Meshtastic

Watchdog Timer as Failsafe

Development
#1

Do any of the Meshtastic devices have a watchdog timer pulsing on an IO?

I was thinking this might be useful for remote solar powered devices. Ive noticed during some tests that if the voltage simply drops below 3.3v the TBeam V1.1 powers down. Simply increasing the voltage above 3.3v does not power it back up. You have to physicaly break the connection to the battery and reattach it for the TBeam to power up again.

I was thinking it would be easy to build a small device using an ATTiny that could monitor the watchdog and when the battery voltage rises above the minimum setpoint it could cycle power to the Radios to power them back up.

Im not sure how likely a lockup could occure, but if the web server were to lockup, this could also trigger a reboot without having to climb a tower or a mountain to power cycle the Meshtastic repeater.

#2

we use the watchdog timer to check for most possible fatal software failures. But the problem you are describing though related is a little bit different:

on the tbeams ttgo used an “axp192” power management chip. Which seems pretty slick based on my read of the datasheet. But if the battery and USB voltage drop low enough that that chip can’t keep itself powered then it will shutdown and only then restart with a button press. Software can’t help.

From my read of the ax912 datasheet I think ttgo could fix this in future revs of the board:

  • They have a backup battery on the GPS, instead of connecting it to the GPS IMO they should instead connect it to the pins for that purpose on the axp192. This would allow the AXP to not lose its brain when the main power gets too low. Section 9.4 of the axp192 datasheet discusses this - that power could also be fed to the GPS.

With this change we could use the I2C registers of the axp192, to automatically enable main CPU power without requiring that button press.

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#3

It looks like the 3.3V is only live when the device is powered. The 5V has power even if the device is powered off. It would be easy enough to monitor the 3.3V to see when the device has powered down and then take action from there.

#4

If you don’t need all the features of the tbeam, the other hardware may be a good alternative. They don’t have the dedicated power management chip and powering them is much more straight forward.

#5

Looks like I forgot to bring my other devices to work today. :thinking: I guess I have to find some work to do and test tomorrow.