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THE END of my involvement with tag2upodi and its maintenance,

This was the most unpleasant codebase I have had to work in my life as a programmer that deep dive through this a profoundly unpleasant experience that took mw months to recover from.

This tool is not recommended anymore

Please consider using the Serial-adapter-as-UPDI prograer. Through its dependance on emulating a device that capped at 115200 baud, jtag2updi speeds in practice are limited to half that, for both writing and verification (because once it receives a page it has to retransmit it over another serial connection of similar speed. (the higher speed of the updi-serial connection approximately cancels the communication overhead; net speeds run around 5k in either direction).

In the past the version of the Serial-to-UPDI included with MegaTinyCore and DxCore was very slow. But there is now a performance optimized version, which is now included with megaTinyCore 2.3.2 and will be in next version of DxCore, hopefully available this week, and I'm hoping it will be added to MegaCoreX soon. The difference in performance is not subtle. It ships with three speed options exposed in the IDE, One at 57600 baud (for unusual circumstances that cause other methods to not work, 230400 (for normal use) and TURBO for maximum speed on 4.5V+ configurations. The slowest of those is comparable in speed to jtag2updi, the others are 3-7 times faster for programming the tiny AVR devices and even faster for the Dx-series parts. The hardware required consists of just a serial adapter and a diode and/or resistor, and there is no buggy, unreliable, firmware involved. In short, it is just a better programmer than this one is. In light of this, no further development work is planned here - PRs will be merged if they look reasonable, but that is all.


This fork of the repo has been stripped down and modified so that folders don't need to be renamed in order to compile with the Arduino IDE.

New Features

This has also has the following additions:

  • Support for DA-series and upcoming DB-series parts; correct mode for reading and writing is deduced by reading the SIB
  • Timeout function - a lack of messages from host for 250ms or the target failing to send an expected response for 100ms will result in a timeout. Timeouts from target are reported to host. Timeouts waiting for host will lead to it attempting to reset a failed status message (on the grounds that "maybe it didn't get through". Four consecutive timeouts waiting for host will cause it to reset to it's initial state and await a new attempt at communication from the host.
  • Improve write speed by disabling the response signature during burst writes (NO_ACK_WRITE).
  • Add debug channel via SPI or second USART to provide rich debugging output.
  • Add support for running on additional processors: 40/44 pin AT mega parts (ones supported by MightyCore), 64/100-pin AT mega parts (ones supported by Megamole), 28/32-pin AT mega parts (ones supported by MiniCore), and mega AVR 0-series parts (supported by MegaCoreX). The tiny AVR 0-series and 1-series parts (supported by megaTinyCore) are also supported.
  • Support for the Lloyd HV updi programming tool. See the following section for more information.

Known issues

There are the following known issues:

  • Timeout system breaks compatibility with terminal mode (-t option), by definition, as a user in terminal mode will inevitably take longer than it is configured to wait. This will be corrected via a planned SUPPORT_TERMINAL_MODE option for those small number of users who use terminal mode.
  • Does not work correctly on LGT-based clones
  • has_pdi does not work correctly (waiting to hear back whether it is broken for all parts or just DA-series parts)
  • Does not run on DA-series parts - This will be corrected by addition of a few more defines, should be easy
  • Does not support targets operating at a low voltage - this could be solved by using an analog comparator for receiving, and using pin in "open drain" mode for transmitting, but I doubt I'll ever bother to implement this.

HV UPDI Programmers

These HV UPDI programming designs are open source and made to work with tinware 0-series and 1-series MCUs, megaTinyCore and the Arduino IDE. This allows using the additional configuration settings for the UPDI pin without the fear of getting locked out from the MCU. Normal workflow when using the Arduino IDE is preserved. This repo has been modified to allow HV UPDI programming and can be installed on various programmer MCUs including the newer AT tiny parts.

Enabling HV UPDI in jtag2updi

Firmware (jtag2updi) support for HV programming is controlled by the USE_HV_PROGRAMMING option in sys.h - uncomment #define USE_HV_PROGRAMMING to enable these features. Be sure that your hardware is consistent with the specifications described herein; when this is used with a non-HV capable programmer, it will generally not function as designed. The mechanism by which these failures occur involves two floating analog pins, and the exact behavior is influenced by the votages they settle on, so ostensibly identical programmers can produce very different output.


HV UPDI Programmer 1. Arduino Nano 2. DIY ATmega4809 3. DIY AT tiny 4. DIY Nano
Feature Click to enlarge Click to enlarge Click to enlarge Click to enlarge
Target Voltage 5V 5V 5V 5V
Programmer's MCU ATmega328P (16MHz) ATmega4809, ATmega4808, ATmega3209, ATmega3208, ATmega1609, ATmega1608, ATmega809 or ATmega808 (16MHz) ATtiny1604, 1614, 1606, 1616, 3216, 1607, 1617 or 3217 (16MHz) ATmega328P (16MHz)
MCU Board Required Yes (Arduino Nano) No No Yes (Arduino Nano)
Max Target Current for Power Cycle 60 mA, PORTC 60 mA, PORTD or external High Side Power Switch High Side Power Switch, User Defined 60 mA, PORTC
5V to 12V Converter Dickson Charge Pump, (10mA) User defined User defined LTC1262CN8 (30mA)

More ...

Original Read me (ElTangas)

This is a firmware, that when uploaded on an atmega328p, or a similar AVR MCU, enables it to interface with avrdude using the Janice Mk2 protocol via a serial link. In particular, you can use an Arduino to host this firmware. It provides a bridge to program the new attiny817 family of MCUs, that use the UPDI interface:

avrdude -> HW Serial interface -> Programmer MCU (e.g. Mega328P) -> SW Serial on PD6 -> Target MCU (e.g. tiny817)

Currently, I have not tested this software with a level shifter, however, since the UPDI pin is high voltage tolerant, it's ok to have V_prog > V_target, but not the reverse.

