The Nuvoton NUC121 and NUC125 are Cortex-M0 based USB microcontrollers capable of crystal-less USB operation. They could be compared to the Atmel/Microchip SAMD11, but have more memory than that device.
At present, source code is provided for these sample devices:
- USB CDC to UART bridge
- USB HID mouse emulator
To their credit, Nuvoton already does provide sample code in their NUC121_Series_BSP_CMSIS download. However, that code is focused on the IAR and Keil toolchains.
This code, in contrast, is written for gcc and clang.
It uses a new USB device stack written specifically for the NUC121/NUC125 and uses an API modeled on vcp. vcp was written for the SAMD11 / SAMD21. Since the USB stack APIs are nearly the same, code can be more easily ported between the ataradov vcp USB stack and this NUC121usb one (as well as NUC126usb). Another advantage of this approach is that the code size is a little more efficient than the Nuvoton reference code.
I've also contributed a NUC121/NUC125/NUC126 driver to TinyUSB. Since TinyUSB runs on many different microcontrollers, code originally written for one device can often be readily adapted to many others.
One approach is to use Rowley Crossworks for ARM to compile this code. It is not free software, but has been my favorite go-to ARM development tool for a decade and counting. Rowley does not officially support the Nuvoton NUC121/NUC125, but you can download an open-source CPU support package for the NUC121/NUC125.
OR
I've modified the build environment provided by mcu-starter-projects to work with the NUC121. With this approach, the code can be built using only open-source software. In Ubuntu-derived distributions, this is likely achieved with as little as:
sudo apt-get install gcc-arm-none-eabi libnewlib-arm-none-eabi build-essential