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Installing Apache NuttX POSIX embedded OS on RISC-V based ESP32-C3 with Arch Linux

Table of content

Apache NuttX logo
* Introduction
* System packages
* NuttX sources and tools
* Configuration of devkit project, compilation and flash
* Connexion to NSH via (USB) serial
* The ostest and other basic sets (UPDATE)
* The apps examples (UPDATE)
* SPIflash, SMARTFS and file fsystems (UPDATE)
* NSH scripting (UPDATE)


Apache NuttX is a POSIX embedded system available on a lot of microcontrollers boards and architectures. After seeing some articles from Lup Yuen Lee, installing and working with NuttX on Bouffalo BL602 and BL604 RISC-V microcontroller boards, I discovered it can be installed on one of my boards. So I tried and managed to install it this evening on my recently acquired 3.5€ ESP32-C3S SoC nodeMCU board. ESP32-C3 is a SoC with RISC-V RV32IMC microcontroller, integrated 2.4GHz WiFi and Bluetooth LTE. The board contains a CH340 serial-USB converter, so it can easily be used/flashed/debugged from a computer. I already made a post about installing ESP-IDF tools and flashing examples on this RISC-V board.

For people that already know Espressif SoCs, here is a table of the power usage of some of their ESP models:

SoC        Modem sleep  Light sleep mode  Deep sleep mode
ESP8266          20 mA          2,000 µA            20 µA
ESP32            20 mA            800 µA            20 µA
ESP32-C3         20 mA            130 µA             5 µA

This article explains the procedure to prepare environment, on Arch Linux in November 2021. This is for x86_64, but should work on ARM too, only RISC-V toolchains are missing on ALARM, can be compiled, by using x86_64 versions of PKGBUILD (riscv32-elf-binutils, riscv64-elf-gcc). You can find the pricompiled binaries in my ArchLinux ARM archives including a little text about the order of compilation (binlib, gcc-bootstrap, newlib, gcc (and optionnaly, gcc and newlib again). Direct link to the three most usefull archives:
* riscv32-elf-binutils-2.36.1-2-armv7h.pkg.tar.xz
* riscv64-elf-gcc-11.1.0-1-armv7h.pkg.tar.xz
* riscv64-elf-newlib-4.1.0-1-any.pkg.tar.xz

Latest GIT version is needed In November 2021 for ESP32-C3, some other RISC-V architectures are already in stable releases. This is followed by an example of flashing and connect to the NSH shell, via serial on USB terminal. The dependencies for Debian based Linux on the official page, some parts could be incomplete. Some aspects of the NuttX, POSIX compatible Filesystem. OStest, GPIO and SPIflash included examples are also shortly described.

System packages

General system dependencies for NuttX:

sudo pacman -S --needed base-devel ncurses5-compat-libs gperf pkg-config \
   gmp libmpc mpfr libelf expat picocom uboot-tools util-linux git wget

Just press enter to select all packages on the base-devel packages group.

You also need some AUR packages, I still use obsolete Pacaur that some say is obsolete:

pacaur -S --needed  isl kconfig-frontends genromfs

Sadly for my case, there is currently a conflict between kendryte-toolchain-bin (used for Kendryte K210 RV64 SoC), that depend on isl19, currently conflicting with isl, I uninstalled kendryte-toolchain-bin package, hope it would still work from binary archive with last isl version (0.24 now), didn't managed to compile kendryte-toolchain from sources with it.

Specific RISC-V and ESP32 tools, riscv64 GCC is used to compile for both RV64 and RV32 architectures, but we need RV32 specific version of binutils here.:

sudo pacman -S --needed esptool riscv64-elf-gcc riscv32-elf-binutils

Specific RISC-V and ESP32 tools, AUR part (we don't use it in this article, but OpenOCD (Open On-Chip Debugger) can be useful for debugging):

pacaur -S openocd-esp32

You will need a special trick with GNU compilation toolchain, as the current version of NuttX search for riscv64-unknown-elf-* and Arch Linux call them riscv64-elf-* (without unknown-. I just created symlinks in /usr/bin/. Need root privileges for this (UPDATE: g++ was also needed by some optional examples applications):

sudo bash
for tool in gcc ar ld nm objcopy g++
  if [ ! -e /usr/bin/riscv64-unknown-elf-${tool} ]; then
    ln -s riscv64-elf-${tool} /usr/bin/riscv64-unknown-elf-${tool}

Or a more radical solution, to have a link for all existing riscv64-elf-* tools:

sudo bash
cd /usr/bin/
ls riscv64-elf-* | while read bin
do tool=${bin//riscv64-elf-}
  if [ ! -e riscv64-unknown-elf-${tool} ]
    ln -s ${bin} riscv64-unknown-elf-${tool}

NuttX sources and tools

Choose a directory where you will install the tools. I choose a directory called nuttx here:

mkdir nuttx
cd nuttx

Some binaries are needed to creating the file system. The booting partition and the partition table. You can compile them by yourself, but I here just chosen to download already compiled ones, I would maybe update this post with compilation process:


We will just use bootloader and partition-table binaries here, but there is also mcuboot binary in the repository, not sure I will need it later, but I downloaded it to have everything for working in my archives:


NuttX sources are also needed:

git clone nuttx
git clone apps

Update November 26, version 10.2.0 stable is out this week with ">esp32-c3 available by default, NuttX and NuttX-apps tarballs (list + download link for each version), Warning they are both called nuttx-version.tar.gz (Github is stupid), but they contain respectively incubator-nuttx-nuttx-version and incubator-nuttx-apps-nuttx-version files trees

So you can just download them this way to avoid problems:

wget -O incubator-nuttx-nuttx-10.2.0.tar.gz
wget -O incubator-nuttx-apps-nuttx-10.2.0.tar.gz

And unarchive them this way:

tar xf incubator-nuttx-nuttx-10.2.0.tar.gz
tar xf incubator-nuttx-apps-nuttx-10.2.0.tar.gz
ln -s incubator-nuttx-nuttx-10.2.0 nuttx
ln -s incubator-nuttx-apps-nuttx-10.2.0 apps

So they can be usable the standard way.

Configuration of devkit project, compilation and flash

We have now all necessaries tools to prepare and install a bootable and functional system. We now enter in the nuttx directory:

cd nuttx

This is time to choose a project. You can see a list of existing ones for ESP32-C3 by typing:

./tools/ -L | grep esp32c3

Description of some of them are available in the ESP32 (not C3) specific part of the doc

I choose "usbconsole" that contain NSH shell and allow to easily connect via USB using ttyUSB.

./tools/ -l esp32c3-devkit:usbconsole

Then you can tune some advanced parameters but I didn't used it, looks like ESP32-C3 is generic enough to not have to do anything special depending on board, so can be passed.

make menuconfig

You can quit it by pressing 2 times esc key.

Time to compile it now. We can compile it with only one CPU core of the computer:


Or to compile faster (if you don't have too low memory, else it could be slower), you can can add for example -j4 to use 4 cores or your computer:

make -j4

A long compilation sequence will be printed, ending by the names of the binaries (in colour here). This will be the system to be flashed.

