Approach for watchdog timers?

[dmurray]

Related to my post here regarding designing a decent architecture for Raspberry Pi, I want to implement a watchdog timer, or timers. My current architecture is shown below. The three main loops that I think I need to monitor are the RPi Scan Engine, RPi Command Handler, and the Data Logger, The Scan Engine reads the system temperature and high-temp alert.The RPi Command Handler accepts commands from a TCP client and updates system settings. The Data Logger just logs the temperature and alert status at 1 second intervals. Regarding watchdog timers, the problem is I've never had a need for one in my usual LV programming, so I'm not quite sure what I want here. If I was using a CRio I could use the built in HW watchdog timers, but that doesn't apply here. So I guess I need some sort of SW timer. What's the general advice here regarding custom-built watchdog SW timers?

 

Edited August 10, 2016 by dmurray

[dmurray]

Not sure why I thought this was difficult. I just added a Watchdog Event Loop to any sub-VI I want to monitor, and send the status to the client. So it's more of a status monitor than a watchdog timer, although it does flag timeouts to the client as well. 

 

 

[ShaunR]

Watchdogs usually give you the option to restart the software if it stops responding and are usually hardware driven. If you have a software one, the chances are your watchdog will hang too so you need an external process  that gets kicked every so often. This external app can then forcefully close the application and restart it. You can communicate with the watchdog via TCPIP or sharedmem and just message it every so often from a dedicated loop in the software

[dmurray]

1 hour ago, ShaunR said:

Watchdogs usually give you the option to restart the software if it stops responding and are usually hardware driven. If you have a software one, the chances are your watchdog will hang too so you need an external process  that gets kicked every so often. This external app can then forcefully close the application and restart it. You can communicate with the watchdog via TCPIP or sharedmem and just message it every so often from a dedicated loop in the software

Okay. So is my thinking correct here?... I could use a HW Real-time clock module and configure it with a timeout. When my SW is running on the RPi, I just 'kick' the module timer periodically to reset the timer. And if the module does time out (because my SW has presumably hung), the module sends a signal back to the RPi which will reboot it. 

[ShaunR]

I don't know much about that module but if it has that capability then yes. you could do that. Most NI modules are quite expensive and now you would have a hardware module dependency for just a watchdog. Most Linux kernels have a watchdog feature if the hardware supports it so I would look into that first. If that's not possible, cost is an issue and it is likely just your application that will hang rather than the OS; then you can spawn a separate process to act as the watchdog.

Edited August 11, 2016 by ShaunR

[dmurray]

2 hours ago, ShaunR said:

I don't know much about that module but if it has that capability then yes. you could do that. Most NI modules are quite expensive and now you would have a hardware module dependency for just a watchdog. Most Linux kernels have a watchdog feature if the hardware supports it so I would look into that first. If that's not possible, cost is an issue and it is likely just your application that will hang rather than the OS; then you can spawn a separate process to act as the watchdog.

It turns out the RPi does have an internal watchdog feature, as you suggested, although apparently it has some issues. But I also found this, which will serve as an external watchdog if I need it. As well as the watchdog functionality, the real time clock itself will also be useful, as the RPi needs to be connected to the internet to keep accurate time (i.e. it doesn't have native or on-board real time clock functionality). So this is perfect for what I want.

Anyway, I'm probably overthinking this, because on reflection the concepts are really simple. But thanks for your help!