“This is a simple High-Voltage UPDI programmer, for programming 0-series, 1-series, and 2-series ATtiny chips to allow you to configure the UPDI pin as a extra general I/O pin.
For simplicity it is designed to work with an existing USB to Serial board, and an external 12V source.
The ATtiny 0-series, 1-series, and 2-series chips allow you to get an extra I/O pin by reprogramming the UPDI pin as a general I/O pin. However, the downside is that you can’t subsequently use a regular UPDI programmer to reprogram the chip. Microchip provide a reset mechanism, which involves applying a short pulse to the UPDI pin before programming it. Although the method is called ‘High-Voltage Activation’, the pulse is actually only 12V, slightly higher than the normal 3.3V or 5V voltage level.
This programmer extends a standard SerialUPDI programmer with extra circuitry to automatically apply the 12V HV pulse, and program an ATtiny chip even when the UPDI pin has been reprogrammed to another function.”