CP: nuttx.hex
CP: nuttx.bin
MKIMAGE: ESP32-C3 binary -c esp32c3 elf2image -fs 4MB -fm dio -ff 40m -o nuttx.bin nuttx v3.2
Generated: nuttx.bin (ESP32-C3 compatible)

For flashing it, you need to first plug your ESP32-C3 based board on your computer, then use flashing command. Parameters needed are chip type, (here an esp32c3), serial port where board is connected to (here ttyUSB0) and transfer rate (921600). Following parameters are the memory address where the binaries will be flashed followed by their names (binary files names are coloured here). So bootloader start at 0, partition table start at 0x8000 and NuttX binary we just compiled start at 0x10000. Warning: bootloader and partition-table need to be flashed only when you install for the first time NuttX, the number of write cycle of a flash is limited so only flash what you need: --chip esp32c3 --port /dev/ttyUSB0 --baud 921600 write_flash \
   0x0 ../bootloader-esp32c3.bin \
   0x8000 ../partition-table-esp32c3.bin \
   0x10000 nuttx.bin

Here is the displayed upload, I coloured (and masked) the MAC address. It can be useful to keep it for later. also colored the flashing of the 3 binaries. v3.2
Serial port /dev/ttyUSB0
Chip is ESP32-C3 (revision 3)
Features: Wi-Fi
Crystal is 40MHz
Uploading stub...
Running stub...
Stub running...
Changing baud rate to 921600
Configuring flash size...
Flash will be erased from 0x00000000 to 0x00004fff...
Flash will be erased from 0x00008000 to 0x00008fff...
Flash will be erased from 0x00010000 to 0x00030fff...
Compressed 19120 bytes to 11416...
Wrote 19120 bytes (11416 compressed) at 0x00000000 in 0.5 seconds (effective 324.4 kbit/s)...
Hash of data verified.
Compressed 3072 bytes to 69...
Wrote 3072 bytes (69 compressed) at 0x00008000 in 0.1 seconds (effective 433.2 kbit/s)...
Hash of data verified.
Compressed 131264 bytes to 51049...
Wrote 131264 bytes (51049 compressed) at 0x00010000 in 1.9 seconds (effective 542.5 kbit/s)...
Hash of data verified.

Hard resetting via RTS pin...

Et voilà! The board is flashed (and restarted). In case of errors here, I wrote few lines about most common problems when flashing microcontrollers or FPGA boards using USB on Linux.

Update: I found a new possible problem. If you are already connected to the board by a terminal, the fuse will fail with the following error:

serial.serialutil.SerialException: device reports readiness to read but returned no data (device disconnected or multiple access on port?)

Just have to quit the terminal application, and restart the previous command, everything should run fine now.

Connexion to NSH via (USB) serial

You can now connect with any serial terminal tool like screen (yes it has this functionality too), gtkterm (with a GUI), or lot of others. I used Picocom as proposed by NuttX documentation.
One of the interesting aspect of NuttX, is that you have an integrated shell called NSH (for NuttShell).

picocom -b 115200 /dev/ttyUSB0

To escape from Picocom you need to type: ctrl-a then ctrl-x. Picocom man page contains all the needed shortcuts.

After connecting to the terminal, just type on enter key to have a NSH prompt, you have an integrated help, with the help command. See the more detailed NSH commands description in the official documentation.

nsh> help
help usage:  help [-v] []

  .         cd        echo      hexdump   mv        set       truncate
  [         cp        exec      kill      printf    sleep     uname
  ?         cmp       exit      ls        ps        source    umount
  basename  dirname   false     mkdir     pwd       test      unset
  break     dd        free      mkrd      rm        time      usleep
  cat       df        help      mount     rmdir     true      xd

Builtin Apps:
  nsh  sh

NuttX is POSIX, so as any UniX environment, every device can be accessed via files. Like on Linux, you have the 2 important directories; /dev containing the devices files, and /proc containing system and devices parameters.

Update: If you are stuck for any reason, you can send a reset signal to the board by pressing two times the F7 function key in the terminal.

nsh> ls
nsh> ls -l /dev
 crw-rw-rw-       0 console
 crw-rw-rw-       0 null
 crw-rw-rw-       0 ttyS0
 crw-rw-rw-       0 zero
nsh> ls -l /proc
 dr--r--r--       0 0/
 dr--r--r--       0 1/
 -r--r--r--       0 meminfo
 dr--r--r--       0 fs/
 dr--r--r--       0 self/
 -r--r--r--       0 uptime
 -r--r--r--       0 version

In the included commands, mkrd allow to create a RAMDISK file system

Update: This was the NuttX version 10.2.0-RC0 c7e604b20b-dirty after cat /proc/version.

The ostest and other basic sets (UPDATE)

The sources of the demo programs can be found in boards/risc-v/esp32c3/esp32c3-devkit/src/ subdirectory and the .config settings, that can also be changed with make menuconfig, for each demo are located in the boards/risc-v/esp32c3/esp32c3-devkit/configs/ directory.

For my second test, I tried esp32c3-devkit:ostest receipe. The system include a command called ostest, that try different systems usage, including memory allocation, timers or multithreading.

If you have already compiled a project and try to compile another one you will have an alert:

Already configured!
Please 'make distclean' and try again.

You only need to follow the instruction and make a distclean to be able to compile another one:

make distclean
[...] <- lot of cleaning
./tools/ -l esp32c3-devkit:ostest

As we already flashed the boot en partition table and number of write cycle of flash are limited (to some thousand of times, look at the flash eprom specification), you can limit the flash to the nuttx.bin part itself: --chip esp32c3 --port /dev/ttyUSB0 --baud 921600 write_flash \
   0x10000 nuttx.bin

You will only be able to see the name of the command by typing help :

nsh> help
help usage:  help [-v] []
[...] <- standard commands
Builtin Apps:
  nsh     ostest  sh

When the ostest finish on this system image, you will see memory used. and the filesystem will have a new directory /var with a subdirectory /var/mqueue added.

nsh> ostest
[...] <= lot of tests
barrier_func: Thread 6 done
barrier_test: Thread 5 completed with result=0
barrier_test: Thread 6 completed with result=0
barrier_test: Thread 7 completed with result=0

End of test memory usage:
======== ======== ========
arena       5d2e0    5d2e0
ordblks         8        8
mxordblk    56710    53ab0
uordblks     5de0     9640
fordblks    57500    53ca0

Final memory usage:
======== ======== ========
arena       5d2e0    5d2e0
ordblks         2        8
mxordblk    58ae0    53ab0
uordblks     47f0     9640
fordblks    58af0    53ca0
user_main: Exiting
ostest_main: Exiting with status 0
nsh> ls var/

As with Linux, you can know the total, used and free memory by displaying the content of /proc/meminfo. The builtin command free do the same thing.

nsh> cat /proc/meminfo
                   total       used       free    largest  nused  nfree
        Umem:     382752       6128     376624     376624     32      1

Among the other tests, some demos that interest me are:
* esp32c3-devkit:gpio to access to GPIO (general purpose Input/Output) via command line.
* esp32c3-devkit:spiflash to access the flash disk using NuttX SMART Flash file system via SPI1. The command mksmartfs can be used to make the FS, and the FS can be mounted the POSIX way (like Linux for example) by typing mount -t smartfs /dev/... /mntpoint. It add mksmartfs command and flash_eraseall and fstest builtin apps.
* esp32c3-devkit:lvgl demo, LVGL is a small graphic and GUI library that help to make interfaces in small memory embedded board systems. Demos include drivers for Sitronix ST7735 (262K Color Single-Chip TFT Controller/Driver) and ST7789 SPI displays controllers in esp32c3_st7735.c and esp32c3_st7789.c in boards/risc-v/esp32c3/esp32c3-devkit/src/. I need to figure out how to connect one. Breadboards will help for this test. Another solution is probably to try it using Qemu?? With this demo the RGB led of the board is set to orange. There is a Youtube video of this demo on an ESP32 (Xtensa) version monothread and multithread, side by side. The ESP32-C3 has only one core. The command in the system is called lvgldemo. Nothing appear on terminal console, so I don't know at all what this demo do. There are some documentation about NuttX and LVGL on the LVGL documentation site. There is also tutorial site.

The apps examples (UPDATE)

We previously downloaded the git apps repository beside the nuttx repository. The apps, contains examples beyond the basic vital functionalities of the system. These resources can be set in the .config file (flags starting with CONFIG_EXAMPLES_) or by using make menuconfig.

