Comment on Washing machine PCB has a USART port. Do repairers use that?
diyrebel@lemmy.dbzer0.com 4 days agoI don’t intend to modify the program. I am just looking to reset the state of the software to get it out of the fault state.
Normally that can be done by using the buttons to enter a secret combination code to:
- enter diagnostic mode
- run various functions/cycles which normally run as part of a program
- see the error code
- reset the board
When the software detects a fault (such as a broken pump), it saves the error code. Then if you fix the pump, the software doesn’t know the pump has been fixed. So the board has to be reset to clear the error code.
The button sequence codes are secret and known only to the manufacturer. They are very protectionist. In Europe, law requires them to make the codes available to other 3rd party technicians – but only in the 1st ten years and they can also charge a fee. Consumers get no access under any circumstances.
My thought was theoretically a pro independent repair service would not want to pay every manufacturer for the secret info for every model they repair – so perhaps they would attach to the USART serial port and have a way to see errors and reset the board. But if it’s as you say, then the USART is disabled and useless to repairers. Which means I’m stuffed because I cannot buy a replacement card for my machine.
If the serial port is not disabled, you conjecture that it is likely a read-only non-interactive mechanism. That still may be useful. I was able to find the secret button combination that is likely giving me an error code which I can guess the meaning of based on leaked docs for other models, but I’m not satisfied with that. I would be useful if I could get more verbose or supplemental info about the error state.
j4k3@lemmy.world 4 days ago
In the last 10 years it has become common for devs to disable and remove or encrypt to prevent reverse engineering and repairs. If the device is older, the debug info from UART is likely still present. How useful that information is may be dubious.
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.