Because the 8-pin ATtinys only have a few GPIO pins available, they are usually operated without an external clock. The internal oscillator does a good job in most applications, but when it comes to precise timing, its +/-10% accuracy is often insufficient. Fortunately, the oscillator can be calibrated, increasing its accuracy to +/-2% or better. There are a few ways to perform this manual calibration, but several steps are required. The TinyCalibrator does this fully automatically by a push of a button. In order to make the device more versatile, a high voltage fuse resetter was also integrated, with which "bricked" ATtinys can be reset to the factory state.

Description and project files: https://github.com/wagiminator/ATtiny84-TinyCalibrator