https://github.com/stnolting/riscv-gcc-prebuilt.

Unpack the downloaded toolchain archive and copy the content to a location in your file system (e.g. `/opt/riscv`). More information about downloading and installing my prebuilt toolchains can be found in the repository’s README.

#### 1.2.2. Use a Third Party Toolchain

Of course you can also use any other prebuilt version of the toolchain. There are a lot RISC-V GCC packages out there - even for Windows. On Linux system you might even be able to fetch a toolchain via your distribution’s package manager.

 Make sure the toolchain can (also) emit code for a `rv32i` architecture, uses the `ilp32` or `ilp32e` ABI and was not build using CPU extensions that are not supported by the NEORV32 (like `D`).

### 1.3. Installation

Now you have the toolchain binaries. The last step is to add them to your `PATH` environment variable (if you have not already done so): make sure to add the binaries folder (`bin`) of your toolchain.

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

You should add this command to your `.bashrc` (if you are using bash) to automatically add the RISC-V toolchain at every console start.

### 1.4. Testing the Installation

To make sure everything works fine, navigate to an example project in the NEORV32 example folder and execute the following command:

``neorv32/sw/example/blink_led$make check`` This will test all the tools required for the generating NEORV32 executables. Everything is working fine if `Toolchain check OK` appears at the end. ## 2. General Hardware Setup This guide shows the basics of setting up a NEORV32 project for FPGA implementation (or simulation only) from scratch. It uses a simplified test "SoC" setup of the processor to keeps things simple at the beginning. This simple setup is intended for evaluation or as "hello world" project to check out the NEORV32 on your FPGA board.  If you want to use a more sophisticated pre-defined setup to start with, check out the `setups` folder, which provides example setups for various FPGA, boards and toolchains. The NEORV32 project features two minimalistic pre-configured test setups in `rtl/test_setups`. Both test setups only implement very basic processor and CPU features. The main difference between the two setups is the processor boot concept - so how to get a software executable into the processor: • `rtl/test_setups/neorv32_testsetup_approm.vhd`: this setup does not require a connection via UART. The software executable is "installed" into the bitstream to initialize a read-only memory. Use this setup if your FPGA board does not provide a UART interface. • `rtl/test_setups/neorv32_testsetup_bootloader.vhd`: this setups uses the UART and the default NEORV32 bootloader to upload new software executables. Use this setup if your board does provide a UART interface. Figure 1. NEORV32 "hello world" test setup (`rtl/test_setups/neorv32_testsetup_bootloader.vhd`)  External Clock Source These test setups are intended to be directly used as design top entity. Of course you can also instantiate them into another design unit. If your FPGA board only provides very fast external clock sources (like on the FOMU board) you might need to add clock management components (PLLs, DCMs, MMCMs, …​) to the test setup or to the according top entity if you instantiate one of the test setups. 1. Create a new project with your FPGA EDA tool of choice. 2. Add all VHDL files from the project’s `rtl/core` folder to your project.  Internal Memories For a general first setup (technology-independent) use the default memory architectures for the internal memories (IMEM and DMEM). These are located in `rtl/core/mem`, so make sure to add the files from `rtl/core/mem` to your project, too. If synthesis cannot efficiently map those default memory descriptions to the available memory resources, you can later replace the default memory architectures by optimized platform-specific memory architectures. Example: The `setups/radiant/UPduino_v3` example setup uses optimized memory primitives. Hence, it does not include the default memory architectures from `rtl/core/mem` as these are replaced by device-specific implementations. However, it still has to include the entity definitions from `rtl/core`. 1. Make sure to add all the rtl files to a new library called `neorv32`. If your FPGA tools does not provide a field to enter the library name, check out the "properties" menu of the added rtl files. 2. The `rtl/core/neorv32_top.vhd` VHDL file is the top entity of the NEORV32 processor, which can be instantiated into the "real" project. However, in this tutorial we will use one of the pre-defined test setups from `rtl/test_setups` (see above).  Make sure to include the `neorv32` package into your design when instantiating the processor: add `library neorv32;` and `use neorv32.neorv32_package.all;` to your design unit. 1. Add the pre-defined test setup of choice to the project, too, and select it as top entity. 2. The entity of both test setups provide a minimal set of configuration generics, that might have to be adapted to match your FPGA and board: Listing 2. Test setup entity - configuration generics `````` generic ( -- adapt these for your setup -- CLOCK_FREQUENCY : natural := 100000000; (1) MEM_INT_IMEM_SIZE : natural := 16*1024; (2) MEM_INT_DMEM_SIZE : natural := 8*1024 (3) );``````  1 Clock frequency of `clk_i` signal in Hertz 2 Default size of internal instruction memory: 16kB 3 Default size of internal data memory: 8kB 1. If you feel like it – or if your FPGA does not provide sufficient resources – you can modify the memory sizes (`MEM_INT_IMEM_SIZE` and `MEM_INT_DMEM_SIZE` – marked with notes "2" and "3"). But as mentioned above, let’s keep things simple at first and use the standard configuration for now. 2. There is one generic that has to be set according to your FPGA board setup: the actual clock frequency of the top’s clock input signal (`clk_i`). Use the `CLOCK_FREQUENCY` generic to specify your clock source’s frequency in Hertz (Hz).  If you have changed the default memory configuration (`MEM_INT_IMEM_SIZE` and `MEM_INT_DMEM_SIZE` generics) keep those new sizes in mind – these values are required for setting up the software framework in the next section General Software Framework Setup. 1. Depending on your FPGA tool of choice, it is time to assign the signals of the test setup top entity to the according pins of your FPGA board. All the signals can be found in the entity declaration of the corresponding test setup: Listing 3. Entity signals of `neorv32_testsetup_approm.vhd` `````` port ( -- Global control -- clk_i : in std_ulogic; -- global clock, rising edge rstn_i : in std_ulogic; -- global reset, low-active, async -- GPIO -- gpio_o : out std_ulogic_vector(7 downto 0) -- parallel output );`````` Listing 4. Entity signals of `neorv32_testsetup_bootloader.vhd` `````` port ( -- Global control -- clk_i : in std_ulogic; -- global clock, rising edge rstn_i : in std_ulogic; -- global reset, low-active, async -- GPIO -- gpio_o : out std_ulogic_vector(7 downto 0); -- parallel output -- UART0 -- uart0_txd_o : out std_ulogic; -- UART0 send data uart0_rxd_i : in std_ulogic -- UART0 receive data );``````  Signal Polarity If your FPGA board has inverse polarity for certain input/output you can add `not` gates. Example: The reset signal `rstn_i` is low-active by default; the LEDs connected to `gpio_o` high-active by default. You can do this in your board top if you instantiate the test setup, or inside the test setup if this is your top entity (low-active LEDs example: `gpio_o ⇐ NOT con_gpio_o(7 downto 0);`). 1. Attach the clock input `clk_i` to your clock source and connect the reset line `rstn_i` to a button of your FPGA board. Check whether it is low-active or high-active – the reset signal of the processor is low-active, so maybe you need to invert the input signal. 2. If possible, connected at least bit `0` of the GPIO output port `gpio_o` to a LED (see "Signal Polarity" note above). 3. Finally, if your are using the UART-based test setup (`neorv32_testsetup_bootloader.vhd`) connect the UART communication signals `uart0_txd_o` and `uart0_rxd_i` to the host interface (e.g. USB-UART converter). 4. Perform the project HDL compilation (synthesis, mapping, bitstream generation). 5. Program the generated bitstream into your FPGA and press the button connected to the reset signal. 6. Done! The LED at `gpio_o(0)` should be flashing now.  After the GCC toolchain for compiling RISC-V source code is ready (chapter General Software Framework Setup), you can advance to one of these chapters to learn how to get a software executable into your processor setup: * If you are using the `neorv32_testsetup_approm.vhd` setup: See section Installing an Executable Directly Into Memory. * If you are using the `neorv32_testsetup_bootloader.vhd` setup: See section Uploading and Starting of a Binary Executable Image via UART. ## 3. General Software Framework Setup To allow executables to be actually executed on the NEORV32 Processor the configuration of the software framework has to be aware to the hardware configuration. This guide focuses on the memory configuration. To enabled certain CPU ISA festures refer to the Enabling RISC-V CPU Extensions section.  If you have not changed the default memory configuration in section General Hardware Setup you are already done and you can skip the rest of this guide. 1. Open the NEORV32 linker script `sw/common/neorv32.ld` with a text editor. Right at the beginning of this script you will find the `MEMORY` configuration listing the different memory section: Listing 5. Cut-out of the linker script `neorv32.ld`: `ram` memory section configuration ``````MEMORY { ram (rwx) : ORIGIN = 0x80000000, LENGTH = DEFINED(make_bootloader) ? 512 : 8*1024 (1) ...``````  1 Size of the data memory address space (right-most value) (internal/external DMEM); here 8kB 1. We only need to change the `ram` section, which presents the available data address space. If you have changed the DMEM (MEM_INT_DMEM_SIZE generic) size adapt the `LENGTH` parameter of the `ram` section (here: `8*1024`) so it is equal to your DMEM hardware configuration.  Make sure you only modify the right-most value (here: 8*1024)! The “512” are not relevant for the application. 1. Done! Save your changes and close the linker script.  Advanced: Section base address and size More information can be found in the datasheet section Address Space. ## 4. Application Program Compilation This guide shows how to compile an example C-code application into a NEORV32 executable that can be uploaded via the bootloader or the on-chip debugger.  If your FPGA board does not provide such an interface - don’t worry! Section Installing an Executable Directly Into Memory shows how to run custom programs on your FPGA setup without having a UART. 1. Open a terminal console and navigate to one of the project’s example programs. For instance, navigate to the simple `sw/example_blink_led` example program. This program uses the NEORV32 GPIO module to display an 8-bit counter on the lowest eight bit of the `gpio_o` output port. 2. To compile the project and generate an executable simply execute: ``neorv32/sw/example/blink_led$ make clean_all exe``
1. We are using the `clean_all` taret to make sure everything is re-build.

