“Sometimes AVRs are operated without an external clock. The internal oscillator does a good job in most applications, but when it comes to precise timing, it is too inaccurate. The accuracy of the oscillator of an AVR is only +/-10%. 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 TinyICOC does this fully automatically. Simply connect the device to the target MCU via the ICSP header and press the button.
Hardware
The TinyICOC is powered by the target board through the ICSP connection. On the one hand, this means that there is no need for a separate power supply and, on the other hand, the oscillator of the target MCU is automatically calibrated at the supply voltage of the target board.
The ATtiny84 was chosen as the microcontroller for the TinyICOC because it has exactly the necessary number of GPIO pins. For accurate frequency measurements, the ATtiny84 is operated with an external 12 MHz crystal. Since the current software version only requires around 3 KBytes, an ATtiny44 can also be used.
The user interface utilizes one button and a 128x32 pixels OLED display.”