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TPS3851: TPS3851 / Reset timeout period is too short

S.Satoshi
Guru 21685 points
Part Number: TPS3851
Other Parts Discussed in Thread: TPS3813

Hi,

My customer confirmed that the tRST was 100usec as attached though it's specified 200msec(typ). So can you confirm following question?

  1. I believe that the duration in right waveform should be more longer (200msec typ). Is my understanding correct?
  2. Can we confirm 200msec when WDI is kept low? In this case, actually the WDI is kept High when Watchdog timeout is detected.

Best Regards,

Satoshi / Japan Disty

  • 0
    TI__Genius 12630 points
    Satoshi,

    1. The 200ms refers to how long before /RESET goes high after VDD comes above the voltage threshold. This delay occurs when /RESET returns to normal high state after returning from reset low condition.

    2. The Reset timeout period of 200ms refers to the /RESET pin. You show the WDI and /WDO pins above. For the /WDO pin, use the Watchdog timeout spec t_wd.

    3. The pulse on WDI pin must be pulsing within the Watchdog timeout spec and must pulse for at least 50ns each pulse for the device to recognize the pulse on WDI. If no pulse occurs within Watchdog timeout then /WDO pin will trigger.

    I hope this answers your question! Please click "This resolved my issue" button if the answer provided resolved your issue. Thanks!

    -Michael
  • 0 in reply to Michael_DeSando
    Guru 21685 points

    Hi Michael,

    Thank you for your support. I believe that you commented about tRST that are circled red in attached, right? But I also can see tRST in circled blue, so I think that /WDO should be low for 200msec (=tRST) when watchdog timer is expired. Is the tRST in blue circle typo, and it should be tWD-setup? I guess because the duration of /WDO Low pulse I attached is similar to tWD-setup (150us).

    Please let me know if I have still misunderstanding.

    Best Regards,

    Satoshi 

  • 0 in reply to S.Satoshi
    TI__Genius 12630 points
    Satoshi,

    You are correct in that if watchdog timer expires (no pulse occurred on WDI within Watchdog timeout) then /WDO triggers and goes low for t_RST. This is the same delay for /RESET too but /WDO and /RESET trigger for different reasons.

    t_WD-setup refers to the time required for the watchdog to be ready to use again after /WDO returns from being triggered. Once /WDO returns back to normal high condition, you must wait 150us before /WDO can be triggered again from pulses on WDI.


    Keep in mind the pulses on WDI must be 50ns wide for them to be recognized.


    I hope this answers your question! Please click "This resolved my issue" button if the answer provided resolved your issue. Thanks!

    -Michael
  • 0 in reply to Michael_DeSando
    Guru 21685 points

    Hi Michael,

    Thank you for your quick response.

    I checked customer's schematic and then they tied /RESET and /WDO together as attached. I'd like to know if this connection is cause of this short Low duration of /WDO. Can you please confirm if such short Low duration is occurred with this connection when watchdog timer is expired?

    Best Regards,

    Satoshi

  • 0 in reply to S.Satoshi
    TI__Genius 12630 points
    Satoshi,

    I believe the connection to be the cause of the short low duration of /WDO. When watchdog expires, /WDO drops low, but since /WDO is connected to /RESET and /RESET is high... /WDO is pulled high by /RESET.

    Please act customer to remove connection and see if problem continues.

    We offer devices with an internally connected /WDO and /RESET which can operate like how customer has the circuit connected above (such as TPS3813).

    -Michael