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TPS3306-18 Watchdog retrigger time

Stefano Bisi
Intellectual 460 points

Hello all,

I have a tricky trouble whit TPS3306-18. I use this component whit a tri state buffer on pin WDI so i can use internal retrigger. Usually when pin is tri stated i see train of short pulse spaced by about 800 mS. This should be the internal retrigger pulse.

Now i found a board where this time is reduced to 1,4 mS and if microprocessor of my board try to toggle WDI pin it goes in reset. ( we have 200 mS square wave to refresh watchdog ).

I try to replace IC but i get the same behaviour. I try to disconnect pin 7 from board and nothing happen; internal refresh seems to be of 1.4 uS

Who is involved in the internal watchdog timing ? Why i found a so quick  retrigger pulse ?

Thank in advance

Stefano

  • 0
    TI__Guru 55940 points

    Hi Stefano,

    Thanks for reaching out!  It looks like you have already done a fair amount of experiments in trying to debug this.  Would you be able to provide any scope shots to help me see the event?  It would be great to have WDI, SENSE1, SENSE2, and /RESET in the scope shot.

    Also, could you provide a schematic of the board exhibiting the behavior?  Is this board different in any way from the boards that have the expected time of 800ms?

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    Intellectual 460 points

    Dear Ryan,

    thanks for your help; here a screenshot of signal on pin 7 ( WDI )

    Today I found what's goes wrong while I add wiring to capture signal on suggested pins.

    I found that this behaviour is caused by a "tombstoned" resistor on the divider network that feeds PFI input.

    So instead 1.5 V i have a much higer voltage on PFI.

    Originally I neglect those pin because on functional block diagram on datasheet it seems not related to watchdog circuitry but in some way an abnormal voltage on input pins speed up watchdog timer.

    Once I fix resistor all goes as expected.

    Thank you very much !

     

  • 0 in reply to Stefano Bisi
    TI__Guru 55940 points

    Hi Stefano,

    Glad to hear you figured it out!

    Very Respectfully,

    Ryan