Внешний супервизор нужен только для такого (errata на мегу161):
Цитата
Brown-out Detecor may cause the Device to be Stuck in Reset State
When the internal Brown-out Detector is enabled, the device may hang in Reset state regardless of the VCC level.
Problem Fix/Workaround
Use external Brown-out Detector and do not enable internal Brown-out Detector
(leave BODEN Fuse unprogrammed).