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  • Author Author: mistertee
  • Date Created: Date Created
  • Views 5875 views
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  • 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.

Parents
  • Like all PCs the Raspberry Pi is likley to hang up during operation.

    Hi Seggy,

     

    Just to flesh out things a bit, perhaps an application could be likely to hang, generally the Linux kernel is not likely to hang unless there is a very severe issue like a build issue, hardware fault or power related issue.

    It is worth mentioning that if the board ever does lock up, this should be investigated; because for the hardware watchdog to have to ever take action on a Linux platform is "severe".

    Possible reasons could be a poor power supply perhaps, or bad cooling strategy, and for these the hardware watchdog action is like a sticking plaster and does not fix the underlying issue(s).

    The underlying issues should therefore be resolved if the watchdog timer actually has to ever take action.

    Finally, if users are experiencing application hangs then these do not require a hardware watchdog timer.

  • in reply to shabaz

    Would using a proper brown-out IC (with the similar pulse before time-out) be preferable than this approach.

     

    I do agree that simply resetting it could mask another issue that requires resolving.

     

    Mark

  • in reply to mcb1

    True - I am sure there is a much better chip out there. 555 timers are like Marmite - you either love it or hate it. I had some in stock (doesn't one always) and couldnt be bothered to order a new IC and pay postage and wait 2 days.

  • in reply to mcb1

    Hi Mark,

     

    That's a good point, power-related issues that require a reset could be handled with brown-out detection driving the reset.

  • in reply to shabaz

    Would you guys be able to recommend one that you've used before?

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