In the menu config the apps are in the last entry. Go down with cursor keys and when you are on the last entry, just press enter:

Then the application configuration submenu will display. Warning, you have generally limited resources on the board, select options with parsimony. The best is to first choose the already prepared set for your platform and then to activate some if they work:
* Cryptographic Library Support contains LibTomCrypt and Mbed TLS Cryptography libraries support.
* The Examples subset contains interesting tools like, audio generator (need external dependencies), uIP web server (implementation on uIP TCP/IP stack, that include a DHCP client), tools to blink LEDs (warning PowerLED needs external dependencies), battery monitor, camera driver, the LVGdemo, pdcurses (a GUI text lib).
* The filesystem utilities contains mkgpt, mkbr (for managing fs), a password file support (need external dependencies) etc... :
* The network utilities contains, a chat tool, a sJSON and CODEC libraries, an FTP client, MQTT-C (a MQTT messages client), and a remote execution server and client.
* The NSH library allow you to tune NSH parameters, including MOTD management, a more heavy and complete Command Line Editor, instead of the minimal readline(), or backslash of characters.
* The System Libraries of NSH add-ons, allow to enable system() interface (and so call of NSH function from C), Terminal Curses control support an hexadecimal file editor, or a minimal CU terminal editor. There are also options to use Emacs, VIM or Ncurses mode interface. Zmodem and GPIO u-Block modem., SPI tools, etc...
* Wireless Libraries and NSH Add-ons, contains Bluetoot and IEEE 802.15.4 applications including both version of "Swiss army knife".


The esp32c3-devkit:gpio add a command to access to GPIO (general purpose Input/Output) via command line. It can be set in make menuconfig by Application configuration ---> Examples ---> [*] GPIO driver example, or by setting the .config file.

To have GPIO activated:

# IO Expander/GPIO Support

To have the command example:


With this a new gpio command appear, if you type it without arguments, it will help you (-h argument works too):

nsh> gpio
ERROR: Missing required arguments
USAGE: gpio [-w ] [-o ] 
       gpio -h
	: The full path to the GPIO pin driver.
	-w : Wait for an signal if this is an interrupt pin.
	-o :  Write this value (0 or 1) if this is an output pin.
	-h: Print this usage information and exit.

You can list the available devices (driver path in the help). Here is a long (-l) listing, and you can see they are character devices files with the initial c:

nsh> ls -l /dev
 crw-rw-rw-       0 console
 crw-rw-rw-       0 gpint0
 crw-rw-rw-       0 gpout0
 crw-rw-rw-       0 gpout1
 crw-rw-rw-       0 null
 crw-rw-rw-       0 ttyS0
 crw-rw-rw-       0 zero

In the example source boards/risc-v/esp32c3/esp32c3-devkit/src/esp32c3_gpio.c, we can see than the pins 1 and 2 are set to the two first gpout, here gpout0 and gpout1 and that only the pin 9 is used for the interrupts gpint0:

/* Pin 1 and 2 are used for this example as GPIO outputs. */

#define GPIO_OUT1  1
#define GPIO_OUT2  2
/* Interrupt pins.  GPIO9 is used as an example, any other inputs could be
 * used.

#define GPIO_IRQPIN  9

Here is a map of the pins on my board, thanks to J-C. François, licence CC-BY-SA:

The pins 1 and 2 are at the upper left, and the pin 9 at the upper right

The current state of a GPIO can be know by a cat. here for example 0 state (the result is without return carriage so just touch by nsh:

nsh> cat /dev/gpout0

Writing a value 1 to the first pin defined (so pin 1) the example display the current value, then change it and verify its state:

nsh> >gpio -o 1 /dev/gpout0
Driver: /dev/gpout0
  Output pin:    Value=0
  Writing:       Value=1
  Verify:        Value=1

SPIflash, SMARTFS and file systems (UPDATE)

The SPIflash recipe ( esp32c3-devkit:spiflash ), add the commands mksmartfs and the builtin apps flash_eraseall and fstest.

To use fstest, you need to prepare the fs and mount it.

In the default included commands, mkrd allow to create a RAMDISK file system. It can be used for tests and avoid to waste flash writing cycles.

The syntax is (help mkrd to display it):

mkrd usage:  mkrd [-m ] [-s ] 

Default secteur size is 512 bytes. We can simply create a 10KB RAM disk, so 512×20 = 10240 bytes = 10 kilobytes bytes, by typing:

mkrd 20

A ramdisk device will appear in /dev called ram0. Filesystems are blocks devices (shown by the initial b, c is character device):

nsh> ls -l /dev
 crw-rw-rw-       0 console
 crw-rw-rw-       0 null
 brw-rw-rw-   10240 ram0
 brw-rw-rw-  983040 smart0
 crw-rw-rw-       0 ttyS0
 crw-rw-rw-       0 zero

You can also see the smart0 device corresponding to the disk, is also available with the SPIflash recipe. sadly RAM disk isn't supported by smartFS so it can't be formatted as is. VFat is supported, but need to add it at configure time. You can activate TMPFS (see below) that also use RAM, as on Linux, for working on temporary RAM disk.

* SmartFS support can be activated by make menuconfig in File Systems ---> -*- SMART file system
* smartfs application can be selected by make menuconfig in Application Configuration ---> File System Utilities ---> mksmartfs.

Flags set in .config, for SMARTFS support:


And for command utilities:


smart0 need to be formated before mounting it. The data will be kept on it after reboot, or flashing a new code.

nsh> mksmartfs /dev/smart0

In POSIX systems, we need to create an empty directory used as a mount point, then mount the partition. The type of filesystem isn't auto-detected, so we need to pass it to mount command here:

mkdir /mnt
mount -t smartfs /dev/smart0 /mnt

It will make a test loop of 100 iterations, of writing and then deleting files in the mountpoint, I put the ouput. If you really want to test the fs you can use it, but you will waste write cycles of the flash :

The commande is:


Here is a sample of the output, I stopped it by resetting the card (2 times F7:

=== FILLING 4 =============================
168. Type[8]: File  Name: iJaUkEIwgSG4cAyl8J
169. Type[8]: File  Name: FNLd0XaTSYF8TV3YtO
170. Type[8]: File  Name: SV0Lrtmigq9Yg4SauoD4f
171. Type[8]: File  Name: GJfbQ0bFaowO0ep
Total file size: 427496

=== DELETING 4 ============================
Deleted some files
  Number of files: 215
  Number deleted:  108
 1. Type[8]: File  Name: PRMIRHz9ZwgHga
 2. Type[8]: File  Name: 9LH4Uf67RL4bFXVCykg3
Total file size: 427496
File System:
  Block Size:      1024
  No. Blocks:      960
  Free Blocks:     457
  Avail. Blocks:   457
  No. File Nodes:  0
  Free File Nodes: 457

End of loop memory usage:
======== ======== ========
arena       58fb0    58fb0
ordblks        41       57
mxordblk    510a0    510a0
uordblks     77d0     6f40
fordblks    517e0    52070[...]

As I stopped it during the loop the files are still on the fs. Wildcard are not supported by the rm command.

As we can see with the df (disk free) command, after deleting some file, this still uses lot of blocks:

nsh> df
  Block    Number
  Size     Blocks       Used   Available Mounted on
  1024        960        725         235 /mnt
     0          0          0           0 /proc

I used the flash_eraseall builtin app to erase evrything:

nsh> flash_eraseall /dev/smart0

This remove the files but don't free the blocks. I needed to force a mksmartfs with the option -f to free the blocks:

It removed the files but didn't cleaned the block. I need to format again the device using:

nsh> mksmartfs -f /dev/smart0

Now All the blocks are available again:

nsh> df
  Block    Number
  Size     Blocks       Used   Available Mounted on
  1024        960         10         950 /mnt
     0          0          0           0 /proc

For example, write a single file to be kept after reboot. we can write the content of the file with cat or echo commands for example. I write here a simple sentence this is kept datas in the test.txt file, using the file redirection character (>):

nsh> echo "this is kept datas" >/mnt/test.txt
nsh> df
  Block    Number
  Size     Blocks       Used   Available Mounted on
  1024        960         11         949 /mnt
     0          0          0           0 /proc

As you can see this took an entire 512 bytes block even if it's an only 18 bytes string file. Every file written use a full block, so try to choose the sector-size when you use mksmartfs.

