The Nuvoton NUC121 and NUC125 are Cortex-M0 based USB microcontrollers capable of crystal-less USB operation.
All devices in the family have 8kBytes of SRAM and 32kB of flash. Additional flash includes 4.5kB of LDROM (memory region that can be mapped in at boot as a bootloader) and 0.5kB of SPROM that can be used for more secure storage.
This device family could be viewed as occupying the space between the Atmel/Microchip SAMD11 and SAMD21, but at a very low price nearer to the SAMD11.
This package was tested with both the SEGGER J-Link and a CMSIS-DAP adapter.