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ATtiny jtag2updi High-Voltage programmer

Published: 20 April 2024
Last updated: 04 May 2024

Why is a High-Voltage programmer needed?

When installing the bootloader, the UPDI pin is programmed as a reset pin. Reprogramming via the UPDI pin is then blocked. It is then no longer possible to burn the bootloader again with the SerialUPDI Programmer, if necessary. This requires the UPDI pin to be reset and this requires high voltage programming. Applying a HV pulse of 12V to the reset pin returns the pin functionality to UPDI.
The name HV (high voltage) is confusing because it is just 12V. The voltage must be between 11.5V and 12.5V.

The HV procedure has changed over time

Attention: Hans Meijdam has discovered that the HV procedure has changed over time for the same ATtiny chips: the HV pulse must appear immediately after power on, previously this was not necessary. The following HV programmers that previously worked, no longer work:

For an HV programmer you can use this from Microchip: MPLAB® PICkit™ 5 In-Circuit Debugger, but I have no experience with this. I have a good experience with the jtag2updi programmer, which I will describe here: 

HV Programmer jtag2updi

Step 1

Build this SerialUPDI programmer on a solderless breadboard: DIY HV UPDI Programmer from Dlloydev on GitHub. I'm using this simpler circuit, without the LTC1262 chip:

jtag2updiATtiny jtag2updi programmer

Using an optocoupler seems illogical, but the DIP package is handy when mounting on a breadboard. Connect A7 to 5V.

Step 2

Perform the Assembly and Setup.

Step 3

Burning the bootloader is done in the same way as described with the SerialUPDI Programmer. The HV programmer also works without the 12V voltage, with the same functionality as the SerialUPDI Programmer. But then the UPDI pin cannot be changed.


Creating the HV pulse with a separate 12V power supply and a switch did not make sense to me. I therefore designed a simple high-voltage pulse generator. If you use this circuit please let me know, I would love to hear about your experience.

Simple UPDO high voltage pulse generatorSimple UPDO high voltage pulse generator