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TPS3705: WDO pull high mechanism

Jerry Chen
Expert 5110 points
Part Number: TPS3705

Hi there,

My customer is using WDO to pull down MR to reset the processor itself which stopped feed WDI. In the WDO output, there is a 0.1uF capacitor that prevents  MR pulled low to its threshold to trigger reset. After removing the capacitor, the issue is closed as reset function works as wanted.

However, in this issue with 0.1uF, we saw something interesting. WDO couldn't go low enough to MR threshold, and it was pulled up after a short time. We don't know how it be pulled up, as no WDI signal(always pulled high in last figure) and it couldn't be the internal 14kohm pull-up resister of MR.

I would like to if there is other mechanism to pull WDO high.

Thanks.

  • 0
    TI__Genius 9375 points

    Hi Jerry,

    In reference to your second statement:

    However, in this issue with 0.1uF, we saw something interesting. WDO couldn't go low enough to MR threshold, and it was pulled up after a short time. We don't know how it be pulled up, as no WDI signal(always pulled high in last figure) and it couldn't be the internal 14kohm pull-up resister of MR.

    How long after did WDO go back high? Was this oscillating behavior?

    Thanks,
    Abhinav.

  • 0 in reply to Abhinav.Sharma
    TI__Expert 5110 points

    Hi Abhinav,

    You could see in the second figure. WDO was pull low from Point A to B then return to high(Point C). In the third figure, you could see the time interval between each WDO spurs is 1.6s, which is exactly the watchdog response time. But, there is no WDI signal to change the WDO status. It would be something internal to pull it back to high then repeat the some action after every 1.6s.

  • 0 in reply to Jerry Chen
    TI__Genius 9375 points

    Hi Jerry,

    I think there is some confusion here. Are you under the impression that while there is a WDI fault, WDO should stay low continuously?

    Thanks,
    Abhinav.

  • 0 in reply to Abhinav.Sharma
    TI__Expert 5110 points

    Hi Abhinav,

    In Figure 3, WDI shops toggling and remain high, at this time , there are spurs in WDO. Zoom in the spur in Figure 2, you could see WDO doesn't go low enough to MR threshold then pulled up by something unknown. It happens only when the 0.1uF cap presents.

  • 0 in reply to Jerry Chen
    TI__Genius 9375 points

    Hi Jerry, 

    Reaching back out as I seem to have missed this last response. Did you find the solution to this issue? If not, let me know and I can help look into it further.

    Thanks,
    Abhinav.