Timonel is an I2C bootloader for ATtiny85/45/25 microcontrollers. It lets you update AVR firmware when there's a more powerful MCU around (ESP8266, ESP32, Arduino, RPi, BeagleBone, etc.) acting as the I2C master, with one or more ATtiny85s as I2C slaves doing peripheral tasks.

Some examples:

• A small robot controlled by an ESP8266, where each limb's tasks are handled by a Tiny85 over an I2C bus.
• Multisensor IoT setups, where each Tiny85 is a node taking care of one or more sensors.
• etc. — you get the idea.

In these setups it's really handy to update the AVRs' firmware from a single entry point: the I2C master. Depending on the main MCU, updates can even go over the air. But back in mid-2018 there wasn't an I2C bootloader that really fit the TinyX5 family and worked around its limitations:

• No dedicated hardware for I2C — just the USI (Universal Serial Interface).
• No protected memory area for the bootloader.
• No way to redirect interrupt vectors to the bootloader.

So this project started...

• Install "timonel.hex" on a Tiny85 (bare chips or Digisparks).
• Build your application as usual, then use "tml-hexparser" to turn the ".hex" file into a byte array "payload.h", and drop it into the "data/payloads" folder of "timonel-mss-esp8266" or "timonel-mms-esp8266".
• Use VS Code + PlatformIO to compile and flash "timonel-mss-esp8266" or "timonel-mms-esp8266" (which includes the payload) on an Arduino-compatible MCU. It's been tested with the ESP8266 ESP01 and NodeMCU. Note: the included ".bin" file has a small demo payload that blinks PB1 on the Tiny85.
• Hook up both chips over I2C (SDA, SCL and ground).
• Open a terminal (e.g. MobaXterm) on the I2C master's serial port (115200 N 8 1).
• Run the "timonel-twim-ss" commands shown on screen to erase and flash new firmware on the Tiny85.
• You can also update the bootloader itself with "timonel-updater" (based on the micronucleus upgrade program).

timonel                           
│
├── timonel-bootloader : Bootloader main folder. Built with "avr-gcc" and "make", using the provided scripts.
│   ├── configs        : Several setups to balance features with memory usage. Called from the "make-timonel.sh" script.
│   ├── releases       : Where the compiled binaries end up.
│   ├── ...
│   ├─ make-timonel.sh : Bootloader build script. Run "./make-timonel.sh --help" for options and parameters.
│   └─ flash-timonel-bootloader.sh : Flashing script. Takes a binary from "releases" and flashes it with "avrdude".
│
├── timonel-bootloader-io : Bootloader as a PlatformIO experimental project.
│   ├── configs           : Several setups to balance features with memory usage. Selected from "platformio.ini".
│   ├── ...
│   └─ platformio.ini     : Controls all the settings and build parameters.
│
├── timonel-bootloader-el : Same bootloader as "timonel-bootloader" (Make version), but with the USI-based, interrupt-free I2C driver as an external library.
│   ├── configs           : Several setups to balance features with memory usage. Called from the "make-timonel.sh" script.
│   ├── releases          : Where the compiled binaries end up.
│   ├── ...
│   ├─ make-timonel.sh    : Bootloader build script. Run "./make-timonel.sh --help" for options and parameters.
│   └─ flash-timonel-bootloader.sh : Flashing script. Takes a binary from "releases" and flashes it with "avrdude".
│
├── timonel-bootloader-ioel : Same bootloader as "timonel-bootloader-el", but as a PlatformIO experimental project.
│   ├── configs           : Several setups to balance features with memory usage. Selected from "platformio.ini".
│   ├── ...
│   └─ platformio.ini     : Controls all the settings and build parameters.
│
├── timonel-hexparser   : Turns a ".hex" file into a ".h" payload for I2C master apps.
│   ├── appl-flashable  : Put your application ".hex" files here.
│   ├── appl-payload    : The converted ".h" payloads land here.
│   ├── ...
│   └─ make-payload.sh  : Hexparser conversion script.
│
├── timonel-updater       : Turns a Timonel binary into a bootloader ".h" update payload for an I2C master.
│   ├── tmlupd-flashable  : Put Timonel bootloader ".hex" files here.
│   ├── tmlupd-payload    : The ".h" bootloader update payloads land here.
│   ├── ...
│   └─ make-updater.sh    : Bootloader updater conversion script.