2. This will compile and link the application sources together with all the included libraries. At the end, your application is transformed into an ELF file (`main.elf`). The NEORV32 image generator (in `sw/image_gen`) takes this file and creates a final executable. The makefile will show the resulting memory utilization and the executable size:

``````neorv32/sw/example/blink_led$make clean_all exe Memory utilization: text data bss dec hex filename 3176 0 120 3296 ce0 main.elf Compiling ../../../sw/image_gen/image_gen Executable (neorv32_exe.bin) size in bytes: 3188`````` 1. That’s it. The `exe` target has created the actual executable `neorv32_exe.bin` in the current folder that is ready to be uploaded to the processor.  The compilation process will also create a `main.asm` assembly listing file in the current folder, which shows the actual assembly code of the application. ## 5. Uploading and Starting of a Binary Executable Image via UART Follow this guide to use the bootloader to upload an executable via UART.  This concept uses the default "Indirect Boot" scenario that uses the bootloader to upload new executables. See datasheet section Indirect Boot for more information.  If your FPGA board does not provide such an interface - don’t worry! Section Installing an Executable Directly Into Memory shows how to run custom programs on your FPGA setup without having a UART. 1. Connect the primary UART (UART0) interface of your FPGA board to a serial port of your host computer. 2. Start a terminal program. In this tutorial, I am using TeraTerm for Windows. You can download it fore free from https://ttssh2.osdn.jp/index.html.en  Any terminal program that can connect to a serial port should work. However, make sure the program can transfer data in raw byte mode without any protocol overhead around it. 1. Open a connection to the the serial port your UART is connected to. Configure the terminal setting according to the following parameters: • 19200 Baud • 8 data bits • 1 stop bit • no parity bits • no transmission/flow control protocol • receiver (host computer) newline on `\r\n` (carriage return & newline) 1. Also make sure that single chars are send from your computer without any consecutive "new line" or "carriage return" commands (this is highly dependent on your terminal application of choice, TeraTerm only sends the raw chars by default). 2. Press the NEORV32 reset button to restart the bootloader. The status LED starts blinking and the bootloader intro screen appears in your console. Hurry up and press any key (hit space!) to abort the automatic boot sequence and to start the actual bootloader user interface console. Listing 6. Bootloader console; aborted auto-boot sequence ``````<< NEORV32 Bootloader >> BLDV: Mar 23 2021 HWV: 0x01050208 CLK: 0x05F5E100 MISA: 0x40901105 ZEXT: 0x00000023 PROC: 0x0EFF0037 IMEM: 0x00004000 bytes @ 0x00000000 DMEM: 0x00002000 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:>`````` 1. Execute the "Upload" command by typing `u`. Now the bootloader is waiting for a binary executable to be send. ``````CMD:> u Awaiting neorv32_exe.bin...`````` 1. Use the "send file" option of your terminal program to send a NEORV32 executable (`neorv32_exe.bin`). 2. Again, make sure to transmit the executable in raw binary mode (no transfer protocol). When using TeraTerm, select the "binary" option in the send file dialog. 3. If everything went fine, OK will appear in your terminal: ``````CMD:> u Awaiting neorv32_exe.bin... OK`````` 1. The executable is now in the instruction memory of the processor. To execute the program right now run the "Execute" command by typing `e`: ``````CMD:> u Awaiting neorv32_exe.bin... OK CMD:> e Booting... Blinking LED demo program`````` 1. If everything went fine, you should see the LEDs blinking.  The bootloader will print error codes if something went wrong. See section Bootloader of the NEORV32 datasheet for more information.  See section Programming an External SPI Flash via the Bootloader to learn how to use an external SPI flash for nonvolatile program storage.  Executables can also be uploaded via the on-chip debugger. See section Debugging with GDB for more information. ## 6. Installing an Executable Directly Into Memory If you do not want to use the bootloader (or the on-chip debugger) for executable upload or if your setup does not provide a serial interface for that, you can also directly install an application into embedded memory. This concept uses the "Direct Boot" scenario that implements the processor-internal IMEM as ROM, which is pre-initialized with the application’s executable during synthesis. Hence, it provides non-volatile storage of the executable inside the processor. This storage cannot be altered during runtime and any source code modification of the application requires to re-program the FPGA via the bitstream.  See datasheet section Direct Boot for more information. Using the IMEM as ROM: • for this boot concept the bootloader is no longer required • this concept only works for the internal IMEM (but can be extended to work with external memories coupled via the processor’s bus interface) • make sure that the memory components (like block RAM) the IMEM is mapped to support an initialization via the bitstream 1. At first, make sure your processor setup actually implements the internal IMEM: the `MEM_INT_IMEM_EN` generics has to be set to `true`: Listing 7. Processor top entity configuration - enable internal IMEM `````` -- Internal Instruction memory -- MEM_INT_IMEM_EN => true, -- implement processor-internal instruction memory`````` 1. For this setup we do not want the bootloader to be implemented at all. Disable implementation of the bootloader by setting the `INT_BOOTLOADER_EN` generic to `false`. This will also modify the processor-internal IMEM so it is initialized with the executable during synthesis. Listing 8. Processor top entity configuration - disable internal bootloader `````` -- General -- INT_BOOTLOADER_EN => false, -- boot configuration: false = boot from int/ext (I)MEM`````` 1. To generate an "initialization image" for the IMEM that contains the actual application, run the `install` target when compiling your application: ``````neorv32/sw/example/blink_led$ make clean_all install
Memory utilization:
text    data     bss     dec     hex filename
3176       0     120    3296     ce0 main.elf
Compiling ../../../sw/image_gen/image_gen
Installing application image to ../../../rtl/core/neorv32_application_image.vhd``````
1. The `install` target has compiled all the application sources but instead of creating an executable (`neorv32_exe.bit`) that can be uploaded via the bootloader, it has created a VHDL memory initialization image `core/neorv32_application_image.vhd`.

2. This VHDL file is automatically copied to the core’s rtl folder (`rtl/core`) so it will be included for the next synthesis.

3. Perform a new synthesis. The IMEM will be build as pre-initialized ROM (inferring embedded memories if possible).

4. Upload your bitstream. Your application code now resides unchangeable in the processor’s IMEM and is directly executed after reset.

The synthesis tool / simulator will print asserts to inform about the (IMEM) memory / boot configuration:

