In a recently arrived package from Seeed Studio there was a Sipeed Longan Nano. I have been eyeing some Sipeed products for a little while, but the Nano was more or less an impulse purchase as explained in a previous post. This is a completely new architecture for me, so this post is about just getting a "Hello World" program running on the device.
Table of Contents
- Longan Nano
- RISC-V ?
- PlatformIO Only
- Compiling Blink
- Uploading With a Serial Link
- Uploading With USB (DFU)
- Adding Serial Output
- Hello World on the LCD
- Downloading the Example Blink Projects
The Longan Nano is a development board based on the GD32VF103CBT6 micro-controller with a RISC-V 32-bit core by GigaDevice. The board purchased came with a 0.96 inch display and a translucent acrylic case which holds the display securely, allows for the manipulation of the two on board push buttons and provides access to 0.1 inch pin headers along the long edges of the board. The headers are supplied with the board but must be soldered. The little screen is a 160x80 pixel colour liquid-crystal display using IPS (in-plane switching) screen technology. The 108 MHz, 32 bit, GD32VF103CBT6 is no slouch with impressive capabilities.
- 128 K bytes of Flash memory
- 32 K bytes of RAM
- 2 ADC (12 bit, 10 channels)
- 2 DAC (12 bit)
- 7 timers (16 bit) of varying capabilities
- a watchdog, a real-time counter/clock (??)
- 3 USART
- 2 I2C
- 3 SPI
- 2 I2S
- 2 CAN
- 1 USB OTG
The following image showing the pin out of the device is from an SVG drawing by Kali Prasad.
While not obvious, there is a mini TF card slot for micro-SD cards on the back side of the board.
The RISC ISA (love or hate it, you have to live with the alphabet soup of electronic technology; RISC stands for Reduced Instruction Set Computer and ISA for Instruction Set Architecture) uses simplified instructions requiring fewer cycles to execute in opposition to the CISC (Complex Instruction Set Computer) ISA where the richer set of complex instructions can take many cycles to complete. As one would expect, the demarcation between CISC and RISC architectures is not very clear. MIPS, SPARC, PowerPC and ARM are well-known families of processors that implement the RISC architecture, while the venerable Intel 8080 and its current x64 successors are in the CISC camp.
RISC-V, pronounced risk-5 and not as in "Hôtel Georges Vé" (see Les Bonbons de Jacques Brel, that's an obscure reference that would mean something only to those that remember French songs of the 1960s), is a relatively recent open standard ISA started at the University of California, Berkeley in 2010. The fact that it is open distinguishes it from the ARM ISA. The GigaDevice introduced its GD32V system in 2019 just as the RISC-V ISA was frozen to allow hardware and software development to proceed. I find it a bit confusing, but it looks like the MCU core, named Bumblebee, was designed by or in cooperation with Nuclei System Technology another Chinese entity. Looking at press releases, it seems that Chinese companies such as GigaDevice are if not embracing RISC-V at least hedging their bets by testing the technology because of the absence of royalties whereas ARM is definitely not free. Furthermore, being open source RISC-V cannot be subjected to export controls by the US government which seems to have happened in the case of ARM. However, things are not so simple as ARM "is (partly) Chinese" and there are two competing RISC-V consortium in China and the MIPS OPEN Initiative making the MIPS ISA freely available is rolled out and ARM is reacting. All this is fascinating, but not germane to my purpose.
For the last three or four weeks, I have been using the Visual Studio Code editor for managing the source files for this site, and using the PlatformIO extension for embedded device development. This was either the second or third attempt to use PlatformIO and so far it is working very well and I feel more productive as a consequence. I was therefore quite pleased to see that the Sipeed Longan Nano among the supported boards. Furthermore, the Sipeed has a Longan Documentation has a Getting Started/Getting to Blinky article in which PlatformIO is used. By the way the introduction to the current post was shamelessly stolen from the Sipeed introduction.
It will be assumed that PlatformIO (PIO for short) is installed and working properly in Visual Studio Code on the desktop, but if not, here are the installation instructions.
I also tried installing the board in the Arduino IDE following the sparse instructions at Sipeed/Longuino. That means that I added the following
in the list of Additional Boards Manager URLs in the IDE Preferences. Then when attempting to install Longuino, I got the following error:
I played around the RISC-V fork of openocd found in the GV32D package installed in PlatformIO and managed to get a different error message when installing Longduino a second time. I gave up at this point and decided to use the Platformio environment which supports the Arduino framework.
One could install the GD32V platform as instructed in PIO configuration. I just started a new project, not because I make it a habit to not read instructions, but because I did not find the documentation until after creating the first Blink sketch.
- If PIO Home is not opened, click on the PIO icon on the left edge,
QUICK ACCESS/PIO Home/Openin the PLATFORMIO LIST in the left pane.
