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Hot!PIC16LF Fail-safe Operation in Low-Power, High-RF Fields

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rbraddy
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2019/07/20 10:52:40 (permalink)
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PIC16LF Fail-safe Operation in Low-Power, High-RF Fields

I have an application whereby the PIC16LF18345 will operate within very high RF field conditions (close proximity to up to 2 KW of HF RF power from immediate antenna system within inches of the MCU).
 
Prior experience running MCU's within high near-field RF conditions like this suggests one should expect periodic lockups; i.e., I have personally observed these lock-up conditions before. To combat this, the MCU / system must detect when it was locked up (even in ultra-low-power sleep conditions) and reboot itself.
 
In my use case, extremely low power consumption from battery operation is required for up to 10 years or longer (using Energizer Ultimate Lithium batteries). During sleep conditions, I am seeing expected < 1 microamp current drain, which is perfect. In my application, wakeups to do actual work occurs infrequently, a few times per hour some days, then weeks or months of total inactivity and no need to do anything but conserve power.
 
With that as the backdrop, given the risks that RF fields pose to potentially interfere with proper long-term operation, I am designing in proper RF filtering for power and GPIO, including the latching relay involved for switching. However, I feel certain that some kind of failsafe mechanism is required.
 
Ideally, this failsafe mechanism would only need to wake up for an extremely brief period of time due to WDT timeout then immediately just go back to sleep. In the case something went wrong, then the application is completely reliant upon the WDT being able to wake up and reboot the MCU in case of failure.  Use of the maximum WDT timer period of 256 seconds (~4.2 minutes) should be sufficient as a maximum outage period, in case of a lockup.
 
So here are my questions as I contemplate design options:
 
Q1. Is the WDT going to address my concerns about potential lock-ups and reboot/recover the MCU?
 
Q2. How much power will the WDT consume if running during low-power sleep?
 
Q3. How do I calculate the required power (I have no available means to measure it) consumed for these WDT wake-ups over time?
 
Q4. Would I be better off with an external timer/reset circuit, such as a 555 timer, that's more impervious to RF?
 
Thanks!
Rick
#1

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    rbraddy
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    Re: PIC16LF Fail-safe Operation in Low-Power, High-RF Fields 2019/07/22 11:41:28 (permalink)
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    Looks like WDT should do what I need, just need guidance on how to determine power usage in connection with low-power sleep. 
     
    It sure would be helpful to see some guidance from Microchip on the power costs of key features relative to low-power sleep, given how critical battery power savings is these days.
    #2
    Jim Nickerson
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    Re: PIC16LF Fail-safe Operation in Low-Power, High-RF Fields 2019/07/22 11:49:57 (permalink)
    +1 (1)
    This is a user forum.
    To directly ask Microchip a question open a support ticket.
    #3
    mbrowning
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    Re: PIC16LF Fail-safe Operation in Low-Power, High-RF Fields 2019/07/22 11:51:00 (permalink)
    +1 (1)
    The information you want is in the datasheet. Based on your questions, it doesn't seem that you've looked.

    Oh well - there's always next year
    #4
    jtemples
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    Re: PIC16LF Fail-safe Operation in Low-Power, High-RF Fields 2019/07/22 11:53:45 (permalink)
    +2 (2)
    IPD_WDT is the symbol to look for in your data sheet.
    #5
    rbraddy
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    Re: PIC16LF Fail-safe Operation in Low-Power, High-RF Fields 2019/07/22 14:34:15 (permalink)
    +1 (1)
    You are correct. Thank you very much.
     
    Yes, I overlooked that information.
    #6
    dan1138
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    Re: PIC16LF Fail-safe Operation in Low-Power, High-RF Fields 2019/07/22 20:19:42 (permalink)
    +1 (1)
    rbraddy
    I have an application whereby the PIC16LF18345 will operate within very high RF field conditions (close proximity to up to 2 KW of HF RF power from immediate antenna system within inches of the MCU).



    Being that close to such a high power RF emitter puts your device well inside the near field too.
     
    What have you done in your design to attenuate the magnetic and electric fields before they can induce currents in your PCB traces?
    post edited by dan1138 - 2019/07/28 10:39:11
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    rbraddy
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    Re: PIC16LF Fail-safe Operation in Low-Power, High-RF Fields 2019/07/23 06:33:19 (permalink)
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    Using capacitors to cancel across key components and traces between ground and power near critical components like MCU power, I/O pins, comparator inputs and across relay contacts.

    Reality is even the “ground” return path will be “floating” in common mode currents... while the MCU is asleep. Not sure how effective a Faraday cage would be but couldn’t hurt.
    #8
    Antipodean
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    Re: PIC16LF Fail-safe Operation in Low-Power, High-RF Fields 2019/07/23 06:46:00 (permalink)
    +2 (2)
    rbraddy
    Using capacitors to cancel across key components and traces between ground and power near critical components like MCU power, I/O pins, comparator inputs and across relay contacts.

    Reality is even the “ground” return path will be “floating” in common mode currents... while the MCU is asleep. Not sure how effective a Faraday cage would be but couldn’t hurt.

    A Faraday cage is a must, you will probably need a 2 stage one in the form of a box inside a box with a metallic connection between them at a single spot where the wiring comes out.
     
    You will also need feed through capacitors on any wiring passing through the boxes, again EACH box will need s set of feed through capacitors. Personally I would put lossy ferrite beads where the connections go between the feedthroughs on the two boxes.

    Do not use my alias in your message body when replying, your message will disappear ...

    Alan
    #9
    oliverb
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    Re: PIC16LF Fail-safe Operation in Low-Power, High-RF Fields 2019/07/23 06:50:38 (permalink)
    +1 (1)
    I presume your PCB has a ground plane?
    Apart from that I'd check out 3 terminal MLCCs, and make sure your PCB layout has a clear dividing line between filtered and unfiltered signals.
    Also how is the PIC powered? A voltage regulator may be succeptable to RF, and could drop its output voltage, possibly enough to put the PIC in "brown out". The output is less likely to go up, but it could happen.
     
    #10
    rbraddy
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    Re: PIC16LF Fail-safe Operation in Low-Power, High-RF Fields 2019/07/23 07:20:49 (permalink)
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    Power is two AAA Ultimate Lithium batteries.
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    rbraddy
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    Re: PIC16LF Fail-safe Operation in Low-Power, High-RF Fields 2019/07/23 19:02:55 (permalink)
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    Battery powered by two AAA cells - no voltage regulator. The PIC spends most of its life asleep, only awakening for up to 30 milliseconds to make switching decisions occasionally.

    The PIC MCU will never be awake and processing during RF transmissions.
    #12
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