• Nb_Micro: Arduino library to control devices that speak the NB command set over an I2C bus.
• Nb_TimonelTwiM: Arduino library for uploading firmware to a chip running the Timonel bootloader. It uses NbMicro to talk to the I2C bus.
• Nb_TwiBus: Arduino library to scan the I2C bus for connected device addresses and data. It uses the TimonelTwiM bootloader object definition.
• nb-twi-cmd: The NB TWI (I2C) command set.

• timonel-mss-esp8266: Timonel I2C master for a single slave. Serial-console app to send commands to a device running the bootloader, from an ESP8266.
• timonel-mms-esp8266: Timonel I2C master for multiple slaves. Serial-console app that loops around flashing, erasing and running a user app on several Tiny85s running the bootloader, from an ESP8266.
• timonel-ota-demo: Shows an ESP8266 I2C master checking a website for updates, grabbing a new firmware file from the internet, and updating an ATtiny85 slave over the I2C bus.

Contributions are welcome! Want to add a feature? Feel free to open a pull request, file an issue, or start a discussion. :o)

v1.6 - 2023-05-22: All multi-byte fields in the protocol now use the same little-endian (LSB first) byte order, as agreed in discussion #28. Data packets bumped up to 64 bytes (a full SPM page on an ATtiny85). Support extended to more USI-based AVR families (ATtinyX4/X5, ATtiny2313, ATtinyX7, ATtiny26, ATtiny43U). Use "|" instead of "+" when building flash addresses, saving 4 bytes each time. Fixed "inline" keyword inconsistencies and improved the scripts (STK500/COM port option, "#!/bin/bash" shebang, path backslashes). Added the "tml-t85-test-comm" configuration for easy startup.

v1.5 - 2020-07-03: Added READEEPR and WRITEEPR to read and write EEPROM data, plus the READDEVS command to read the device signature, fuses and lock bits. A few code fixes and a smaller "pre-main" startup file free up an extra flash page for apps. The repo was reorganized: I2C libraries and examples now live in their own repos so they can be versioned independently. Added an experimental PlatformIO project for more structured builds, though the Make version is still the recommended one.

v1.4 - 2019-10-29: Big memory savings by inlining the TWI driver functions — "tml-t85-small" now takes under 1 kB, leaving 7 kB for user apps. Speed boost by sending 32-byte packets (half a page of memory on a Tiny85). "autorun" for user apps is now optional, and internal clock config support got better. Interactive master improved with slimmer libs (see it working), plus a new multi-slave master (see it working).

v1.3 - 2019-06-06: Inline bootloader functions (smaller code) and low-fuse auto clock tweaking. Support for 1, 2, 8 and 16 MHz clocks in user-app mode. Refactored the TWI master's UploadApplication, which now supports both page address calculation types and both APP_USE_TPL_PG modes. Several bug fixes.

v1.2 - 2019-05-15: "Good-neighbor" behavior stops data dumps from interfering between Timonel devices on a multi-device bus. The TWI master code was packed into a couple of Arduino libraries to make the many config options easier to handle, and moved to PlatformIO.

v1.1 - 2018-10-29: Added the optional ReadFlash command. Minor tweaks to run Timonel at 8 MHz.

v1.0 - 2018-10-05: Optional features implemented.

v0.9 - 2018-09-29: Functional pre-release: temporary page buffer and other minor issues fixed.

v0.8 - 2018-09-16: First functional pre-release.

v0.7 - 2018-09-07: Non-functional.

Thanks to the folks at AVRFreaks.net, especially joeymorin and clawson, for all the knowledge and technical advice. And many thanks to Donald Papp at Hackaday for posting about this.