``````NEORV32 PROCESSOR CONFIG NOTE: Boot configuration: Direct boot from memory (processor-internal IMEM).
NEORV32 PROCESSOR CONFIG NOTE: Implementing processor-internal IMEM as ROM (3176 bytes), pre-initialized with application.``````

## 7. Setup of a New Application Program Project

1. The easiest way of creating a new software application project is to copy an existing one. This will keep all file dependencies. For example you can copy `sw/example/blink_led` to `sw/example/flux_capacitor`.

2. If you want to place you application somewhere outside `sw/example` you need to adapt the application’s makefile. In the makefile you will find a variable that keeps the relative or absolute path to the NEORV32 repo home folder. Just modify this variable according to your new project’s home location:

``````# Relative or absolute path to the NEORV32 home folder (use default if not set by user)
NEORV32_HOME ?= ../../..``````
1. If your project contains additional source files outside of the project folder, you can add them to the `APP_SRC` variable:

``````# User's application sources (add additional files here)
APP_SRC = $(wildcard *.c) ../somewhere/some_file.c`````` 1. You also can add a folder containing your application’s include files to the `APP_INC` variable (do not forget the `-I` prefix): ``````# User's application include folders (don't forget the '-I' before each entry) APP_INC = -I . -I ../somewhere/include_stuff_folder`````` ## 8. Enabling RISC-V CPU Extensions Whenever you enable/disable a RISC-V CPU extensions via the according `CPU_EXTENSION_RISCV_x` generic, you need to adapt the toolchain configuration so the compiler can actually generate according code for it. To do so, open the makefile of your project (for example `sw/example/blink_led/makefile`) and scroll to the "USER CONFIGURATION" section right at the beginning of the file. You need to modify the `MARCH` variable and eventually the `MABI` variable according to your CPU hardware configuration. ``````# CPU architecture and ABI MARCH = -march=rv32i (1) MABI = -mabi=ilp32 (2)``````  1 MARCH = Machine architecture ("ISA string") 2 MABI = Machine binary interface For example, if you enable the RISC-V `C` extension (16-bit compressed instructions) via the `CPU_EXTENSION_RISCV_C` generic (set `true`) you need to add the `c` extension also to the `MARCH` ISA string in order to make the compiler emit compressed instructions.  Privileged Architecture Extensions Privileged architecture extensions like `Zicsr` or `Zifencei` are "used" implicitly by the compiler. Hence, according instruction will only be generated when "encoded" via inline assembly or when linking according libraries. In this case, these instruction will always be emitted (even if the according extension is not specified in `MARCH`). I recommend to not specify any privileged architecture extensions in `MARCH`.  ISA extension enabled in hardware can be a superset of the extensions enabled in software, but not the other way around. For example generating compressed instructions for a CPU configuration that has the `c` extension disabled will cause illegal instruction exceptions at runtime. You can also override the default `MARCH` and `MABI` configurations from the makefile when invoking the makefile: ``$ make MARCH=-march=rv32ic clean_all all``
 The RISC-V ISA string for `MARCH` follows a certain canonical structure: `rev32[i/e][m][a][f][d][g][q][c][b][v][n]…​` For example `rv32imac` is valid while `rv32icma` is not.

## 9. Application-Specific Processor Configuration

Due to the processor’s configuration options, which are mainly defined via the top entity VHDL generics, the SoC can be tailored to the application-specific requirements. Note that this chapter does not focus on optional SoC features like IO/peripheral modules. It rather gives ideas on how to optimize for overall goals like performance and area.

 Please keep in mind that optimizing the design in one direction (like performance) will also effect other potential optimization goals (like area and energy).

### 9.1. Optimize for Performance

The following points show some concepts to optimize the processor for performance regardless of the costs (i.e. increasing area and energy requirements):

• Enable all performance-related RISC-V CPU extensions that implement dedicated hardware accelerators instead of emulating operations entirely in software: `M`, `C`, `Zfinx`

• Enable mapping of compleX CPU operations to dedicated hardware: `FAST_MUL_EN ⇒ true` to use DSP slices for multiplications, `FAST_SHIFT_EN ⇒ true` use a fast barrel shifter for shift operations.

• Implement the instruction cache: `ICACHE_EN ⇒ true`

• Use as many internal memory as possible to reduce memory access latency: `MEM_INT_IMEM_EN ⇒ true` and `MEM_INT_DMEM_EN ⇒ true`, maximize `MEM_INT_IMEM_SIZE` and `MEM_INT_DMEM_SIZE`

• Increase the CPU’s instruction prefetch buffer size: `CPU_IPB_ENTRIES`

• To be continued…​

### 9.2. Optimize for Size

The NEORV32 is a size-optimized processor system that is intended to fit into tiny niches within large SoC designs or to be used a customized microcontroller in really tiny / low-power FPGAs (like Lattice iCE40). Here are some ideas how to make the processor even smaller while maintaining it’s general purpose system concept and maximum RISC-V compatibility.