After rebooting the device (2 times F7) or unplug/replug it. You can still see the data on the file. Note that after each reboot, you need to mount afain the filesystem:

nsh> mount -t smartfs /dev/smart0 /mnt
nsh> cat /mnt/test.txt
this is kept datas

Removing files by rm command effectively free the blocks:

nsh> rm test.txt
nsh> df
  Block    Number
  Size     Blocks       Used   Available Mounted on
  1024        960         10         950 /mnt

Among the other available file systems in NuttX, there is LittleFS, CNX-Software made an article about it. It is very compact and reliable, but there is no utilies to manage them in applications, need to use libs or code a shell compatible command. The Source Git repository shows an exemple usage in C

By make menuconfig select Board Selection ---> [*] Mount SPI Flash MTD on bring-up (LittleFS) ---> (X) LittleFS (Only one file system can be set at a time here), and

In the .config, if the file sustem is not set for the board:



For the more general LITTLEFS support, by make menuconfig, in File Systems ---> -*- LITTLEFS File System (several can be selected in this part).

Options in .config:


With this setting (after selecting SPIflash), there is a esp32c3flash device that replace smart0.

And The smartFS can't be mounted. The error message is a bit erroneous:

nsh> mount -t smartfs /dev/esp32c3flash /mnt
nsh: mount: mount failed: No such device


If you choose in make menuconfig the option File Systems -> [*] TMPFS file system

Default TMPFS option in .config


You will see an automounted /tmp, it is 512 blocks wide on my board (so 512×512/1024=512/2=256KB of RAM disk. 1 block is used for the file system root directory (mounted in /tmp):

nsh> df
  Block    Number
  Size     Blocks       Used   Available Mounted on
     0          0          0           0 /proc
   512          1          1           0 /tmp

TMPfs allow to use the free memory, but don't really use it until your write files inside:

nsh> cat /proc/meminfo
                   total       used       free    largest  nused  nfree
        Umem:     364464      10016

354448 354448 38 1

I cd (change directory) in /tmp here and write a file, it will take one more 512 bytes block and so, 10016 + 512 (block) + 176 (probably some references in filesystem dictionnary) = 10704 bytes of memory will be used. The size of the refs vary it took only 96 bytes on another test.

nsh> cd /tmp
nsh> echo "This is a test" >test
nsh> df
  Block    Number
  Size     Blocks       Used   Available Mounted on
     0          0          0           0 /proc
   512          2          0           2 /tmp
nsh> cat /proc/meminfo
                   total       used       free    largest  nused  nfree
        Umem:     364464      10704     353760     353744     42      2

NSH scripting (UPDATE)

You can make simple shell script in NSH. For exemple this one lie exemple test if /dev/ram0 exists and then print the result of the test.

if [ -e /dev/ram0 ]; then echo "ram0 exists"; else echo "no ram0"; fi

Variable can be set and unset

nsh> set foo bar
nsh> echo $foo

script can be put in files. I didn't found an easy way to put them. The cat function, seems to be limited to go from one file to another, it doesn't manage STDIN a standard way, and there is no editor. The only solution is to echo line by line, or to transfer files by network or serial (need to search more).
If you have activated the TMPFS as explained previously or formatted and mounted SMARTFS partition, you can write files inside. Here is an exempla with TMPFS, available in /tmp by default I removed the initial nsh> here, so it will be easier to copy paste:

echo if [ -e /dev/ram0 ] >/tmp/ 
echo then >>/tmp/
echo "  echo ram0 exists" >>/tmp/
echo else >>/tmp/
echo "  echo ram0 doesn't exist" >>/tmp/
echo fi >>/tmp/

The >> symbol means you append lines to an existing file instead of overwriting it.
you can now see the script by a cat:

nsh> cat /tmp/
if [ -e /dev/ram0 ]
  echo ram0 exists
  echo ram0 doesn't exist

And it can be executed by two means, sh or source, the difference,

nsh> sh /tmp/
ram0 doesn't exist

Now, if we create ram0 by using mkrd:

nsh> mkrd 20
nsh> sh /tmp/
ram0 exists

Using ESP-IDF with RISC-V ESP32-C3 nodeMCU module on a Linux system

>NodeMCU ESP32-C3S”></a></p>
<h2 id=Table of Content

* Introduction
* Step 1, install dependencies
* Step 2, install ESP-IDF
* Step 3, setup working environment
* Step 4, test “Hello World” example
* Step 5, tune the target, if needed
* Step 6, flash the project
* Step 7, viewing the output
* Troubleshooting
** Invalid value for ‘{esp32|esp32s2}’: ‘esp32c3′ is not one of ‘esp32′, ‘esp32s2′
** libsodium/crypto_box/curve25519xchacha20poly1305/box_seal_curve25519xchacha20poly1305.c
** Flashing errors
*** Detection, power, and device that disappear
*** usbfs: interface 0 claimed by ch341 while ‘brltty’ sets config #1
** Problem with CA certificates bundle


The list of existing ESP32-C3 SoC modules (with their specs) is very interesting, This one RISC-V core SoC has lower computing power than the ESP32-S3 (dual-core Xtensa ISA based, with a RISC-V co-processor core for “ultra low power” (ULP) mode), but RISC-V architecture is also far more efficient and will for sure have longer lifetime support and evolution due to its openness. ESP32-C3 use an RV32IMC (Integer, Multiplication/division and Compressed extensions) core.

I bought for my test a ESP-C3-32S-Kit on AliExpress (for ~3.4€). There is also an interesting option, where you can use really tiny 3~4 € ESP-C3-01M-Kit programmer interface/mother board, with a SoC included alone on a daughter (ESP32-C3M with an on PCB antenna). This allows, to flash several pads one only one programmer board. This reduce size of the needed system for final application to just the SoC and it’s power supply and peripherals. Pads of the SoC daughter board are wide enough to be easily soldered.

ESP-IDF is a tool to develop in C language on Espressif platforms, including ESP8266 and ESP32 series. It uses FreeRTOS free and open source embedded real-time OS for its SoCs. Sadly the official documentation for installing it is incomplete with the current state (November 2021) of GIT repository. Here is the mean to install a working ESP-IDF for this SoC series on Linux. This will be more compact and straight forward that the official with Linux+Windows installation.

About RISC-V more generally, I created this week a list of available Open Source RISC-V implementations, and open sources tools to build them or work with them.

Step 1, install dependencies

Instruction for Arch Linux, CentOS and Debian, based Linux systems:

* Debian/Ubuntu/…

sudo apt-get install git wget flex bison gperf python3 python3-pip python3-setuptools cmake \
  ninja-build ccache libffi-dev libssl-dev dfu-util libusb-1.0-0

* CentOS

sudo yum -y update && sudo yum install git wget flex bison gperf python3 python3-pip python3-setuptools \
  cmake ninja-build ccache dfu-util libusbx

* Arch Linux/Manjaro/…

sudo pacman -S --needed gcc git make flex bison gperf python-pip cmake ninja ccache dfu-util libusb

Step 2, install ESP-IDF

This will take at least 1.2GB of disk space (size of the git clone at the time I write this article).

mkdir -p ~/esp
cd ~/esp
git clone --recursive

They didn’t mention, that 4.3 is required to use ESP32-C3. At the time I tried it, version 4.2 is currently (November 2021) the default branch, you can see the current version by this commands. Warning: This start the environment, so you will probably need to log again or simply start another shell to avoid problems when finishing the installation.:

cd ~/esp/esp-idf
. ./ --version

So you need to checkout the 4.3 branch to have a working version with ESP32-C3, and then update submodule to match the new version:

cd ~/esp/esp-idf
git checkout release/v4.3
git submodule update --init --recursive

Then you need to install some binaries/libs. Warning, it will create a ~/.expressif directory to place all downloaded files inside, don’t forget to create the directory + a link to a partition with enough place (about 1GB needed for all versions).

cd ~/esp/esp-idf
./ esp32c3

After the installation, the following command is suggested to enter in the working (python pyenv) environment (don’t forget the first . and space)

Step 3, setup working environment

. ./

A message will be displayed that explain to you how to compile a program:

Done! You can now compile ESP-IDF projects.
Go to the project directory and run: build

They suggest in the documentation to add an alias to your .bashrc:

alias get_idf='. $HOME/esp/esp-idf/'

So you will only have to type get_idf to get ready to use it.

