ATtiny 0, 1, and 2-Series High-Voltage Programmer

Microchip's ATtiny 0, 1, and 2-series chips each have a UPDI programming pin, which can be repurposed to a general purpose I/O. The downside, of course, is that it can no longer be programmed normally via UPDI, unless you use a “high-voltage” pulse to reset this functionality. As noted in David Johnson-Davies’ Tiny UPDI-HV Programmer post, the electrical potential for this reset pulse is a mere 12V.

Depending on your frame of reference, this number may be disappointing, but it is well beyond the normal 3.3 or 5V level these chips typically experience. This allowed Johnson-Davies to create his own reset-programmer without taking electrical linesman-level precautions.

The device is fairly simple in function, using a MAX4544 SPDT analog switch to convert a USB-to-serial programmer's DTR signal to a roughly 100µs 12V pulse. For simplicity, 12V is provided by an external source, e.g. a battery or power supply.

This pulse procedure resets the UPDI pin for programming duties, enabling the chip to be programmed in the standard fashion. New programs can even include reusing the UPDI pin for GPIO duties, though you’ll again need to use a special HV pulsing programmer to make changes.

The project write-up lists USB-to serial adapters that have been tested with this programmer, though other devices may work as well. There’s also an example flashing procedure to help get you started in the world of “extra GPIO” ATTiny 0/1/2 series devices!