SoC

• This is obvious, but exclude all unused optional IO/peripheral modules from synthesis via the processor configuration generics.

• If an IO module provides an option to configure the number of "channels", constrain this number to the actually required value (e.g. the PWM module `IO_PWM_NUM_CH` or the external interrupt controller `XIRQ_NUM_CH`).

• Reduce the FIFO sizes of implemented modules (e.g. `SLINK_TX_FIFO`).

• Disable the instruction cache (`ICACHE_EN ⇒ false`) if the design only uses processor-internal IMEM and DMEM memories.

• To be continued…​

CPU

• Use the embedded RISC-V CPU architecture extension (`CPU_EXTENSION_RISCV_E`) to reduce block RAM utilization.

• The compressed instructions extension (`CPU_EXTENSION_RISCV_C`) requires additional logic for the decoder but also reduces program code size by approximately 30%.

• If not explicitly used/required, constrain the CPU’s counter sizes: `CPU_CNT_WIDTH` for `[m]instret[h]` (number of instruction) and `[m]cycle[h]` (number of cycles) counters. You can even remove these counters by setting `CPU_CNT_WIDTH ⇒ 0` if they are not used at all (note, this is not RISC-V compliant).

• Reduce the CPU’s prefetch buffer size (`CPU_IPB_ENTRIES`).

• Map CPU shift operations to a small and iterative shifter unit (`FAST_SHIFT_EN ⇒ false`).

• If you have unused DSP block available, you can map multiplication operations to those slices instead of using LUTs to implement the multiplier (`FAST_MUL_EN ⇒ true`).

• If there is no need to execute division in hardware, use the `Zmmul` extension instead of the full-scale `M` extension.

• Disable CPU extension that are not explicitly used (`A`, `U`, `Zfinx`).

• To be continued…​

### 9.3. Optimize for Clock Speed

The NEORV32 Processor and CPU are designed to provide minimal logic between register stages to keep the critical path as short as possible. When enabling additional extension or modules the impact on the existing logic is also kept at a minimum to prevent timing degrading. If there is a major impact on existing logic (example: many physical memory protection address configuration registers) the VHDL code automatically adds additional register stages to maintain critical path length. Obviously, this increases operation latency.

In order to optimize for a minimal critical path (= maximum clock speed) the following points should be considered:

• Complex CPU extensions (in terms of hardware requirements) should be avoided (examples: floating-point unit, physical memory protection).

• Large carry chains (>32-bit) should be avoided (constrain CPU counter sizes: e.g. `CPU_CNT_WIDTH ⇒ 32` and `HPM_NUM_CNTS ⇒ 32`).

• If the target FPGA provides sufficient DSP resources, CPU multiplication operations can be mapped to DSP slices (`FAST_MUL_EN ⇒ true`) reducing LUT usage and critical path impact while also increasing overall performance.

• Use the synchronous (registered) RX path configuration of the external memory interface (`MEM_EXT_ASYNC_RX ⇒ false`).

• To be continued…​

 The short and fixed-length critical path allows to integrate the core into existing clock domains. So no clock domain-crossing and no sub-clock generation is required. However, for very high clock frequencies (this is technology / platform dependent) clock domain crossing becomes crucial for chip-internal connections.

### 9.4. Optimize for Energy

There are no dedicated configuration options to optimize the processor for energy (minimal consumption; energy/instruction ratio) yet. However, a reduced processor area (Optimize for Size) will also reduce static energy consumption.

To optimize your setup for low-power applications, you can make use of the CPU sleep mode (`wfi` instruction). Put the CPU to sleep mode whenever possible. Disable all processor modules that are not actually used (exclude them from synthesis if the will be never used; disable the module via it’s control register if the module is not currently used). When is sleep mode, you can keep a timer module running (MTIME or the watch dog) to wake up the CPU again. Since the wake up is triggered by any interrupt, the external interrupt controller can also be used to wake up the CPU again. By this, all timers (and all other modules) can be deactivated as well.

 Processor-internal clock generator shutdown If no IO/peripheral module is currently enabled, the processor’s internal clock generator circuit will be shut down reducing switching activity and thus, dynamic energy consumption.

## 10. Customizing the Internal Bootloader

The NEORV32 bootloader provides several options to configure and customize it for a certain application setup. This configuration is done by passing defines when compiling the bootloader. Of course you can also modify to bootloader source code to provide a setup that perfectly fits your needs.

 Each time the bootloader sources are modified, the bootloader has to be re-compiled (and re-installed to the bootloader ROM) and the processor has to be re-synthesized.
 Keep in mind that the maximum size for the bootloader is limited to 32kB and should be compiled using the base ISA `rv32i` only to ensure it can work independently of the actual CPU configuration.
Parameter Default Legal values Description

Serial console interface

`UART_EN`

`1`

`0`, `1`

Set to `0` to disable UART0 (no serial console at all)

`UART_BAUD`

`19200`

any

Baud rate of UART0

Status LED

`STATUS_LED_EN`

`1`

`0`, `1`

Enable bootloader status led ("heart beat") at `GPIO` output port pin #`STATUS_LED_PIN` when `1`

`STATUS_LED_PIN`

`0`

`0` …​ `31`

`GPIO` output pin used for the high-active status LED

Boot configuration

`AUTO_BOOT_SPI_EN`

`0`

`0`, `1`

Set `1` to enable immediate boot from external SPI flash

`AUTO_BOOT_OCD_EN`

`0`

`0`, `1`

Set `1` to enable boot via on-chip debugger (OCD)

`AUTO_BOOT_TIMEOUT`

`8`

any

Time in seconds after the auto-boot sequence starts (if there is no UART input by user); set to 0 to disabled auto-boot sequence

SPI configuration

`SPI_EN`

`1`

`0`, `1`

Set `1` to enable the usage of the SPI module (including load/store executables from/to SPI flash options)

`SPI_FLASH_CS`

`0`

`0` …​ `7`

SPI chip select output (`spi_csn_o`) for selecting flash

`SPI_FLASH_SECTOR_SIZE`

`65536`

any

SPI flash sector size in bytes

`SPI_FLASH_CLK_PRSC`

`CLK_PRSC_8`

`CLK_PRSC_2` `CLK_PRSC_4` `CLK_PRSC_8` `CLK_PRSC_64` `CLK_PRSC_128` `CLK_PRSC_1024` `CLK_PRSC_2024` `CLK_PRSC_4096`

SPI clock pre-scaler (dividing main processor clock)

`SPI_BOOT_BASE_ADDR`

`0x08000000`

any 32-bit value

Defines the base address of the executable in external flash

Each configuration parameter is implemented as C-language `define` that can be manually overridden (redefined) when invoking the bootloader’s makefile. The according parameter and its new value has to be appended (using `+=`) to the makefile’s `USER_FLAGS` variable. Make sure to use the `-D` prefix here.