Step 4, test “Hello World” example

I would personally make it this way:

cd ~/esp/
mkdir tests
cd tests
cp -a ~/esp/esp-idf/examples/get-started/hello_world .
cd hello_world set-target esp32c3

Errors possible, see:
* Invalid value for ‘{esp32|esp32s2}’: ‘esp32c3′ is not one of ‘esp32′, ‘esp32s2′
* libsodium/crypto_box/curve25519xchacha20poly1305/box_seal_curve25519xchacha20poly1305.c

Step 5, tune the target, if needed

You need now to use menuconfig if you want to setup some parameters, default one should just work fine. menuconfig

This show you an interactive cli menu interface like for the Linux kernel, that allow you to tune some parameters. You don’t need to tune them for this test.

You can navigate with cursor keys, validate or enter submenus with enter, and quit them without modification with esc key. When you quit it, it will say you if you didn’t done any modification:

No changes to save (for '~/esp/test/hello_world/sdkconfig')

Step 5 build the project

Now as everything is setup, you only need to build the project, this could take some time, as it, at first time, compile, the whole embedded system: build

It will end with a message displaying the location of the binary to be flashed and the instruction to type to flash it, the long explicit way and the short implicit way (I coloured them here):

Generated >/data/arc/esp/test/hello_world/build/hello-world.bin

Project build complete. To flash, run this command:
/home/popolon/.espressif/python_env/idf4.3_py3.9_env/bin/python ../../esp-idf/components/esptool_py/esptool/ -p (PORT) -b 460800 --before default_reset --after hard_reset --chip esp32c3  write_flash --flash_mode dio --flash_size detect --flash_freq 80m 0x0 build/bootloader/bootloader.bin 0x8000 build/partition_table/partition-table.bin 0x10000 build/hello-world.bin
or run ' -p (PORT) flash'

* Problem with CA certificates

Step 6, flash the project

If you didn’t have plugged your board in the USB port it’s the time to do. Then (by default white colour RGB led is enlighten. Type then the suggested command. I choose the short way here: --port /dev/ttyUSB0 flash

It should now do the flashing sequence. I show only some important steps. In color you MAC address is given, it can be useful to detect if everything run fine.

Serial port /dev/ttyUSB0
Chip is ESP32-C3 (revision 3)
Features: Wi-Fi
Crystal is 40MHz
Uploading stub...
[...] <- lot of output
Writing at 0x00010000... (20 %)
[...] <- lot of output
Hard resetting via RTS pin...

Read this if you have some troubles at flashing.

Step 7, viewing the output

You can connect to the board, still with its ttyUSB0 port, need to connect at speed 115200. I previously wrote an article about how to control a remote board, including serial

I use TinySerial here:

com /dev/ttyUSB0 115200

You can also use to monitor the output like this ( monitor shortcuts and launching GDB with GDBStub: -p /dev/ttyUSB0 monitor

In both case, you will see a booting sequence, an Hello World! some characteristics of your board, then a 10 seconds counter and a reboot. This is looping until you unplug or flash again the board.

Hello world!
This is esp32c3 chip with 1 CPU core(s), WiFi/BLE, silicon revision 3, 2MB external flash
Minimum free heap size: 327588 bytes
Restarting in 10 seconds...
Restarting in 9 seconds...
Restarting in 8 seconds...

You can quit by typing: the 2 keys ctrl-c followed then by a

Building, flashing and monitor can be combined by one command: -p /dev/ttyUSB0 flash monitor


I don’t want to pollute too much the reading of the workflow and keep it as reference, so people can found answer when they encouter the same errors.

Invalid value for ‘{esp32|esp32s2}’: ‘esp32c3′ is not one of ‘esp32′, ‘esp32s2′

If you have the following error, you didn’t checkout the 4.3 branch at Step 2

Usage: /data/arc/esp/esp-idf/tools/ set-target [OPTIONS] {esp32|esp32s2}
Try '/data/arc/esp/esp-idf/tools/ set-target --help' for help.

Error: Invalid value for '{esp32|esp32s2}': 'esp32c3' is not one of 'esp32', 'esp32s2'.


If during the set-target you have the following error:

CMake Error at ~/esp/esp-idf/tools/cmake/component.cmake:475 (add_library):
  Cannot find source file:


This mean you didn’t made the git submodules update in Step 2. So return in ~/esp/esp-idf and git submodule update.

You will then need to remove the hello_world directory and copy it again in tests: cd ~/esp/tests/; rm -R hello_word; cp -a ~/esp/esp-idf/examples/get-started/hello_world ..

Flashing errors

There are several problems possible for flashing part, most of them are about ttyUSB0 detection/usage.

Detection, power, and device that disappear


First look if your board is detected by lsusb.

* If the device isn’t detected at all, verify if you USB wire is not a powering only wire. Try by connecting other USB devices, if all of them aren’t detected by the wire, this is probably the case, if you use a wire for your phone with the same kind of connector, try if it detected with it, then your board.

* Another case, a board that worked perfectly the first day, was not detected any more the second day. I looked at it with artisanal microscope using phone zoom + cheap lens, and seen a hair or wire, like on the detail picture at left, just above two pins of the USB connector. After removing it, the board work again.

* If the interface is displayed by lsusb but disappear when flashing it, try to disable power saving (autosuspend) for this device or all USB devices, lots of old mother boards have problems with USB devices power saving. Some mouses keep stuck, some flashable devices disappear, etc…

usbfs: interface 0 claimed by ch341 while ‘brltty’ sets config #1

If it fails with a long error message including around the 10th line:

FileNotFoundError: [Errno 2] No such file or directory: '/dev/ttyUSB0'

Then there is a problem with the USB detection in your system configuration.

You can dig in kernel message with dmesg | less then search for ttyUSB0 (just type /ttyUSB0 to found the problem.

In my case for example, the brtty (tty for braille) driver tried to get the serial device and then disconnected it:

[36767.028312] usb 1-1.6: ch341-uart converter now attached to ttyUSB0
[36767.252867] audit: type=1130 audit(1637441179.890:468): pid=1 uid=0 auid=4294967295 ses=4294967295 msg='unit=brltty-device@sys-devices-pci0000:00-0000:00:1a.0-usb1-1\x2d1-1\x2d1.6 comm="systemd" exe="/usr/lib/systemd/systemd" hostname=? addr=? terminal=? res=success'
[36767.878792] audit: type=1130 audit(1637441180.513:469): pid=1 uid=0 auid=4294967295 ses=4294967295 msg='unit=brltty@-sys-devices-pci0000:00-0000:00:1a.0-usb1-1\x2d1-1\x2d1.6 comm="systemd" exe="/usr/lib/systemd/systemd" hostname=? addr=? terminal=? res=success'
[36767.883002] usb 1-1.6: usbfs: interface 0 claimed by ch341 while 'brltty' sets config #1
[36767.883975] ch341-uart ttyUSB0: ch341-uart converter now disconnected from ttyUSB0
[36767.883998] ch341 1-1.6:1.0: device disconnected

You can in this case not the device id with the following command (the ID in the result is coloured here:

lsusb | grep CH340
Bus 001 Device 005: ID 1a86:7523 QinHeng Electronics CH340 serial converter

Then edit the 90-brltty-device.rules:

>code>sudo vi /usr/lib/udev/rules.d/90-brltty-device.rules

Search the line containing 1a86/7523 by typing /1a86/7523, then comment the line by adding a # at start:

#ENV{PRODUCT}=="1a86/7523/*", ENV{BRLTTY_BRAILLE_DRIVER}="bm", GOTO="brltty_usb_run"

save and quit by typing :x if you use vi/vim/nvim… and then reload udev rules by typing:

systemctl reload systemd-udevd

It should now be ok, by just unplugging and replugging the USB wire from your computer.