- Click on + New Project i the PIO Home page.
Project Wizard window will pop up.
- Set the project name. I chose
- Select the board. I entered "nano" in the box and then scrolled down the list of boards with "nano" somewhere in their name.
- Change the framework from
GigaDevice GD32V SDKto
- Click on the button.
PIO will obtain the platform toolchain and set up the project as a directory with assorted subdirectories all shown in the EXPLORER pane on the left.
- Click on the
srcdirectory and then on the
main.cppfile name in the EXPLORER pane.
- The typical bare source template will be displayed in the
main.cpptab. Replace the bare
loop()functions with those in the standard Arduino Blink sketch. Do not remove the
#include <Arduino.h>directive at the top of the file; it is necessary in PlatformIO when using the Arduino platform, it is implicit in the Arduino IDE. Note that I shortened the LED on time by an order of magnitude for a reason to be explored below.
- Click on the
PlatformIO: Buildicon. It is the check mark on the tool bar along the bottom of the application window.
The image says it all, the compilation was a success. Now there remained the task of uploading the binary to the Nano.
Above is a photograph, borrowed from a Mikrozone Server video showing the
BOOT0 buttons to the left of the USB-C connector and the 8 pin header at the other end. The top 4 pins provide a serial connection that can be used to upload the compiled firmware with a 3.3 volt USB to TTY converter and to power the Nano. The pinout is shown below.
It's best to power off the USB to TTY converter and disconnect the Nano from all devices. Connect the ground of both the converter and the Nano together and connect the 3.3 volt line of the converter to the 3.3 volt pin of the Nano. My USB to TTY converter has a jumper to select either 5 or 3.3 volts and I set it to 3.3 volts. Make sure your converter will output 3.3 volt signals otherwise the Nano will be damaged. Finally, connect the converter Tx signal to the RX0 pin of the Nano and the converter Rx signal to the TX0 pin of the Nano.
When the USB converter is connected to the desktop or portable computer, the factory installed firmware, which flashes different coloured LEDS will probably start up. The board has to be reset and restarted in download mode. The sequence of button presses to do that is simple and not time critical.
- Press and hold down the
- Press and release the
RESETbutton. The flashing LEDs will go off.
- Release the
If PlatformIO is running on a Linux system, then it will help to install PlatformIO UDEV rules as instructed in 99-platform-udev.rules in the PIO Troubleshooting FAQ.
serial is the default upload protocol, but it will not hurt to confirm it. If PIO cannot find the correct serial port, then it should be specified in the configuration file. It was not necessary to do so on my system. After all this preparation, it is now possible to upload the compiled firmware. Click on the
PlatformIO: Upload icon. It is the right pointing arrow on the tool bar along the bottom of the application window. Here is the result.
The upload was successful and a red LED, partly hidden under the LCD display started blink. However the on and off times showed that the I/O pin controlling the LED is active LOW. In other words, the pin has to be set to 0 volt to turn on the LED. I rewrote the Blink sketch to reflect this and added a
for loop to quickly blink the LED before turning it off for a longer delay. By changing the number of short consecutive on-off cycles, it will be easier to ascertain that a new version has been uploaded to the device.
From now, uploading modified versions of this firmware is a two-step procedure: (1) reset the Nano in download mode and (2) press on the upload icon in PIO which will recompile the firmware automatically.
It is possible to power the Longan Nano and upload sketches over the USB port just as done with ESP boards such as the nodeMCU or Lolin/Wemos D1 mini. I was keen to try that since it is marginally easier than with a USB-serial converter as shown above.
Before connecting the USB cable, remove the USB-serial cable or at least disconnect the 3.3 volt. This might be wrong, but I assume that it's not a good idea to have two power sources to the device. Then connect the USB-C cable to the desktop and Nano and change the upload protocol from serial to udf in the
platformio.ini configuration file.
DFU (Device Firmware Upgrade) is a mechanism for upgrading the firmware of USB devices. As far as I know, dfu-util is the only way to support this mechanism on a desktop running Linux. But then, what do I know? I had to look up DFU yesterday!. In any case, PIO installs and uses this utility in the GD32V package to upload and download firmware with the
Restart the Nano in download mode.
- Press and hold down the
- Press and release the
RESETbutton to reset the Nano.
- Release the
Before uploading the firmware with this protocol, it is not a bad idea to check that the Nano now shows up as an anonymous USB device with ID 28e9:0189 and to check that there is a
UDEV rule for the device. You can even check that the installed
dfu utility recognizes the Nano.
Change the LED flashing parameters to something quite different to those currently on the Nano, say
so that it will be quite obvious if the firmware upload worked or not. Try to upload the modified binary by
click on the
PlatformIO: Upload icon on the tool bar along the bottom of the application window. If it worked,
then congratulations. Chances are that the sizes and dates of the
dfu-* files in the GD32V package are different from those shown above and PIO has been updated. By all means skip the rest of this section.