For example, to configure a UART Baud rate of 57600 and redirecting the status LED to output pin 20 use the following command (in the bootloader’s source folder `sw/bootloader`):

Listing 9. Example: customizing, re-compiling and re-installing the bootloader
``$make USER_FLAGS+=-DUART_BAUD=57600 USER_FLAGS+=-DSTATUS_LED_PIN=20 clean_all bootloader``  The `clean_all` target ensure that all libraries are re-compiled. The `bootloader` target will automatically compile and install the bootloader to the HDL boot ROM (updating `rtl/core/neorv32_bootloader_image.vhd`). ### 10.1. Bootloader Boot Configuration The bootloader provides several boot configurations that define where the actual application’s executable shall be fetched from. Note that the non-default boot configurations provide a smaller memory footprint reducing boot ROM implementation costs. #### Default Boot Configuration The default bootloader configuration provides a UART-based user interface that allows to upload new executables at any time. Optionally, the executable can also be programmed to an external SPI flash by the bootloader (see section Programming an External SPI Flash via the Bootloader). This configuration also provides an automatic boot sequence (auto-boot) which will start fetching an executable from external SPI flash using the default SPI configuration. By this, the default bootloader configuration provides a "non volatile program storage" mechanism that automatically boot from external SPI flash (after `AUTO_BOOT_TIMEOUT`) while still providing the option to re-program SPI flash at any time via the UART interface. #### `AUTO_BOOT_SPI_EN` The automatic boot from SPI flash (enabled when `AUTO_BOOT_SPI_EN` is `1`) will fetch an executable from an external SPI flash (using the according SPI configuration) right after reset. The bootloader will start fetching the image at SPI flash base address `SPI_BOOT_BASE_ADDR`. Note that there is no UART console to interact with the bootloader. However, this boot configuration will output minimal status messages via UART (if `UART_EN` is `1`). #### `AUTO_BOOT_OCD_EN` If `AUTO_BOOT_OCD_EN` is `1` the bootloader is implemented as minimal "halt loop" to be used with the on-chip debugger. After initializing the hardware, the CPU waits in this endless loop until the on-chip debugger takes control over the core (to upload and run the actual executable). See section Debugging using the On-Chip Debugger for more information on how to use the on-chip debugger to upload and run executables.  All bootloader boot configuration support uploading new executables via the on-chip debugger.  Note that this boot configuration does not load any executable at all! Hence, this boot configuration is intended to be used with the on-chip debugger only. ## 11. Programming an External SPI Flash via the Bootloader The default processor-internal NEORV32 bootloader supports automatic booting from an external SPI flash. This guide shows how to write an executable to the SPI flash via the bootloader so it can be automatically fetched and executed after processor reset. For example, you can use a section of the FPGA bitstream configuration memory to store an application executable.  This section assumes the default configuration of the NEORV32 bootloader. See section Customizing the Internal Bootloader on how to customize the bootloader and its setting (for example the SPI chip-select port, the SPI clock speed or the flash base address for storing the executable). ### 11.1. SPI Flash The bootloader can access an SPI compatible flash via the processor top entity’s SPI port. By default, the flash chip-select line is to `spi_csn_o(0)` and uses 1/8 of the processor’s main clock as clock frequency. The SPI flash has to support single-byte read and write, 24-bit addresses and at least the following standard commands: • READ `0x03` • READ STATUS `0x05` • WRITE ENABLE `0x06` • PAGE PROGRAM `0x02` • SECTOR ERASE `0xD8` • READ ID `0x9E` Compatible (FGPA configuration) SPI flash memories are for example the "Winbond W25Q64FV2 or the "Micron N25Q032A". ### 11.2. Programming an Executable 1. At first, reset the NEORV32 processor and wait until the bootloader start screen appears in your terminal program. 2. Abort the auto boot sequence and start the user console by pressing any key. 3. Press u to upload the executable that you want to store to the external flash: ``````CMD:> u Awaiting neorv32_exe.bin...`````` 1. Send the binary in raw binary via your terminal program. When the upload is completed and "OK" appears, press `p` to trigger the programming of the flash (do not execute the image via the `e` command as this might corrupt the image): ``````CMD:> u Awaiting neorv32_exe.bin... OK CMD:> p Write 0x000013FC bytes to SPI flash @ 0x00800000? (y/n)`````` 1. The bootloader shows the size of the executable and the base address inside the SPI flash where the executable is going to be stored. A prompt appears: Type `y` to start the programming or type `n` to abort.  Section Customizing the Internal Bootloader show the according C-language `define` that can be modified to specify the base address of the executable inside the SPI flash. ``````CMD:> u Awaiting neorv32_exe.bin... OK CMD:> p Write 0x000013FC bytes to SPI flash @ 0x08000000? (y/n) y Flashing... OK CMD:>`````` 1. If "OK" appears in the terminal line, the programming process was successful. Now you can use the auto boot sequence to automatically boot your application from the flash at system start-up without any user interaction. ## 12. Packaging the Processor as IP block for Xilinx Vivado Block Designer 1. Import all the core files from `rtl/core` (including default internal memory architectures from `rtl/core/mem`) and assign them to a new design library `neorv32`. 2. Instantiate the `rtl/wrappers/neorv32_top_axi4lite.vhd` module. 3. Then either directly use that module in a new block-design ("Create Block Design", right-click → "Add Module", thats easier for a first try) or package it ("Tools", "Create and Package new IP") for the use in other projects. 4. Connect your AXI-peripheral directly to the core’s AXI4-Interface if you only have one, or to an AXI-Interconnect (from the IP-catalog) if you have multiple peripherals. 5. Connect ALL the `ACLK` and `ARESETN` pins of all peripherals and interconnects to the processor’s clock and reset signals to have a unified clock and reset domain (easier for a first setup). 6. Open the "Address Editor" tab and let Vivado assign the base-addresses for the AXI-peripherals (you can modify them according to your needs). 7. For all FPGA-external signals (like UART signals) make all the connections you need "external" (right-click on the signal/pin → "Make External"). 8. Save everything, let VIVADO create a HDL-Wrapper for the block-design and choose this as your Top Level Design. 9. Define your constraints and generate your bitstream.  Guide provided by GitHub user `AWenzel83` from https://github.com/stnolting/neorv32/discussions/52#discussioncomment-819013 ## 13. Simulating the Processor The NEORV32 project includes a core CPU, built-in peripherals in the Processor Subsystem, and additional peripherals in the templates and examples. Therefore, there is a wide range of possible testing and verification strategies. On the one hand, a simple smoke testbench allows ensuring that functionality is correct from a software point of view. That is used for running the RISC-V architecture tests, in order to guarantee compliance with the ISA specification(s). On the other hand, VUnit and Verification Components are used for verifying the functionality of the various peripherals from a hardware point of view. ### 13.1. Testbench A plain-VHDL (no third-party libraries) testbench (`sim/simple/neorv32_tb.simple.vhd`) can be used for simulating and testing the processor. This testbench features a 100MHz clock and enables all optional peripheral and CPU extensions except for the `E` extension and the TRNG IO module (that CANNOT be simulated due to its combinatorial (looped) architecture). The simulation setup is configured via the "User Configuration" section located right at the beginning of the testbench’s architecture. Each configuration constant provides comments to explain the functionality. Besides the actual NEORV32 Processor, the testbench also simulates "external" components that are connected to the processor’s external bus/memory interface. These components are: • an external instruction memory (that also allows booting from it) • an external data memory • an external memory to simulate "external IO devices" • a memory-mapped registers to trigger the processor’s interrupt signals The following table shows the base addresses of these four components and their default configuration and properties:  Attributes: `r` = read `w` = write `e` = execute `a` = atomic accesses possible `8` = byte-accessible `16` = half-word-accessible `32` = word-accessible Table 2. Testbench: processor-external memories Base address Size Attributes Description `0x00000000` `imem_size_c` `r/w/e, a, 8/16/32` external IMEM (initialized with application image) `0x80000000` `dmem_size_c` `r/w/e, a, 8/16/32` external DMEM `0xf0000000` 64 bytes `r/w/e, !a, 8/16/32` external "IO" memory, atomic accesses will fail `0xff000000` 4 bytes `-/w/-, a, -/-/32` memory-mapped register to trigger "machine external", "machine software" and "SoC Fast Interrupt" interrupts  The simulated NEORV32 does not use the bootloader and directly boots the current application image (from the `rtl/core/neorv32_application_image.vhd` image file).  UART output during simulation Data written to the NEORV32 UART0 / UART1 transmitter is send to a virtual UART receiver implemented as part of the testbench. Received chars are send to the simulator console and are also stored to a log file (`neorv32.testbench_uart0.out` for UART0, `neorv32.testbench_uart1.out` for UART1) inside the simulation’s home folder. Please note that printing via the native UART receiver takes a lot of time. For faster simulation console output see section Faster Simulation Console Output. ### 13.2. Faster Simulation Console Output When printing data via the UART the communication speed will always be based on the configured BAUD rate. For a simulation this might take some time. To have faster output you can enable the simulation mode for UART0/UART1 (see section Documentation: Primary Universal Asynchronous Receiver and Transmitter (UART0)). ASCII data sent to UART0|UART1 will be immediately printed to the simulator console and logged to files in the simulator execution directory: • `neorv32.uart?.sim_mode.text.out`: ASCII data. • `neorv32.uart?.sim_mode.data.out`: all written 32-bit dumped as 8-char hexadecimal values. You can "automatically" enable the simulation mode of UART0/UART1 when compiling an application. In this case, the "real" UART0/UART1 transmitter unit is permanently disabled. To enable the simulation mode just compile and install your application and add UART?_SIM_MODE to the compiler’s USER_FLAGS variable (do not forget the `-D` suffix flag): ``sw/example/blink_led$ make USER_FLAGS+=-DUART0_SIM_MODE clean_all all``

