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Some GBA games, like Pokemon Ruby, contained an internal RTC with a small button battery to power it. When the cartridges were inserted and the GBA was powered on, did the RTC continue to run on the internal battery or did the system's battery power it while it was connected?

What about GB/GBC cartridges which contained battery-backed SRAM for save data?

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To answer this with absolute certainty, you'd have to examine each cartridge on its own, particularly those that weren't made by Nintendo. I'm not sure anyone has ever done that, but people have looked at quite a number of cartriges (e.g., here and here), and Nintendo cartridges for the GB/GBC all appear to use the Mitsumi MM1134 system supervisor IC (or something pin compatible) to switch between power from the Gameboy and battery backup.

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  • So for the GB/GBC at least, the answer is "yes, for Nintendo games"?
    – forest
    Feb 14 at 23:23
  • Let's put it this way: When you look at reports from people who have looked inside Nintendo-made GB/GBC cartridges with battery-backed SRAM, they've always found the MM1134. It's also good design practice to use a system supervisor to block spurious writes during power-down / power-up. So it's very probably true. Feb 15 at 7:05

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