I followed the instructions by dr-br to downoad and compile the latest version of
configure command given had been ./configure --prefix=$HOME as suggested by dr-br, the utilities would have ended up in a
~/bin which is not in the search path on my system. I checked that the utility was installed in my
.local/bin directory and then uploaded the firmware with it.
Despite that ominous
bad page size message and the final reported error the firmware did get onto the Nano.
I copied the three
dfu-* files in
/.local/bin/ on top of the files in
~/.platformio/packages/tool-gd32vflash/, changed the LED blinking frequency again and uploaded.
PIO gave an even scarier FAILED message, but the new firmware was uploaded to the Nano.
What is a sketch without some serial output? It is not too difficult to do, but the usual
Serial class used in Arduino is not available. Instead, USART0 will be initialized explicitly and then the
_put_char() function used by
printf will be modified to send the output to the UART. The source for this is a Sipeed documentation page entitled UART. The document is not available in English, so thank you Google translate.
To see the output it will be necessary to connect a USB to TTY converter to the Nano. This is described above. It remains possible to upload the firmware with the
udf protocol, but in that case do not connect the 3.3 volt line of the USB - TTY converter to the Nano header. Set the baud for the communication software (miniterm) to 115200 to match the rate set for the USART in the firmaware. This is done in the
platformio.ini configuration file.
The output will be sent to the serial port just as soon as the firmware is uploaded. If the terminal is not visible then click on the PlatformIO: Serial Monitor icon, which looks like a 2 prong plug facing up on the bottom tool bar. To view the beginning of the output, press on the RESET button on the Nano.
I could have printed "Hello World" and that would have fulfilled the initial boast of this post. Of course, it would be better to do it on the LCD.
It only makes sense to send any text output to the liquid crystal display supplied with the Nano. Based on the work of others, notably the
micros example in longan-nano-experiments by Stephan Mühlstrasser, here is what I came up with.
The firmware blinks the on-board red LED and echoes the state of the LED to the LCD. It also displays and updates a loop counter. And, of course, the first line on the LCD says "Hello World!".
As can be seen, all the details of the SPI interface with the LCD is taken care of in a library called
lcd appropriately enough. My knowledge of Mandarin, whether simplified or otherwise, is nil, but with the help of Google, I added English comments in
lcd.c. Someone probably did that already, however it's not a bad way to familiarize oneself with some code.
The complete PIO projects can be downloaded with the following links.
- nanoblink.zip: simple LED blink.
- nanoblink2.zip: LED blink with serial output.
- nanoblink3.zip: LED blink with output to LCD.
Just extract the content of the archive somewhere in the PlatformIO projects directory. Here is one way to do it in Linux.
Note that I decided to put the
nanoblink example project in a subdirectory, called
longan_nano, of the PlatformIO project directory. The
-p option of the
mkdir command creates all needed directories along the specified path. In other words, it created directory
longan_nano before creating
blink1 inside of it.
To start using a downloaded project
- Click on the Open Project button in the PIO Home page
- Click on the
Projectsfolder in the Places section of the Favorites pane
- Navigate to the desired project directory such as
- Click on the button.
The project can now be compiled. Adjust upload protocol in the
platformio.ini configuration file before trying to upload new firmware to the Nano.
Then Sipeed Longan Nano has not been available for a long time. Some sites that mimic an unwrapping video merely repeating the product release announcement. However, there is something to learn in just about every site with practical example code for the Nano. Here is a non-exhaustive list of sites that should be looked at.
- longan-nano-experiments by Stephan Mühlstrasser (smuehlst)
- Already cited above, this is a good source of projects many of which are built on the Arduino framework as done here.
- Tests, Getting familiar with the Sipeed Longan Nano board by Andreas Müller (MuellerA)
- A more extensive list of projects built on the gd32vf103-sdk framework.
- Longan Nano (GD32VF103) by Kevin Sangeelee
- This post serves as a brain, link, and file dump – an disorganised reference for anything [Kevin needs] to keep regarding the Nano
- Hands On with RISC-V, Part 1 – An Introduction and Hands On with RISC-V, Part 2 – Serial Communication by Dale Wheat
- An introduction to the RISC-V computer instruction set architecture using a Longang Nanoe and then describing how to use one of the on-board serial ports for both device programming and communication with the host PC.
Do a search of "longan-nano" and "sipeed nano" on GitHub for projects with a more focused topic.
Of course there are "official sites" that should be consulted.
- Sipeed forum
- Sipeed Telegram Group
- Sipeed Longan Documentation (the Chinese site contains more information)
- Sipeed datasheets, user manual...
- GD32VF103RISC-V32-bit MCUFirmware LibraryUser Guide GigaDevice Semiconductor Inc.