The provided define will change the default UART0/UART1 setup function in order to set the simulation mode flag in the according UART’s control register.

 The UART simulation output (to file and to screen) outputs "complete lines" at once. A line is completed with a line feed (newline, ASCII `\n` = 10).

### 13.3. Simulation using a shell script (with GHDL)

To simulate the processor using GHDL navigate to the `sim/simple/` folder and run the provided shell script. Any arguments that are provided while executing this script are passed to GHDL. For example the simulation time can be set to 20ms using `--stop-time=20ms` as argument.

``neorv32/sim/simple$sh ghdl_sim.sh --stop-time=20ms`` ### 13.4. Simulation using Application Makefiles (In-Console with GHDL) To directly compile and run a program in the console (using the default testbench and GHDL as simulator) you can use the `sim` makefile target. Make sure to use the UART simulation mode (`USER_FLAGS+=-DUART0_SIM_MODE` and/or `USER_FLAGS+=-DUART1_SIM_MODE`) to get faster / direct-to-console UART output. ``````sw/example/blink_led$ make USER_FLAGS+=-DUART0_SIM_MODE clean_all sim
[...]

#### 13.4.1. Hello World!

To do a quick test of the NEORV32 make sure to have GHDL and a [RISC-V gcc toolchain](https://github.com/stnolting/riscv-gcc-prebuilt) installed. Navigate to the project’s `sw/example/hello_world` folder and run `make USER_FLAGS+=-DUART0_SIM_MODE MARCH=-march=rv32imac clean_all sim`:

