cross-posted from: https://lemmy.dbzer0.com/post/38015770

A washing machine is trapped in a fault state even though all the components function (AFAICT). The controller board has two ports:

  • ISP (to attach an ISP programmer to flash new software)
  • USART (4-pin serial port: 0v, TX, RX, 5v)

I’m guessing the ISP port is useless without whatever proprietary software is needed. But what can the USART do for me? Can that be used to obtain the error code and clear it, or reset the board to the factory state? Has anyone done that, without documentation?

  • @j4k3English
    2•7 days ago

    You are correct and I misspoke in my wording and stated logic. I had intended to constrain my logic to eliminating the potential of the entire script being stored or somehow altered and reloaded. Storing an error code is entirely feasible.

    Now the machine will not even attempt to run because it is apparently trapped in an error state.

    You might trace out the pins that go to any buttons. I am not super familiar with the 32L, but IIRC usually old Atmel chips only have a couple of hardware interrupts available.

    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.