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I have a microcontroller (Arduino-like IoT device) and would like to connect a low power external hardware watchdog to be able to recover from failures. I know that a software watchdog is already incorporated, but it's just not sufficient in this case.

The requirements are:

  1. WD's output to start on "on" state and be able to stay in this state for at least one day in the absence of external trigger.
  2. If it receives a high signal from the microcontroller, it should reset (i.e. extend the "on" state for another day).
  3. If it doesn't receive a high signal from the microcontroller within the predefined time period (e.g. one day), it should timeout and switch its output to "off" for at least 10 seconds.
  4. It would be nice if the on and off periods are configurable (hardware-wise, e.g. through a potentiometer).
  5. Operation voltages 2-6V.
  6. Price does matter, as it's for a relatively cheap IoT microcontroller.

To give an example of what I'm trying to achieve, the best solution that I've come across that should cover my needs is this one: https://www.ebay.com/itm/DC-3V-24V-automatic-re-trigger-cycle-delay-time-timer-switch-module-max-20days/303098128782 (unfortunately, it exhibits instabilities and even worse the IC on it is not labeled).

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  • \$\begingroup\$ Is this just a "shopping" question ie you want us to search for you? See : electronics.stackexchange.com/help/on-topic \$\endgroup\$ – Solar Mike Jul 5 at 7:15
  • \$\begingroup\$ I see your point, thank you for the on-topic link you provided. I don't think that this is really a shopping question, i.e. I don't need somebody to suggest a link for me to buy from, I don't see it that way at least. Perhaps what you say arises from the fact that I provided a link, but I only did so to help others understand what I'm trying to build. I'd say that my question is more of a "a specific electronics design problem": I have a very specific problem within my (quite larger) circuit that I can't solve. I'm seeking for help on this specific problem. \$\endgroup\$ – dkomna Jul 5 at 7:32
  • \$\begingroup\$ Well, you give a list of requirements to be met and also point out that price matters. But others will decide... \$\endgroup\$ – Solar Mike Jul 5 at 7:38
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    \$\begingroup\$ look at a 4060 IC, oscillator and 14 stage counter. They're designed to implement long delays with low power. Maybe cascade two of them to get out to days. \$\endgroup\$ – Neil_UK Jul 5 at 7:53
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    \$\begingroup\$ Then install a reset generator eg. MCP102 ww1.microchip.com/downloads/en/DeviceDoc/20001906D.pdf \$\endgroup\$ – Bruce Abbott Jul 5 at 21:49
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A 32 bit timer at 32768 Hz will give you that kind of time period (up to 36 hours.)

Maybe a Silego GPAK3 could do it. It has logic, counters and an internal clock reference that together could make that long of a resettable timeout. In volume they’re about 15 - 20 cents. And they’re really small.

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  • \$\begingroup\$ After reading some answers and searching for reading material, I'm starting to understand better the oscillators and counters for building a watchdog. I can't say that I fully comprehend how to build a WD, but I'm on it. GPAK may be a good starting point as well. Thank you very much! \$\endgroup\$ – dkomna Jul 5 at 20:24
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You could use a small microcontroller such as an ATTINY85 or similar, which requires no crystal or other external components, apart from a decoupling capacitor, and it is very cheap, and with low power consumption, if configured correctly. A solution like this would allow you to adjust its function to your needs.

However, if you are doing this circuit commercially, you will have to take into considerations that you will have one more device that requires programming during the manufacturing stage, which might or might not affect the manufacturing costs, depending on the complexity of the system.

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  • \$\begingroup\$ Thank you very much @Elsemito. It's not a commercial circuit, for the time being at least. However, you are absolutely right, I would highly prefer to avoid such a solution, due to the extra programming complexity that it imposes. However, I haven't yet written it off. \$\endgroup\$ – dkomna Jul 5 at 19:56

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