Reset pin not working in Argon

We’d like to use a Xenon like a HW watchdog to reset an Argon by taking the Argon’s reset pin low. However, we just can’t make it work.

In a nutshell, the Argon is creating a heartbeat by toggling D7 high/low every half second. The Xenon watches this thru an input pin, and if the Argon’s pin state doesn’t toggle for a certain period of time, the Xenon takes the Argon’s reset pin low for 1 second. However, the Argon doesn’t reset.

But if we use the same code to reset a Xenon, it works every time. Also, if the Argon’s reset pin is connected directly to ground, it does reset.

Any thoughts on why the Xenon can be reset by taking its reset pin low by another controller, but the Argon can’t?

Update: I added a 470 ohm pulldown from the Argon’s reset pin to ground, and now when the other controller takes the Argon’s reset pin low, it resets. Why doesn’t the Xenon need this? Or a better question is why does the Argon need this?

Are both grounds of the xenon and argon connected together?

Yes, the grounds are connected. Plus I can unplug the Argon from the breadboard and plug a Xenon in its place and the Xenon will reset every time. Or I can put a 470 ohm resistor from the Argon’s reset pin to ground and using the otherwise-same everything else, it will reset every time.

Unfortunately, adding a pulldown resistor to the circuit isn’t a good solution in this case and doesn’t explain why Argons and Xenons would behave differently.

@retupmoc1, both the Argon and the Xenon share identical RESET hardware design. Both have a button which connects to GND when pressed, a 10K pull-up resistor and a 0.1uF cap to GND to slow the rise time going to the REST pin of the nRF52840. To reset the Argon, a GPIO pin from the Xenon must be set LOW and be able to sink 330uA or so from the pull-up resistor, which it can. NOTE: The GPIO pin used to connect to the Argon RESET pin must be set to pinMode(OUTPUT) with no pull-up or pull-down specified.

Adding a 470 ohm resistor to GND on the Argon creates a voltage divider, producing a 0.15V voltage at the RESET pin. Since there is no typical “LOW” voltage specified for the nRF52840, the 0.15V may hold the RESET pin at the very cusp of resetting. This may explain why it works. Why it works without resistor on a Xenon is not clear to me.

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@peekay123 - thanks for the detailed info. I’ve tried this with multiple Argons, and none will reset without the 470 ohm resistor in place. Multiple Xenons work, so it doesn’t appear to be an isolated case.

Thanks again.

@retupmoc1, I’m going to have to test this on my bench.

Remove your 470 ohm resistor. Change your reset time to 5 seconds so you have time to make a measurement. What is the voltage on the Argon’s reset pin when you have it pulled low.