+1 for AVR128DA28 (avrdx/breadxavr)Done a quick test with a stress application and source from git - ran for over an hour, no issues.
Memory region Used Size Region Size %age Used
flash: 64774 B 128 KB 49.42%
sram: 798 B 16 KB 4.87%
eeprom: 0 B 512 B 0.00%
rodata: 2317 B 4 KB 56.57%
CP: nuttx.hex
CP: nuttx.asm
+94 bytes of flash compared to 13.0.0 (with serial driver and pipe)
