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  • Author Author: mistertee
  • Date Created: Date Created
  • Views 5920 views
  • Likes 1 like
  • Comments 19 comments
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Watchdog Timer for the Raspberry Pi

mistertee

Like all PCs the Raspberry Pi is likley to hang up during operation due to unexpected events not being catered for in the code. In an unattended application where the program automatically starts this can be a problem as there is no one around to restart the RPi. If there is no keyboard or HDMI monitor this is by power cycling. The Raspberry Pis internal watchdog timer has a poor reputaion - there are many comments about it being unreliable. This post presents a simple external watchdog timer.

 

Although there are dedicated power supply monitoring chips this solution uses components that are likley to be in most electronic hobbyists kit - based around the popular 555 timer.

 

image

 

The programme has to supply a Hi going pulse every few seconds. If there is no pulse for more than about 60 seconds the 555 output on Pin 3 will go low. This is coupled through to the on-board reset pin of the Raspberry Pi through a capacitor to issue a low going pulse which resets the Pi.

 

A capcitor is used here so that only a pulse going from Hi to Low will reset the raspberry Pi. A constant Lo voltage which is present while the raspberry Pi is starting up will have no effect.

 

If a shorter time is required then either reduce the value of R1 (1Mohm) or reduce the value of C1 (100 uF).

 

The LED pulses every time a Hi Pulse is issued to provide a visual indication to the user that everything is fine. The 555 timer does not easily provide a retriggerable monostable function that is required for a watchdog timer. Diode D1 is used to discharge the capacitor every time a pulse is received.

 

The reason the delay before a reset is set at more than 60 seconds is that the Raspberry Pi takes more than 30 seconds to start up.

 

Click here for a guide to setting the Raspberry Pi to autostart.

 

image

 

The image above shows the connection to the Reset pin of the Raspberry Pi.

 

Summary

 

Althogh the Raspberry Pi has an in-built watchdog timer, this has a poor reputaion. This watchdog timer is a reliable way of resetting the Raspberry Pi in remote applications where there is a risk of it hanging up due to the code not handling unexpected events correctly.

  • in reply to mistertee

    or just have a lot of spare time

    Haha ... less since I frequented this place. image

  • in reply to shabaz

    OK thanks. I will change the post now to make this more explicit.

  • in reply to mistertee

    We do not work for E14, but like you, we actively use the Pi (and other SBCs).

    Your statement  regarding it being "likely to hang during operation" was something to be concerned about.

     

    Digging deeper now that it turns out it was actually a Python application issue which would not result in the board hanging, my concern disappears.

  • in reply to shabaz

    Thanks for both your comments. Out of interest do both you guys work for Element 14 or just have a lot of spare time?

  • in reply to mistertee

    Actually the best answer would be to not do this, until the Python code is fully baked and hardened. Otherwise you're at risk of code restarting under your feet, making life very difficult in debugging.

    A restart such as this needs to be an extremely rare thing, in my opinion to the point that you have sufficient time to investigate each single restart even if you have hundreds of customers using your software non-stop.

     

    Your process monitor can be as simple or as sophisticated as required, there might be examples via google, ranging from scripts to complete applications (I've not checked). The only time I've written one was for a commercial app so I can't share it, but it was in C++ (the language doesn't matter). It is at heart a state machine that needs the ability to detect actual application status, desired status (running or stopped, etc - since sometimes you may wish the application not to run), be able to kill and restart the application (e.g. using a kill or pkill command).