Notice, however, that the logic levels need to be compatible for successful programming: V_target cannot be lower than about 60% of V_prog (60% will likely work, 70% is guaranteed to work). Therefore, it will not be possible to program a 2.5V target with a 5.0V programmer, because communication errors will surely occur (but no electrical damage), but if V_target is 3.3V (66% of 5.0V) chances are good.

V_prog V_target ±+ ±+ | | +---------± +--------------------± | | +-------------------± | PC | | Programmer ±+ ±+ Target | | avrdude | | | +---------± | | | TX +---------± RX PD6 +-----± 4k7 +--------± UPDI | | | | | +---------± | | | RX +---------± TX | | | | | | | | | | | | | | | | | | ±-+ ±-+ | +---------± +--------------------± | | +-------------------± JTAG ICE MkII ±+ UPDI ±+ Protocol GND

Drawing adapted from:

If you use an Arduino as host for this program, be sure that, after burning the software, you disable its auto-reset feature, using one of the techniques described here:

Alternatively, you can use an Arduino without integrated USB/serial adapter, like the pro-mini; in that case, just disconnecting the DTR wire will disable the auto-reset. Just remember the UPDI chip must be connected to the same supply voltage as the Arduino's MCU!

Building with Arduino IDE

The program can be built as if it was an Arduino sketch.

Open jtag2updi.ino in the jtag2updi folder, and upload to your micro controller.

The Arduino IDE will automatically set the correct MCU model and F_CPU, but if you want to change the speed of the UPDI link, you will have to edit UPDI_BAUD directly in the source code.

Using with avrdude

If using megaTinyCore, you can ignore all this stuff

You will find a modified avrdude.conf file in the base folder. This is based on the current avrdude.conf file from:*checkout*/avrdude/trunk/avrdude/

It has been modified to work with avrdude 6.3, by removing (actually, commenting out) some incompatible stuff, and adding the "jtag2updi" programmer type.

The definitions for UPDI chips were slightly modified so that avrdude thinks they use the PDI programming interface instead of UPDI (i.e., avrdude thinks they are some kind of Omegas).

This allows the Janice mk2 protocol to be used for programming UPDI chips, since this protocol predates UPDI and is not formally compatible with it. Originally, I had planed to use the STK500v2 protocol, and emulate the ISP interface, and I actually wrote an ISP version of the programmer software.

However, this would require entirely new definitions for the UPDI chips inside the avrdude.conf file, while using jtagice2 requires only very slight changes to the definions provided by Atmel (now Microchip).

Janice mk2 is the most advanced of Atmel's programming protocols that still supports a UART serial connection instead of USB, making it easily compatible with any Arduino you choose to host this software, and any OS you run avrdude on.

It's major limitation is speed; it can't go over 115200 Baud, because the protocol lacks definitions for higher speeds. It's actually inferior to the STK500v2 protocol in this respect, this older standard can run at any speed avrdude instructs it to.

Fortunately, the current UPDI chips do not have very large flash memories, so I think this isn't a major issue.

Example command line (windows):

avrdude -c jtag2updi -P com7 -p t1614

If all the connections are correct and the target is indeed an unlocked tiny1614, the output will be:

avrdude: AVR device initialized and ready to accept instructions

Reading | ################################################## | 100% 0.03s

avrdude: Device signature = 0x1e9422 (probably t1614)

avrdude done. Thank you.

If the chip is locked, the output will be:

avrdude: jtagmkII_reset(): bad response to reset command: RSP_ILLEGAL_MCU_STATE avrdude: initialization failed, rc=-1 Double check connections and try again, or use -F to override this check.

avrdude: jtagmkII_close(): bad response to sign-off command: RSP_ILLEGAL_MCU_STATE

avrdude done. Thank you.

To unlock the chip, you need to erase it. Currently, the "-e" option is not working with jtag2updi, let's call it a known bug, so you need to enter interactive mode, using "-t", and "-F" to override the error:

avrdude -c jtag2updi -P com7 -p t1614 -U flash -t -F

You will enter the avrdude prompt:

avrdude: jtagmkII_reset(): bad response to reset command: RSP_ILLEGAL_MCU_STATE avrdude: initialization failed, rc=-1 avrdude: AVR device initialized and ready to accept instructions avrdude: Device signature = 0xffff00 avrdude: Expected signature for ATtiny1614 is 1E 94 22 avrdude: NOTE: Programmer supports page erase for Xmega devices. Each page will be erased before programming it, but no chip erase is performed. To disable page erases, specify the -D option; for a chip-erase, use the -e option. avrdude>

Enter "erase" then "quit" and the chip will be unlocked (and erased).

avrdude> erase

erase avrdude: erasing chip avrdude> quit quit

avrdude done. Thank you.


If you have triple-checked all the connections but still getting errors, the problem might be the speed of the serial links. I have set the jtag2updi entry on the avrdude configuration file to run at 115200 baud by default. This baud rate can cause errors, for example, if your MCU is running at 8MHz.

This can be changed with the avrdude "-b" option. Valid baud rates are 2400, 4800, 9600, 14400, 19200, 38400, 57600 and 115200. You can make the setting permanent by editing the jtag2updi entry on "avrdude.conf".

If the trouble is on the UPDI link, a slower speed can be selected by changing UPDI_BAUD and recompiling.


UPDI programmer software for Arduino (targets ATtiny 417 / 814 / 816 / 817 / 1614 / 1616 / 1617 and similar MCUs)






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