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TPS3851: About WDI toggle control

user5088412
Prodigy 85 points
Part Number: TPS3851

Tool/software:

Hello,

We have two questions:

1.Regarding toggle control:
The datasheet specifies a “Minimum WDI pulse duration” of 50 ns. We also understand that this IC is cleared on the low edge of WDI.
If we set the WDI pulse width to a value significantly different from 50 ns, would that cause any issues?
For example, if we simplify the timing such that both the high period and the low period are 100 ms each, would that be problematic?

Incidentally, we are currently controlling WDI as shown below. We are considering whether this can be toggled.

2.Regarding the “Minimum WDI pulse duration”:
The datasheet shows a nominal value of 50 ns but does not define a minimum or maximum value.
Could you please provide the minimum and maximum specifications for this parameter?

  • 0
    TI__Mastermind 18710 points

    Hello,

    The 50ns is the minimum so any thing wider than a 50ns pulse is fine for this device. A 100ms square wave is definitely perfect as long as the falling edge is within the configured watchdog timeout.  

    From bench testing the shortest pulse is around 20ns, this is not advised though because the state machine can intermittently ignore the pulse causing a false watchdog assertion. I would advice 50ns to be set as your minimum. As for the maximum, it is the duration of the configured watchdog time minus 20% for accuracy and tolerance. 

    Jesse  

  • 0 in reply to Jesse Villanueva
    Prodigy 85 points

    Thank you for your response.

    >As for the maximum, it is the duration of the configured watchdog time minus 20% for accuracy and tolerance.
    Could you please clarify this point further?

    Example: When the watchdog time is set to 300 ms,

    ・The maximum WDI pulse duration would be 240 ms (300 ms × 80%).

    For instance, let’s assume the watchdog time is set to 300 ms and the design specification requires clearing the watchdog at least twice within this period. In that case, we would clear WDI at 150 ms intervals. Under these conditions, is our understanding correct that we do not need to worry about the maximum value?

  • +1 in reply to user5088412
    TI__Mastermind 18710 points

    Yes, your understanding is correct, you should not worry about the maximum value as long as the falling edge is inside the timeout period. So in your example, timeout period at 300ms, a falling edge at 150ms will work fine. 

    Jesse