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Thread: Questions on Watchdog IC Interface to reset Teensy 4.0

  1. #1
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    Questions on Watchdog IC Interface to reset Teensy 4.0

    Hello!

    I'm a student and I'll be using a Teensy 4.0 in the electronics design for a CubeSat. I'm worried about potential latch up issues on the microcontroller caused by single event upsets. Previous flight heritage dictates that space ready electronics boards utilize a hardware configured watchdog circuit to monitor the microcontroller. Ideally, I would use a backup microcontroller, or a backup FPGA, or a radiation hardened microcontroller, but unfortunately the design is time and money constrained.

    After reading some other forum posts here, it seemed like there was a bunch of software solutions. However, I am looking to avoid anything software related to have a totally hardware based backup. I've thus designed the below circuit (see attached photo) using a MIC1832 watchdog I.C. Since there is no reset pin on the Teensy 4.0, the watchdog will short the power input to the Teensy. It seems that Paul recommended this in another forum post.

    My main concern is timing characteristics. The watchdog timer is 1.2 seconds. If the watchdog triggers, then the RST pin will go high for 250ms. I'm wondering two things about timing:

    1) Will the 250ms short on the power line be enough to reset the microcontroller? The input voltage is DC.
    2) Will the microcontroller be able to reboot in the 1.2 seconds and start sending the /ST signal before the watchdog trips again? I don't want it to get stuck in a constant loop.

    This is my first time designing a watchdog circuit, and my first time designing with the Teensy 4.0. I'm also quite new to electrical engineering, so please feel free to explain even basic concepts if there is something I am missing. Thanks you all so much!
    Jashan

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  2. #2
    Senior Member PaulStoffregen's Avatar
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    Shorting the power seems like a pretty bad way to go. Maybe it can be made to work, but if your end goal is reliability, my gut feeling is this strays pretty far in the wrong direction!

    Before you go a lot farther on this path, maybe read chapter 59 of the IMXRT1060 reference manual, starting on page 3213.

    https://www.pjrc.com/teensy/datasheets.html

    If you don't use the EWM and you *really* want to interrupt the power, I'd recommend using a P-channel mosfet which normally passes the power but you can turn off when you want to interrupt the power. Don't short the power supply with a N-channel transistor.

  3. #3
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    Hi Paul, thanks for the quick response. I just had a similar recommendation from a professor, and it makes much more sense. However, I will look into the chapter on the external watchdog monitor that you recommended. Thank you so much!

  4. #4
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    Teensy 4 has 4 watchdogs, each with similar behaviour, yet still different. As long as you keep feeding it you shouldn't need to worry about it firing into a reboot loop. It is also capable of resetting if you feed it too fast (if you use window mode). the Pioneer NEX runs a similar processor, MX6, the watchdog timeout on it is 30 seconds. It is all adjustable and built into the hardware, so long as you set a feeding time that just exceeds loading time, that should give you enough processing time before the feeding begins.

  5. #5
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    Hey tonton, thanks for the response. Is the feeding time set in software, and if so, how? The main concern with using an onboard watchdog is that anything inside the microcontroller could theoretically be hit by a charged particle in space and cause a latch up. Since there are four of them though, that does provide good redundancy. Maybe it is appropriate to say that I do not need an external watchdog monitor?

  6. #6
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    you could hard fault the cpu it'll still restart, try it out, checkout WDT_T4, you'll have to checkout the differences with each watchdog in the release manual like Paul said

    The 4 different watchdogs are WDT1 WDT2 WDT3 and EWM (external watchdog monitor).
    Last edited by tonton81; 10-27-2020 at 06:22 PM.

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