Generally all the kind of (tty)USB connection problem can be solved by tuning /etc/udev/rules.d/

Problem with CA certificates bundle

With the 4.2 version of ESP-IDF, some FAILED are thrown in relation with a x509_crt_bundle and cacrt_all.pem invalid certificate.

Updating the Certificate Bundle section has a link to himself as solution. ^^. Looks like the problem is on ESP-IF 4.2, 4.3 runs just fine, see install + upgrade process.

My system CA-certificates are already up to date but it want a specific version to put somewhere (not documented) to be able to push it on the board firmware. Ca-certificates, are certificates about CA (Certificate Authorities) that you need to be sure that this authorities are still be considered as valid. anyway ESP-IF 4.3 works just fine.

Short and practical introduction to FPGA, Verilog and Verilator and few words about SystemVerilog

→ Version en français ici

Warning, I’m a less than one month beginner in Verilog, Verilator et FPGA, studied as a hobby, if there are some mistakes I will correct it. You can contact me on The Fediverse.

Table of Content

* Introduction
** Real world full process
** Things to know and understand
** How to code a FPGA
* Verilog
** Values
** Types
** Gates
** Modules
** Simple example, writing an “and” gate
** Initial and always blocks
* Simple example with Verilator
** Make the test with Verilator
** Tracing example and GTKWave
** About Verilator examples
** Basic practical example with Verilator
* Further reading

The text in strong are here to help diagonal reading.


Still continue to go to lower layers with the world of FPGA (Field Processors Gateway Array). This is a reprogrammable development electronic tool used to build and test processor. After the processor is validated in FPGA, you can start to build ASIC (Application-specific integrated circuit), actual hardwired processors that we use every-days in our computing devices. FPGA are also used as is in several industrial appliance (avionics, audio or video processing, etc…) for their parallelism, so the fact they are faster than a general purpose ASIC and a piece of software in these cases, and the ability to update them easily in case of problem. This post is a little introduction about FPGA, the popular IEEE standard Verilog HDL (“Hardware description language”) language and how to test it with free and open source software (FOSS) Verilator simulator. If you want to use VHDL, GHDL is FOSS simulator for VHDL.

Real world full process

The steps to implement a circuitry on an FGPA after it’s design are (Opensource softwares are given as reference for each step):
* Implementation of the logic (on an HDL, “Hardware description language”), so Verilog here, any text or code editor can be used (I use amphibian VIM and NeoVIM and sometime Geany light GUI but powerful IDE.
* SimulationVerilator here, making testbench, it’s already a good step in verification too.
* Formal verificationYoSYS is an open source verification and synthesizer system, so it’s more formal verification than Verilator. We will not go through this point here.
, Synthesis — making routes and placement of logic blocks, with physical and time constraints (see basic principle and today implementations) blocks from HDL script and output a bitstream in the FPGA format, YoSYS again.
* FlashingopenFPGAloader make the job on most FPGA.
* Real world testing on FPGA.

We will only test on simulator here, for understanding the basis. When you are ok, with this, you can go further, choose a FPGA, and start working with it.

Things to know and understand

The main concept to understand and be able to manipulate to build a basic integrated circuit are:
* 0, 1: electricity moving across a wire or not, that can be intrepted as 1 and 0 sate, true and false, etc…
* Boolean operators (for logical gateway), that is the main tool to manipulate them. You don’t need to have other notions of electronics. Most basics one are OR, AND, NOT, and XOR (exclusive OR). You can made all this operator with a NAND gateway (AND+NOT), but this not the more efficient way of doing them. If you master algebraic properties of booleans and know how to simplify them, you will be able to produce simpler circuit and do more efficient one, but it is not required to make circuitry.
* ALU (arithmetics logical unit), allowing simple additions
* flip-flop: special circuitry used to memorize information and change it depending on input and clock edge.
* latch: sequential logic circuitry does not depend on clock
* clock: Time, operation are time dependant and need to be synchronized to have a coherent behaviour. Clocks are like the drummer in a music band, It give the rhythm basis necessary to every time related construction, other instruments, singers, and dancers follow his time signals. Called clock signals here.
* Multiplexers (MUX) that can aggregate several lines in one, and demultiplexer that made the opposite.
These functions are all integrated in each logical block of a FPGA.

Other important part of a FPGA are:
* One to few PLL (Phase-locked loop) that allow to have secondary clocks ticks at frequencies multiples of the main clock or to sync on external clocks.
* A flash memory that allow to keep the bitstream on the board after a shutdown, it will be automatically loaded at start.
* Lots of peripherals I/O blocks to communicate with the outside world.

How to code a FPGA

There are three main methods for developing FPGA circuitries, and both method can be mixed:
* Drawing them with graphical tool representing this logical doors and other basic tools mentioned (flip-flop, latch, MUX, etc).
* Using a HDL (hardware description language), like Verilog or VHDL for the most famous. Compilers are here to build all the stuff behind this languages, as there are assembler to transform human easily readable assembly code to machine code, or higher level language (C/C++, Python, Lua, JavaScript, FORTRAN, BASIC, Pascal, Camel, LISP, etc) compiler or interpreter to convert them to machine code. There are also higher level language like Chisel, that are higher level description languages.
* Using OpenCL (Open Computing Language), designed mainly for intensive computing, that can be spread simultaneously on GPGPU and CPUs.

When using HDL, tools are here to automatically computing the routing circuitry for you. Tools like VTR (Verilog To Routing) and NextPNR, optimize their placement for you.

Symbiyosys is a verification tool.

OpenFPGAloader is a FOSS software used to program/flash/fuse the FPGA itself.

You can do very interesting things only few hours after start study of HDL, it allows to design simple circuitry with code, near from some higher level languages, with variables, constants, registers, conditional tests and basic booleans transformations, additions and bits manipulations. A register is already itself a circuit of several, basic logical gates. They are generaly made as ASIC inside FPGA, to accelerate circuit and reduce power usage.


So we choose here Verilog (Official IEEE Std 1364-2001 specifications), it is well spread with lot of tools, VHDL is also a very famous one, anyway it is always interesting to look at alternatives, that could have interesting aspects. I choose it as a starting language, because used in few projects and examples I’m interested to, and, there are all the FOSS tools needed to write, simulate, verify. There are also several light RISC-V implementations for embedded (SERV and PicoRV32 (Github) (including Sipeed Lichee Tang (20€ FPGA board) implementation (I actually choose a 12€ Sipeed Tang Nano 4K as first board, The Tang Nano (without 4K) cost 3€ but only have 1000 logical blocks. There are also the very convenient and efficient Verilator simulator and VTR (Verilog to Routing, route optimizer.


By defaults, numbers are integers, it is possible to force the numbers of bits (before the single quote ' character) and choose signed (s character, signed meaning able to use minus sign) or unsigned (default). Values can be in b binary, o octal, h hexadecimal, and even force d decimal (case by default). The datas can also be given in string format.

