macsehw/firmware/rtc/README.md

RTC

This directory contains a fully functional firmware for implementing a drop-in replacement for the RTC on ATTiny85 microcontroller. Unfortunately, ATTiny85 cannot be a true drop-in replacement, but you can get pretty close. The main caveat is time drift due to counting seconds entirely from the internal oscillator, the crytsal oscillator unfortunately cannot be used (efficiently/effectively) by the ATTiny85.

The AVR core clock is run from an internal oscillator to generate 8 MHz, so set the fuse bits accordingly when programming. See the source code of MacRTC.c for more information on electrical specifications and the like.

Reference source, Visited 2020-08-05:

• https://www.reddit.com/r/VintageApple/comments/91e5cf/couldnt_find_a_replacement_for_the_rtcpram_chip/e2xqq60/
• https://pastebin.com/baPZ4nN4