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So, when a simple CPU core is running, there are various ways to force it to stop what it is doing and divert attention elsewhere for things that are more important. At the general level there are flags that can be set to indicate higher priority tasks need to be completed inside the CPU. This is stuff like a block of serial communication is received and needs to be processed so that the buffer doesn’t get too full. Or, some timer expired and triggered some code to run next. Hardware interrupts are like these flags but are usually setup as the highest priority interrupts in the physical hardware. Like a person can make any Input/Output capable pin into an interrupt by turning it into an input, and simply checking the state of that pin in the code that is running. However, the hardware interrupt is very powerful and forces the CPU to only pay attention to whatever code is associated exclusively with that interrupt. Typically in the code, one would only use this hardware interrupt to set a flag somewhere quickly and return to execution of whatever was happening. There are a lot of gotchas that need to be taken care of if one wants to do something more complicated because the hardware interrupt isn’t like multithreading code in a desktop CPU where all the registers and states are saved. This is like, stop in the middle of a word on the exact letter you are pronouncing mid sentence while talking to someone about something important the moment that interrupt happens. It is quite likely that the key combo to reset your device is related to one of the hardware interrupt pins. It would be reasonable in the code to check if another pin is low when the interrupt happens. You know the device can be reset by someone. It will be just a combination of keys. If there are a lot of keys, this should limit the number of possibilities to something manageable. Write this stuff down and test methodically. Also be sure to check Louis Rossman’s new documentation project website and do a search on the EEVBlog forum if you have not already done so.

Most of what you are saying about functionality is not in line with how a microcontroller works. The microcontroller is not sophisticated like a real computer. It is like a very simple state machine running on the device. Basically it is like a single long script that is not running on any kind of abstraction layer. When the device is turned on, there is the script. The script is not executed by some operating system or any other complication. The entire software stack is the script. Only, the script is actual hardware registers and flags and operation codes. There is a tiny amount of RAM that actually runs the script, like 16kb. There is the programmed memory, like 32kb. Then there is a tiny amount of electrically erasable and reprogrammable memory, like 4kb. The eeprom is the only section of memory that a microcontroller can access to write persistent data, meaning something that will still be present when power is cycled. I doubt many people use eeprom to save any kind of error. This is more like where a serial number might be saved, or more likely the calibration values for some internal control algorithm. It is not anywhere near large enough to save something significant like the script. It is far more likely that the script is just a state machine and is reaching an error state because of some missing or bad signal that it needs to continue running the script. This is likely your problem and the issue is like 99% likely to be hardware and not the microcontroller. Resetting such a device is just an interrupt signal that restarts the script. There is nothing dynamic about this. From the perspective of the original dev, the ISP programming interface has the functionality of brute forcing running code to step through each machine instruction and halt further execution while monitoring or changing any value present in any register of the internal microprocessor. This is super powerful but requires a pricy setup from the manufacturer in most cases, and a thorough understanding of the internals of a microprocessor’s registers, op codes, memory addressing, interrupts, clocks, and the ALU (arithmetic logic unit). In this type of device, text strings are far too expensive to have much, if any, value. Any such debug info is likely to be very terse. Text strings take an enormous amount of memory space to encode, so in general, these are not used very much. A microcontroller is a far simpler device than this. It is a computer in the sense of a device that can do Input/Outputs and run a control algorithm like a temperature controller with a PID loop.

Most of the info you will find about using UART to hack around is related to embedded Linux where UART is a full terminal interface. Devices capable of running embedded Linux are the next level above something like an ESP32 microcontroller. The datasheet for the ATMega32L is not really relevant to you unless you want to rebuild the entire software stack from scratch. Reverse engineering and rebuilding is a major project even for a pro dev. The manufacturer as you perceive a brand is likely just a logistics and label maker with little to no relevance to your actual device. The components PR entire machine were likely out sourced and contract manufactured. In the world of contract manufactured goods, the software is usually just a checklist of features. As soon as the checklist is complete and verified, the dev gets paid and is never seen again. If the company wants something more or something changed it will cost a fortune for the old dev to return or a new one to read-in to the code and make changes or just start over. This kind of thing might happen when versioning is required because some part is not available or something like that during production runs. Each run is like a one time thing. Some capital is put together and parts are sourced, then the contractor makes like 5k of the thing over the course of 2 weeks. These go in shipping containers around the world with whatever brand stickers applied. When each run is done, the software and any unique tooling are consumables to the whole affair and not something anyone cares about or keeps like a real asset of value. It is probably archived in several private databases on various employee’s computers, but these are not connected in any way to aid or support your needs. It is in the best interest of the company to never give out this info. Also getting the whole microcontroller toolchain working is a major pain in the ass. That large datasheet is because a microcontroller is a whole computer all integrated into a single chip. It is just a very simple type of computer. You need to understand all of the peripheral parts of computing to really make sense of it in practice. It is not just a microprocessor. Embedded Linux means that there is a kernel and user space abstraction layer that makes the unique hardware irrelevant and abstracted away. Without this abstraction layer, you need to know and understand that actual hardware assembly, or use a language like C and a compiler for the actual hardware. This is the level you are facing.