32'hFFFF00FF  // 32 bits unsigned hexadecimal opaque yellow (full red+green+alpha) RGBA
8'b00101010   // 42 (8bit unsigned binary)
8'b0010_1010  // 42 (8bit unsigned binary) easier to read version
8'sb1010_1010 // -86 (8bit signed binary)
-8'sd42       // -42 (8bit signed decimal)
12'o0754      // -rwxr-x-r-- rights (12 bits unsigned octal) on Unix compatible filesystems.
"Hello"       // a character string

Constants can be defined by preceding them by simple quote followed by define 'define:

'define myvalue -8'sd42;


There are two different class of types, network and logic. We cover here :
* wire, that is of network type, just implementing a wire between components.
* reg, that is a logical type, register, that can be set or unset to memorize values.

wire w; // a simple wire between 2 components called w
reg r0; // a 1 bit memory called r0 (Register 0)

Value of a wire is set dynamically by the components linked to it, but it is possible to hard set them a permanent value with the assign function:

assign a = 1;

There are also higher level types that will use several of previous items, as for example:

integer i,j; // Two signed 32bits integers called i and j
real num;    // one 64bits float called num
time t;      // one 64bits unsigned integer representing time (in clock ticks)
realtime rt; // one 64bits float representing time

About time, it is interesting to know that #integer can be used to wait n clock ticks. No semi-column (;) must be put after the value, instead the instruction that follow the delay is generally wrote on the same line. For example:

#20 out = a & b;// wait 20 ticks then compute out value from a AND b

Simple bit elements, wire and reg can be organized in vectors, their bits are noted from MSB (most significant bit) to LSB (less significant bit).

wire [7:0] Bus0; // a 8 bits bus called Bus0 connected to the system
reg [31:0] R0;   // a 32 bits register called R0

It is possible to use arithmetic expressions to define them, in this cases, using array (see bellow) is better:

reg[8*256:1] string;   // 256 bytes string, * to simplify usage, start from 1, as will not be accessed by bit
reg[16*16-1:0] sprite; // a 16×16 monochrome sprite, better to use -1 and start by 0

After these declaration, we can access to unique bits or groups of bits the following way:

Bus0[0];   // Access to LSB of Bus0
Bus0[7:5]; // Access to 3 MSB of Bus0 (defined as 8 bits from 0 to 7)

Verilog also allows to concatenate several bits ranges to another one. For example, here, to convert from 16 bit Little Endian to 16 bit Big Endian data format:

BigEndian[15:0] = {LittleEndian[7:0], LittleEndian[15,8]};

If the destination bus is 16 bits here, then [15:0] is not mandatory for the left part as we concatenate two 8 bits values).

And all this types can be organized in arrays.

Both complex types (in the general meaning, not mathematics complex numbers) and vectors can be organized in arrays.

reg[7:0] cursor[0:7];            // 8x8 pixels monochrome cursor bitmap, Here, cursor is the name
reg[15:0] sprite[0:15];          // 16*16 pixels monochrome sprite bitmap
reg[31:0] palette[0:15];         // 16 RGBA8 colours palette
reg[31:0] FrameBuffer[0:307199]; // 640×480 RGBA8 screen frame-buffer

Arrays are noted from their lower address to higher one.


Here is available logical gates (or operators) in the language, they are bitwise, but can be applied bit-to-bit to all bits of a bit vector:

~a     // NOT
a & b  // AND
a | b  // OR
a ^ b  // XOR
a ~^ b // XNOR, can also be wrote ^~

Gates Look-up tables

There are also two bitwise shift operators available:

>> // Right shift
<< // Left shift


Verilog is organized by module (like classes in object programming, or an integrated circuit on a PCB). A module has:
* A module name
* It’s interface between its inside and outside world (think to public variables or methods in object programing or IC pins in electronics), is a list of ports defined between parenthesis as for functions in a functional language module module_name (wire a, wire b, wire out); ... endmodule.
* By default, declaration are logic blocks, executed in parallel.
* For executing code sequentially and conditionally, always (and initial in testbenchs) sub-block must be used (see below for more details).

Simple example, writing an “and” gate

Basic operators are already present in Verilog, but see here how it could be wrote. In Verilog boolean functions, the left argument is the output. So an and gate could be reimplemented this way:

module and(f,a,b); // The ; is not an error here
  output f;
  input a,b;

  assign f = a & b; // ^ is an AND gate

This example should be put in an "and.v" file.
* The default type is wire
* Since Verilog 2005, input output can be put in the module header like this:

module and(output f, input a,b);

This example is a permanent unconditional gate, you can use them to do full circuitry, but Verilog is an high level language allowing to code more complex things a simple way. The always blocks is here for more complex gatewares.

Initial and always blocks

There are two types of blocks to know to start in Verilog:
* always block can be synthesized, so used in circuitry and executed each time conditions are met, so triggered by an event given between parenthesis always @(event).
* initial block can’t be synthesized, so used in testbench only and executed only one time and unconditionally. They allow to give initial values of the test. It is possible to have several initial blocks. This also allow to test/prototype quickly a simple function with text output, before put in a circuitry always block.

They both allow to:Posedge and negedge on clock signal
* execute code sequentially
* if else conditionals
* while, for, loops

A typical event to trigger an always can be, for example, a positive clock edge (posedge) or negative clock edge (negedge)

always @(posedge clk)

but can be a conditional logic test

always @(a or b)

Simple example with Verilator

A simple example given with Verilator:

module our (clk);
  input clk;  // Clock is required to get initial activation
  always @(posedge clk)
    begin $display("Hello World"); $finish;

Here, the always block is triggered at first positif clock edge (posedge), it print ($display) an “Hello World” string on the standard output, and finish ($finish) the simulation.

It is possible to test and validate lot of things with Verilator before going to further in FPGA process (Formal verification, converting to bitstream (that can be very slow with routing process) and flashing the FPGA). A good start is to make a copy of the above given example, and to modify the top.v file.

Make the test with Verilator

The example are installed on:
* Arch Linux or Manjaro: /usr/share/verilator/examples/
* Debian or Ubuntu: /usr/share/doc/verilator/examples/

So to test it, simply copy the directory to your home directory, where you will have write access, and then, build and execute with a simple make (Verilator transpile it in C and then compile it, this allow to have a very fast simulation):

On Arch Linux based distribution:

cp -a /usr/share/verilator/examples/make_hello_c ~/

On Debian based distribution:

cp -a /usr/share/doc/verilator/examples/make_hello_c ~/

Then on any distribution:

cd ~/make_hello_c

make allows to rebuild from the sources if needed, and then execute the testbench immediatly.

The output of the bench itself:

code>-- RUN ---------------------
Hello World!
- top.v:11: Verilog $finish
-- DONE --------------------

The following output text (I didn’t paste here) is about compilation and suggestiong to use next tutorial.

Tracing example and GTKWave

GTKwave is companion tool to Verilator. It allows you to see the chronogram resulting from the simulation, to understand the temporal functioning of the circuit, and to see what could be the source of a possible problem. It uses the .vcd (Value Change Dump) file, for tracing. It is used as source to draw the chronogramme (see make_tracing_c example, output can be found in the logs/ subdir. The file is logs/vlt_dump.vcd, created after using the make command).

To use the tracing example with GTKwave, you need to install the gtkwave package, then, as for first example, duplicate the directory to a place you can write into:
* On Arch Linux based distribution:

cp -a /usr/share/verilator/examples/make_tracing_c ~/

* On Debian based distribution:

cp -a /usr/share/doc/verilator/examples/make_tracing_c ~/

Then on any distribution:

cd ~/make_tracing_c
gktwave logs/vlt_dump.vcd


The text output during the make phase give interesting informations too:

-- RUN ---------------------
obj_dir/Vtop +trace
[1] Tracing to logs/vlt_dump.vcd...

[1] Model running...

[1] clk=1 rstl=1 iquad=1234 -> oquad=1235 owide=3_22222222_11111112
[2] clk=0 rstl=0 iquad=1246 -> oquad=0 owide=0_00000000_00000000
[3] clk=1 rstl=0 iquad=1246 -> oquad=0 owide=0_00000000_00000000
[4] clk=0 rstl=0 iquad=1258 -> oquad=0 owide=0_00000000_00000000
[5] clk=1 rstl=0 iquad=1258 -> oquad=0 owide=0_00000000_00000000
[6] clk=0 rstl=0 iquad=126a -> oquad=0 owide=0_00000000_00000000
[7] clk=1 rstl=0 iquad=126a -> oquad=0 owide=0_00000000_00000000
[8] clk=0 rstl=0 iquad=127c -> oquad=0 owide=0_00000000_00000000
[9] clk=1 rstl=0 iquad=127c -> oquad=0 owide=0_00000000_00000000
[10] clk=0 rstl=1 iquad=128e -> oquad=128f owide=3_22222222_11111112
[11] clk=1 rstl=1 iquad=128e -> oquad=128f owide=3_22222222_11111112
[12] clk=0 rstl=1 iquad=12a0 -> oquad=12a1 owide=3_22222222_11111112
[13] clk=1 rstl=1 iquad=12a0 -> oquad=12a1 owide=3_22222222_11111112
[14] clk=0 rstl=1 iquad=12b2 -> oquad=12b3 owide=3_22222222_11111112
[15] clk=1 rstl=1 iquad=12b2 -> oquad=12b3 owide=3_22222222_11111112
[16] clk=0 rstl=1 iquad=12c4 -> oquad=12c5 owide=3_22222222_11111112
*-* All Finished *-*
- sub.v:29: Verilog $finish
[17] clk=1 rstl=1 iquad=12c4 -> oquad=12c5 owide=3_22222222_11111112

You can see here, each steps of the execution with the clock (clk) switching between 0 and 1 state.

