Is a power off sufficient to avoid problems with the Watchdog on Arduino Mega 2560?

Question

I'd like to offer some context about what I'd like to do because I might have misundertood something.

I'm trying to reduce power consumption on my device, an Arduino Mega 2560, and I've found this library

https://github.com/rocketscream/Low-Power

which uses the Watchdog to put Arduino on power saving for a predetermined interval of time; after that it reactivates what has been disabled and let the program continue its flow.

I searched for more information about the Watchdog and I found out several warnings about dealing with Watchdog on an Arduino Mega 2560, stating that it would only work with a modified bootloader: This scared me a bit because I'm not very keen on risking to brick my Arduino (I absolutely need it to work on the next few days).

Now, I only have two Arduino Mega 2560 at my disposal and if I'm not mistaken, flashing the bootloader on an Arduino Mega 2560 using another Arduino Mega 2560 as a programmer can't be done because the procedure ends with errors:

( see http://forum.arduino.cc/index.php?topic=126160.0 )

Hence my most important question: if I test the watchdog functionality on my code (the sketch) and the Arduino happens to continually reset, can I fix the situation by powering off the Arduino?

(if you can help with my other issues as well, I'd be very grateful, I'm not sure I completely understood how these things work)

Answer 1

The problem you have is that once the watchdog is turned on it is very hard to turn it off again. There are certain procedures you have to go through to ensure it's turned off.

The default bootloader doesn't turn the bootloader off, and a reset doesn't turn it off either. The watchdog then fires whilst the bootloader is still waiting for communication from the PC thus making it reboot before your sketch has had a chance to run. Hence the change needed in the bootooader.

The only way to clear the watchdog is to remove the power. However as soon as your sketch runs and turns the watchdog back on you are then back at square one.

So you need to upload a new sketch to stop that happening, but that is tricky, since the watchdog keeps resetting and interrupting the bootloader. So what do you do? Well, there's a specific sequence you need to do.

1. Unplug the mega
2. Compile the new sketch
3. Hold the reset button on the mega
4. Plug in the mega
5. Press upload
6. As soon as avrdude starts release the reset button.

That way you have a chance of the bootloader running without your sketch having run first and enabled the watchdog. The bootloader runs just the once when the computer is tryung to upload the sketch, and with luck that will happen instead of your sketch running.