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I have never been able to get my Arduino IDE to recognise an ATtiny device. I thought you had the solution but following your instructions did not cure the problem. Even installing the full version of the URL you provided into the 'Preferences' > 'Additional Boards Manager URLs' did not result in the IDE giving me an ATtiny option. Can you confirm that this is the correct address of the URL? https://raw.githubusercontent.com/damellis/attiny/ide-1.6.x-boards-manager/package_damellis_attiny_index.json