 The simulator will output some sanity check notes (and warnings or even errors if something is ill-configured) right at the beginning of the simulation to give a brief overview of the actual NEORV32 SoC and CPU configurations.
``````stnolting@Einstein:/mnt/n/Projects/neorv32/sw/example/hello_world$make USER_FLAGS+=-DUART0_SIM_MODE MARCH=-march=rv32imac clean_all sim ../../../sw/lib/source/neorv32_uart.c: In function 'neorv32_uart0_setup': ../../../sw/lib/source/neorv32_uart.c:301:4: warning: #warning UART0_SIM_MODE (primary UART) enabled! Sending all UART0.TX data to text.io simulation output instead of real UART0 transmitter. Use this for simulations only! [-Wcpp] 301 | #warning UART0_SIM_MODE (primary UART) enabled! Sending all UART0.TX data to text.io simulation output instead of real UART0 transmitter. Use this for simulations only! | ^~~~~~~ Memory utilization: text data bss dec hex filename 4612 0 120 4732 127c main.elf Compiling ../../../sw/image_gen/image_gen Installing application image to ../../../rtl/core/neorv32_application_image.vhd Simulating neorv32_application_image.vhd... Tip: Compile application with USER_FLAGS+=-DUART[0/1]_SIM_MODE to auto-enable UART[0/1]'s simulation mode (redirect UART output to simulator console). Using simulation runtime args: --stop-time=10ms ../rtl/core/neorv32_top.vhd:347:3:@0ms:(assertion note): NEORV32 PROCESSOR IO Configuration: GPIO MTIME UART0 UART1 SPI TWI PWM WDT CFS SLINK NEOLED XIRQ ../rtl/core/neorv32_top.vhd:370:3:@0ms:(assertion note): NEORV32 PROCESSOR CONFIG NOTE: Boot configuration: Direct boot from memory (processor-internal IMEM). ../rtl/core/neorv32_top.vhd:394:3:@0ms:(assertion note): NEORV32 PROCESSOR CONFIG NOTE: Implementing on-chip debugger (OCD). ../rtl/core/neorv32_cpu.vhd:169:3:@0ms:(assertion note): NEORV32 CPU ISA Configuration (MARCH): RV32IMACU_Zbb_Zicsr_Zifencei_Zfinx_Debug ../rtl/core/neorv32_cpu.vhd:189:3:@0ms:(assertion note): NEORV32 CPU CONFIG NOTE: Implementing NO dedicated hardware reset for uncritical registers (default, might reduce area). Set package constant <dedicated_reset_c> = TRUE to configure a DEFINED reset value for all CPU registers. ../rtl/core/neorv32_imem.vhd:107:3:@0ms:(assertion note): NEORV32 PROCESSOR CONFIG NOTE: Implementing processor-internal IMEM as ROM (16384 bytes), pre-initialized with application (4612 bytes). ../rtl/core/neorv32_dmem.vhd:89:3:@0ms:(assertion note): NEORV32 PROCESSOR CONFIG NOTE: Implementing processor-internal DMEM (RAM, 8192 bytes). ../rtl/core/neorv32_wishbone.vhd:136:3:@0ms:(assertion note): NEORV32 PROCESSOR CONFIG NOTE: External Bus Interface - Implementing STANDARD Wishbone protocol. ../rtl/core/neorv32_wishbone.vhd:140:3:@0ms:(assertion note): NEORV32 PROCESSOR CONFIG NOTE: External Bus Interface - Implementing auto-timeout (255 cycles). ../rtl/core/neorv32_wishbone.vhd:144:3:@0ms:(assertion note): NEORV32 PROCESSOR CONFIG NOTE: External Bus Interface - Implementing LITTLE-endian byte order. ../rtl/core/neorv32_wishbone.vhd:148:3:@0ms:(assertion note): NEORV32 PROCESSOR CONFIG NOTE: External Bus Interface - Implementing registered RX path. ../rtl/core/neorv32_slink.vhd:161:3:@0ms:(assertion note): NEORV32 PROCESSOR CONFIG NOTE: Implementing 8 RX and 8 TX stream links. ## ## ## ## ## ## ## ######### ######## ######## ## ## ######## ######## ## ################ #### ## ## ## ## ## ## ## ## ## ## ## ## ## #### #### ## ## ## ## ## ## ## ## ## ## ## ## ## ## ###### ## ## ## ## ######### ## ## ######### ## ## ##### ## ## #### ###### #### ## ## ## ## ## ## ## ## ## ## ## ## ## ## ###### ## ## #### ## ## ## ## ## ## ## ## ## ## ## #### #### ## ## ######### ######## ## ## ## ######## ########## ## ################ ## ## ## ## ## Hello world! :)`````` ### 13.5. Advanced Simulation using VUnit VUnit is an open source unit testing framework for VHDL/SystemVerilog. It allows continuous and automated testing of HDL code by complementing traditional testing methodologies. The motto of VUnit is "testing early and often" through automation. VUnit is composed by a Python interface and multiple optional VHDL libraries. The Python interface allows declaring sources and simulation options, and it handles the compilation, execution and gathering of the results regardless of the simulator used. That allows having a single `run.py` script to be used with GHDL, ModelSim/QuestaSim, Riviera PRO, etc. On the other hand, the VUnit’s VHDL libraries provide utilities for assertions, logging, having virtual queues, handling CSV files, etc. The Verification Component Library uses those features for abstracting away bit-toggling when verifying standard interfaces such as Wishbone, AXI, Avalon, UARTs, etc. Testbench sources in `sim` (such as `sim/neorv32_tb.vhd` and `sim/uart_rx*.vhd`) use VUnit’s VHDL libraries for testing NEORV32 and peripherals. The entrypoint for executing the tests is `sim/run.py`. ``````# ./sim/run.py -l neorv32.neorv32_tb.all Listed 1 tests # ./sim/run.py -v Compiling into neorv32: rtl/core/neorv32_uart.vhd passed Compiling into neorv32: rtl/core/neorv32_twi.vhd passed Compiling into neorv32: rtl/core/neorv32_trng.vhd passed ...`````` See VUnit: User Guide and VUnit: Command Line Interface for further info about VUnit’s features. ## 14. Building the Documentation The documentation (datasheet + user guide) is written using `asciidoc`. The according source files can be found in `docs/…​`. The documentation of the software framework is written in-code using `doxygen`. A makefiles in the project’s `docs` directory is provided to build all of the documentation as HTML pages or as PDF documents.  Pre-rendered PDFs are available online as nightly pre-releases: https://github.com/stnolting/neorv32/releases. The HTML-based documentation is also available online at the project’s GitHub Pages. The makefile provides a help target to show all available build options and their according outputs. ``neorv32/docs$ make help``
Listing 10. Example: Generate HTML documentation (data sheet) using `asciidoctor`
``neorv32/docs$make html``  If you don’t have `asciidoctor` / `asciidoctor-pdf` installed, you can still generate all the documentation using a docker container via `make container`. ## 15. FreeRTOS Support A NEORV32-specific port and a simple demo for FreeRTOS (https://github.com/FreeRTOS/FreeRTOS) are available in the `sw/example/demo_freeRTOS` folder. See the according documentation (`sw/example/demo_freeRTOS/README.md`) for more information. ## 16. RISC-V Architecture Test Framework The NEORV32 Processor passes the according tests provided by the official RISC-V Architecture Test Suite (V2.0+), which is available online at GitHub: https://github.com/riscv/riscv-arch-test All files required for executing the test framework on a simulated instance of the processor (including port files) are located in the `sw/isa-test` folder of the NEORV32 repository. The test framework is executed via the `sim/run_riscv_arch_test.sh` script. Take a look at the provided `sim/README.md` (online at GitHub) file for more information on how to run the tests and how testing is conducted in detail. ## 17. Debugging using the On-Chip Debugger The NEORV32 on-chip debugger allows online in-system debugging via an external JTAG access port from a host machine. The general flow is independent of the host machine’s operating system. However, this tutorial uses Windows and Linux (Ubuntu on Windows) in parallel.  See datasheet section On Chip Debugger (OCD) for more information.  This tutorial uses `gdb` to directly upload an executable to the processor. If you are using the default processor setup with internal instruction memory (IMEM) make sure it is implemented as RAM (INT_BOOTLOADER_EN generic = true). ### 17.1. Hardware Requirements Make sure the on-chip debugger of your NEORV32 setups is implemented (ON_CHIP_DEBUGGER_EN generic = true). Connect a JTAG adapter to the NEORV32 `jtag_*` interface signals. If you do not have a full-scale JTAG adapter, you can also use a FTDI-based adapter like the "FT2232H-56Q Mini Module", which is a simple and inexpensive FTDI breakout board. Table 3. JTAG pin mapping NEORV32 top signal JTAG signal FTDI port `jtag_tck_i` TCK D0 `jtag_tdi_i` TDI D1 `jtag_tdo_o` TDO D2 `jtag_tms_i` TMS D3 `jtag_trst_i` TRST D4  The low-active JTAG test reset (TRST) signals is optional as a reset can also be triggered via the TAP controller. If TRST is not used make sure to pull the signal high. ### 17.2. OpenOCD The NEORV32 on-chip debugger can be accessed using the RISC-V port of OpenOCD. Prebuilt binaries can be obtained - for example - from SiFive. A pre-configured OpenOCD configuration file (`sw/openocd/openocd_neorv32.cfg`) is available that allows easy access to the NEORV32 CPU.  You might need to adapt `ftdi_vid_pid`, `ftdi_channel` and `ftdi_layout_init` in `sw/openocd/openocd_neorv32.cfg` according to your interface chip and your operating system.  If you want to modify the JTAG clock speed (via `adapter speed` in `sw/openocd/openocd_neorv32.cfg`) make sure to meet the clock requirements noted in Documentation: Debug Transport Module (DTM). To access the processor using OpenOCD, open a terminal and start OpenOCD with the pre-configured configuration file. Listing 11. Connecting via OpenOCD (on Windows) ``````N:\Projects\neorv32\sw\openocd>openocd -f openocd_neorv32.cfg Open On-Chip Debugger 0.11.0-rc1+dev (SiFive OpenOCD 0.10.0-2020.12.1) Licensed under GNU GPL v2 For bug reports: https://github.com/sifive/freedom-tools/issues 1 Info : Listening on port 6666 for tcl connections Info : Listening on port 4444 for telnet connections Info : clock speed 1000 kHz Info : JTAG tap: neorv32.cpu tap/device found: 0x0cafe001 (mfg: 0x000 (<invalid>), part: 0xcafe, ver: 0x0) Info : datacount=1 progbufsize=2 Info : Disabling abstract command reads from CSRs. Info : Examined RISC-V core; found 1 harts Info : hart 0: XLEN=32, misa=0x40801105 Info : starting gdb server for neorv32.cpu.0 on 3333 Info : Listening on port 3333 for gdb connections`````` OpenOCD has successfully connected to the NEORV32 on-chip debugger and has examined the CPU (showing the content of the `misa` CSRs). Now you can use `gdb` to connect via port 3333. ### 17.3. Debugging with GDB This guide uses the simple "blink example" from `sw/example/blink_led` as simplified test application to show the basics of in-system debugging. At first, the application needs to be compiled. We will use the minimal machine architecture configuration (`rv32i`) here to be independent of the actual processor/CPU configuration. Navigate to `sw/example/blink_led` and compile the application: Listing 12. Compile the test application ``.../neorv32/sw/example/blink_led$ make MARCH=-march=rv32i clean_all all``

This will generate an ELF file `main.elf` that contains all the symbols required for debugging. Furthermore, an assembly listing file `main.asm` is generated that we will use to define breakpoints.

Open another terminal in `sw/example/blink_led` and start `gdb`. The GNU debugger is part of the toolchain (see Software Toolchain Setup).

Listing 13. Starting GDB (on Linux (Ubuntu on Windows))
``````.../neorv32/sw/example/blink_led\$ riscv32-unknown-elf-gdb
GNU gdb (GDB) 10.1
Copyright (C) 2020 Free Software Foundation, Inc.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Type "show copying" and "show warranty" for details.
This GDB was configured as "--host=x86_64-pc-linux-gnu --target=riscv32-unknown-elf".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<https://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.

