I have been using a prototype atmega128rfa1 on a protoboard, powered with 3v3 from that output of a Gabotronics XMEGA Xprotolab, but with UART1 connected to the level shifters of an STK500 to the computer COM1 port. A recent firmware bug caused an endless reset and the rs232 output was repeatedly printing "Reset", perhaps for a long time.
When I eventually ran a terminal program and saw that string, I unplugged the power connector to the Xprotolab. Much to my surprise the string continued to be output, the rfa1 now being powered by the tx, rx, and ground connections to the STK500. The printing stopped when those were disconnected (Duh!) but started upon reconnection. The only way to reset was to disconnect the rs232, power from the external 3v3, and then reconnect.
All is now ok as long as the rfa1 does not watchdog reset, but this behavior has now become common when I power off the 3v3.
Is the rfa1 now suspect? Or is this a known phenomenon when using level shifters?
I should add, for a couple of weeks I was running the STK500 at a target voltage of 5v0, and everything seemed ok even with the 3v3 power to the rfa1. It's possible that blew the protection diodes on the rfa1 port pins.