The ATmega328P-based Arduino like Nano, Pro Mini, and UNO have a Brown-out Detector (BOD) to monitor power supply voltage during the sleep modes. The BOD can be turned OFF or kept ON using BOD_OFF or ...
This version has been replaced by a new version : https://github.com/SHWotever/SimHub/wiki/SIMHUB-STANDARD-ARDUINO-PRO-MICRO-sketch-V2/ This firmware (arduino pro ...