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TPS3813: Watchdog immediately armed after power-up

Michael Kapitzke
Prodigy 50 points
Other Parts Discussed in Thread: TPS3813

HI,

we are looking for a window-watchdog supervisory and are currently evaluating the TPS3813K33. It was found that the window-watchdog is armed right after power-up whereas the datasheet claims the window-watchdog is inactive until a first rising edge at WDI occurs. An armed window-watchdog needs proper WDI signals otherwise a /RESET will be generated. Debugging/programming the uController is not possible with periodic /RESET signals generated by the '3813 right after power-up.

Our test setup is:

WDI: Connected to GND; WDR/WDT: Connected to VDD; VDD=3V3

Result:

Though WDI is tied to GND, period /RESET signals (tp=2.4sec) are generated right after power-up

 

Any idea?

Michael

  • 0
    TI__Guru**** 150790 points

    Have you connected WDI to GND or WDR?  WDI is the watch dog input that you need to pulse.  WDR is the digital input to configure the watchdog.

    Also, do you have a pull-down resistor on the WDI input as recommended on page 9 of the datasheet?

  • 0 in reply to Chris Glaser
    Prodigy 50 points

    Hi Chris,

    our design uses a port pin and a pull-down resistor to trigger the WDI input. But it turned out that the TPS3813 generates periodic /RESET signal right after power-on without any activity on WDI. This behaviour is in contrast to the datasheet.

    In order to find the root cause of this misbehaviour, I breadboarded the TPS3813K33 with the setup described in my initial message. Result: See above

    WDI: GND

    WDT, WDR: VDD

    VDD: 3V3

    You will agree that with the WDI input tied to GND, in theory no /RESET shall ever be generated by the '3813 due to the watchdog. The test setup is simple and allows reproducing the misbehaviour in no time.

    Michael

  • 0 in reply to Michael Kapitzke
    TI__Guru**** 150790 points

    Thank you for confirming your test setup.  After re-reading your first post, it was clearly stated there also.

    Holding WDI low will not disable the watchdog.  You still need to pulse it high before the upper window timeout period to avoid a reset.  After the reset is let go (goes high), the lower window is disabled until the first rising edge of WDI.  However, the watchdog cannot be disabled on this IC.

  • 0 in reply to Chris Glaser
    Prodigy 50 points

    Chris,

    disabling the watchdog is not intended in our application. But the datasheet seems to indicate that the '3813 and it's timers are not armed until the first rising edge of WDI. See:

    D/s page 5, bottom.

    The lower boundary of the watchdog window starts with the rising edge of the WDI trigger pulse. At the same

    time, all internal timers will be reset

     

    D/s page 8, bottom.

    After the reset of the supervisor is released, the lower boundary of the first WDI window is disabled. However,

    after the first WDI pulse low-to-high transition is detected, the lower boundary function of the window is enabled.

    All further WDI pulses will need to fit into the configured window frame

     

    My test setup clearly shows that the '3813 is expecting proper WDI right after /RESET even though no rising edge of WDI occured. The only possible explanation is that the upper boundary timer is already enabled after /RESET. Can you confirm this?

    Michael

  • 0 in reply to Michael Kapitzke
    TI__Guru**** 150790 points

    Yes, this is correct.  This is why you are generating a reset after ~2.5 s.  This is the time limit for the WDI pulse as you have it setup.

    Only the lower boundary is disabled when the part powers up.  The upper boundary is always active and needs a WDI.

  • 0 in reply to Chris Glaser
    Prodigy 50 points

    Thanks for the clarification.

    Michael

  • 0 in reply to Michael Kapitzke
    Prodigy 35 points
    Thank you for this thread. I see the exact same thing on my setup and now I know why. I was also expecting what Michael expected. I hope we can solve this in our product anyway. // Daniel, INFICON (Sweden)
  • 0 in reply to Daniel Arvelund
    TI__Guru 55940 points
    Hi Daniel,

    I responded on the other thread that you mentioned, but it appears you have already found the answer. We do have devices that do have a watchdog that can be disabled temporarily but unfortunately TPS3813 is not one of them. Please see the following application note that describes the devices that can be disabled:

    www.ti.com/.../slva145.pdf

    Very Respectfully,
    Ryan