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28 lines
1.2 KiB
Markdown
28 lines
1.2 KiB
Markdown
# RTC
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This directory contains a fully functional firmware for implementing a
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drop-in replacement for the RTC on ATTiny85 microcontroller.
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Unfortunately, ATTiny85 cannot be a true drop-in replacement, but you
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can get pretty close. The main caveat is time drift due to counting
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seconds entirely from the internal oscillator, the crytsal oscillator
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unfortunately cannot be used (efficiently/effectively) by the
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ATTiny85.
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The AVR core clock is run from an internal oscillator to generate 8
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MHz. The required fuse bytes setting is included in the generated ELF
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object file in the `.fuse` section, in the order (low, high,
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extended). See the source code of MacRTC.c for more information on
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electrical specifications and the like.
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Reference source, Visited 2020-08-05:
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* https://www.reddit.com/r/VintageApple/comments/91e5cf/couldnt_find_a_replacement_for_the_rtcpram_chip/e2xqq60/
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* https://pastebin.com/baPZ4nN4
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## Device Programming
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Please note: For the ATTiny85 form factor, the external RESET pin must
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be disabled since it is used for the 1-second interrupt output.
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Therefore, after the initial programming, it will only be possible to
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reprogram via high-voltage serial programming.
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