The time is in tick, and in testbench, can be defined by timescale directive, that allow more precise resolution granular time. We don’t need to use it for now.

The .vcd (Value Change Dump) file is defined in top.v:


And you will have all the step in the logs/annotated/ dir and a coverage file in logs/coverage.dat:

-- COVERAGE ----------------
verilator_coverage --annotate logs/annotated logs/coverage.dat
Total coverage (2/31) 6.00%
See lines with '%00' in logs/annotated

-- DONE --------------------

The last one is defined in sim_main.cpp.

About Verilator examples

In each example, there is a small c++ code interface, modifying c++ code isn’t needed for basic testings. When basic concept are understood, c++ part can be tuned to interface with system libraries, to simulate devices communications, audio, graphics, etc.

In these simples starting examples, the most interesting ones for beginning are all available in Verilog:
* make_hello_c Verilog Hello World with Makefile
* make_hello_sc Verilog Hello World using SystemC with Makefile
* cmake_hello_c Verilog Hello World with cmake.
* cmake_hello_sc Verilog Hello World using SystemC with cmake.

SystemC is set of C++ classes and macros which provide an event-driven simulation interface, simulate concurrent processes in a real-time environment.

The other examples are (with their variants cmake|make and c|sc):
* make_tracing_c for a better understanding at start and for debugging purpose.
* make_protect_lib For creating a DPI protected library. DPI (Direct Programming Interface), is an interface between SystemVerilog and functions in a language like C or C++.

System Verilog is an advanced evolution of Verilog so it’s also a HDL and sometime view as a HVL (Hardware Verification Language). It add several types, including block types, has some C aspects, and can be programmed in object-oriented programming.

Some common file extensions with Verilog, System Verilog and related simulators are:
* .v as Verilog.
* .vc (verilog ???) or .f as File, used for large projects, containing arguments to give to Verilator (or other simulator), including, flags, includes directory, linked libs for the simulation.
* .vcd as Value Change Dump file, contains tracing output of simulation for analysis (see below).
* .vo as Verilog Output file.
* .sv as System Verilog,

Basic practical example with Verilator

So we will reuse the simple make example (you can choose cmake example instead in this case you will have to call cmake instead of make.

I copied make_hello_c example in my work directory:

cp -a /usr/share/verilator/examples/make_hello_c verilator_test
cd verilator_test

and then just edited top.v ( vim, emacs, gedit, or any text editor of your choice can be used) and replaced it’s content.

Example of simple module after my test, I would like to understand how to access to a simple (verilog) registers, different kind of datatypes, including sprite one (the one displayed beside the title of this section), you can download this version of top.v here:

module top;
 reg [8*11:1] str1;
 reg [8*25:1] str2;  // filled by spaces at left
 reg a,b,c;
 reg [7:0] sprite[0:7];  // use one byte by line
 reg [15:0] sprite2;  // sprite as bitfield
 integer i;

 initial begin
   str1 = "Hello World"; // string initialisation tests
   str2 = "Hello World";

   a = 1'b1; // bit initialisation tests
   b = a^1;
   c = a^0;

   sprite[0] = 8'b10011000; // sprites initialisation tests
   sprite[1] = 8'b00100100;
   sprite[2] = 8'b01000010;
   sprite[3] = 8'b10011001;
   sprite[4] = 8'b10011001;
   sprite[5] = 8'b01000010;
   sprite[6] = 8'b00100100;
   sprite[7] = 8'b00011000;

   sprite2[ 7:0] = 8'b10011000;
   sprite2[15:8] = 8'b00100100;

   $display ("str1 = %s", str1); // strings display
   $display ("str2 = %s", str2);

   $display ("a = %d", a);       // bits display
   $display ("b = a^1 = %d", b);
   $display ("c = a^0 = %d", c);

   for ( i=0; i<8; i=i+1) begin  // sprites display
    $display ("sprite[%2d] = %b",i,sprite[i]);
   for ( i=0; i<2; i=i+1) begin  // try to read bitranges, including out of bounds
    $display ("sprite2[8*i(%1d) +: 8] = %8b",i,sprite2[8*i +:8]);
    $display ("sprite2[8*i(%1d) -: 8] = %8b",i,sprite2[8*i -:8]);
    $display ("sprite2[4*i(%1d) -: 8] = %8b",i,sprite2[4*i -:8]);

To test it, just, apply a make, here in blue are comment of the output added in blog post only:

-- VERILATE & BUILD -------- 
[...]  This part is the compilation log
-- RUN ---------------------   This part is interesting one
str1 = Hello World
str2 =               Hello World    str2 with filled 25 chars
a = 1
b = a^1 = 0
c = a^0 = 1
sprite[ 0] = 10011000    Eight lines of the sprite
sprite[ 1] = 00100100
sprite[ 2] = 01000010
sprite[ 3] = 10011001
sprite[ 4] = 10011001
sprite[ 5] = 01000010
sprite[ 6] = 00100100
sprite[ 7] = 00011000
sprite2[8*i(0) +: 8] = 10011000      some bitrange access tests
sprite2[8*i(0) -: 8] = x0010010      We go out of registers here with x displayed
sprite2[4*i(0) -: 8] = x0010010
sprite2[8*i(1) +: 8] = 00100100
sprite2[8*i(1) -: 8] = 01001100
sprite2[4*i(1) -: 8] = xxxxx001

Further reading

Relatively complete documentation online:

About Verilog:
* A complete Verilog manual at Chip Verify
* Verilog TUTORIAL for beginners on
* Verilog overview in few presentation screens (PDF) at
* Verilog Quick Reference Card.pdf
Verilog HDL Quick reference Guide
* French: Verilog syntax at

Testbenchs, SystemC and System Verilog:
* How to write a basic verilog TestBench, using a C++ template. The site FPGA tutorial has 3 main subjects, VHDL, Verilog & System Verilog.
* Tutorials about Systemverilog and SystemC.
* Some tutorials about System Verilog.
* French: System Verilog in 13 minutes on
* Verilog Simulation with Verilator and SDL using SDL library to simulate VGA graphics output, based on Verilator and some part of SystemVerilog.

Some interesting FPGA projects:
* From Nand To Tetris, all the step to do in few exercises a computer based on a 16 bit CPU, using only NAND gate (interesting game), then building a Tetris with this small system. Their use their own HDL, but there are already some port as examples, as this one in Verilog using Ikarus Verilog simulator (older free software simulator), one in Verilog for opensourced Lattice ICE40 FPGA (tested on an open harware Olimex board) and Opensource tools (YoSYS as synthesizer), in Verilog for De0-nano (Altera Cyclone IV FPGA).
Tang nano MIDI Sounder, a MIDI synthesizer, with nice 8 bits like sound.
* Speech256, speech synthesizer.
* (Japanese) SERV RISC-V procesossor for Tang Nano
* (Japanese) Tetris for Tang Nano in Verilog
* ZipCPU a blog for this CPU with a nice Verilator tests bench examples, using C++ templates for generic testbench.
* UART, Serial Port, RS-232 Interface FPGA implementation (both in Verilog and VHDL).
* Consolite, a light console in Verilog for FPGA, assembly programmable. An emulator is also available.
* FloPoCoFPU for FPGA devcelopped by INRIA.

For Free software loving hackers:
* Apicula project to open Gow1n FPGA array bitstreams