For help, type "help".
Type "apropos word" to search for commands related to "word".
(gdb)``````

Now connect to OpenOCD using the default port 3333 on your local machine. Set the ELF file we want to debug to the recently generated `main.elf` from the `blink_led` example. Finally, upload the program to the processor.

 The executable that is uploaded to the processor is not the default NEORV32 executable (`neorv32_exe.bin`) that is used for uploading via the bootloader. Instead, all the required sections (like `.text`) are extracted from `mail.elf` by GDB and uploaded via the debugger’s indirect memory access.
Listing 14. Running GDB
``````(gdb) target remote localhost:3333 (1)
Remote debugging using localhost:3333
warning: No executable has been specified and target does not support
determining executable automatically.  Try using the "file" command.
0xffff0c94 in ?? () (2)
(gdb) file main.elf (3)
A program is being debugged already.
Are you sure you want to change the file? (y or n) y
Transfer rate: 43 KB/sec, 2132 bytes/write.
(gdb)``````
 1 Connect to OpenOCD 2 The CPU was still executing code from the bootloader ROM - but that does not matter here 3 Select `mail.elf` from the `blink_led` example 4 Upload the executable

After the upload, GDB will make the processor jump to the beginning of the uploaded executable (by default, this is the beginning of the instruction memory at `0x00000000`) skipping the bootloader and halting the CPU right before executing the `blink_led` application.

#### 17.3.1. Breakpoint Example

The following steps are just a small showcase that illustrate a simple debugging scheme.

While compiling `blink_led`, an assembly listing file `main.asm` was generated. Open this file with a text editor to check out what the CPU is going to do when resumed.

The `blink_led` example implements a simple counter on the 8 lowest GPIO output ports. The program uses "busy wait" to have a visible delay between increments. This waiting is done by calling the `neorv32_cpu_delay_ms` function. We will add a breakpoint right at the end of this wait function so we can step through the iterations of the counter.

Listing 15. Cut-out from `main.asm` generated from the `blink_led` example
``````00000688 <__neorv32_cpu_delay_ms_end>:
688:	01c12083          	lw	ra,28(sp)
690:	00008067          	ret``````

The very last instruction of the `neorv32_cpu_delay_ms` function is `ret` (= return) at hexadecimal `690` in this example. Add this address as breakpoint to GDB.

 The address might be different if you use a different version of the software framework or if different ISA options are configured.
Listing 16. Adding a GDB breakpoint
``````(gdb) b * 0x690
Breakpoint 1 at 0x690``````

Now execute `c` (= continue). The CPU will resume operation until it hits the break-point. By this we can "step" from increment to increment.

Listing 17. Iterating from breakpoint to breakpoint
``````Breakpoint 1 at 0x690
(gdb) c
Continuing.

Breakpoint 1, 0x00000690 in neorv32_cpu_delay_ms ()
(gdb) c
Continuing.

Breakpoint 1, 0x00000690 in neorv32_cpu_delay_ms ()
(gdb) c
Continuing.``````

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 NEORV32 RISC-V Processor
HQ: https://github.com/stnolting/neorv32
Contact: stnolting@gmail.com

### Proprietary Notice

• "GitHub" is a Subsidiary of Microsoft Corporation.

• "AXI", "AXI4-Lite" and "AXI4-Stream" are trademarks of Arm Holdings plc.

• "ModelSim" is a trademark of Mentor Graphics – A Siemens Business.

• "Quartus Prime" and "Cyclone" are trademarks of Intel Corporation.

• "Windows" is a trademark of Microsoft Corporation.

• "Tera Term" copyright by T. Teranishi.

• Timing diagrams made with WaveDrom Editor.

• Documentation made with `asciidoctor`.

### Disclaimer

This project is released under the BSD 3-Clause license. No copyright infringement intended. Other implied or used projects might have different licensing – see their documentation to get more information.

This document contains links to the websites of third parties ("external links"). As the content of these websites is not under our control, we cannot assume any liability for such external content. In all cases, the provider of information of the linked websites is liable for the content and accuracy of the information provided. At the point in time when the links were placed, no infringements of the law were recognizable to us. As soon as an infringement of the law becomes known to us, we will immediately remove the link in question.

### Citing

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

 Contributors ❤️ Please add as many contributors as possible to the `authors` field 😉. This project would not be where it is without them. Full names can be found in the repository’s `.mailmap`.
Listing 18. BibTeX
``````@misc{nolting20,
author       = {Nolting, S.},
title        = {The NEORV32 RISC-V Processor},
year         = {2020},
publisher    = {GitHub},
journal      = {GitHub repository},
howpublished = {\url{https://github.com/stnolting/neorv32}}
}``````
 DOI This project also provides a digital object identifier provided by zenodo: https://zenodo.org/record/5121427

### Acknowledgments

A big shoutout to all contributors, who helped improving this project! ❤️

RISC-V - instruction sets want to be free!