The Micro Chip Curiosity Nano board for the AVR128da48 is a nice dev board. It comes with a built in programmer that works with their MPLAB IDE. However, the board does have an issue with the built in power supply. Normally, the board is powered off of the USB port. The on board voltage regulator can be set to anything from 1.8v to 5.5 volts under control of the IDE, and the board will 'remember' this voltage setting the next time it is plugged in. Since the built in debugger is built on a SAM chip, it runs off of a separate 3.3 volt regulator, and a level converter is used to interconnect the processor with the debugger. The debugger also includes a USB to Serial converter, and this provides a USB Serial (ACM0 device) on the USART 1 device (port pins PC0 and PC1).
The one fly in this ointment, is that the CPU end of the level shifter is powered by the on board target regulator. While you can disable the target regulator by grounding one pin of the Nano, and then provide external target power, with the internal regulator turned off the level converter doesn't work! I wondered why my application would not work with the serial port when externally powered, this is the reason!
The end result is that you can't easily built a project around the Nano, and externally power it, not if you want to use the USB serial port anyway! There may be a work around here, but I fear it will involve micro surgery to attach a bodge wire. Anyone else run into this?