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nPORRST and TMS570

Donald Hughes
Prodigy 30 points
Other Parts Discussed in Thread: TMS5700432, TMS570LS0432

2 part question:

With the TMS570

What happens if nPORRST is open? is there an internal pull-down?

If the nPORRST pin is shorted to +VCC or VCCIO, is the internal voltage monitor circuitry going to gracefully shutdown if the VCC or VCCIO goes out of spec, or is the short the controlling signal?

Thanks

  • 0
    TI__Guru 68830 points

    Donald,

    >> What happens if nPORRST is open? is there an internal pull-down?

    The device datasheet lists whether there is a pull up or down on a particular pin, in the terminal functions table.

    So please check the datasheet of the *specific* part # that you are inquiring about - but generally yes nPORRST has a weak pulldown.

    >>If the nPORRST pin is shorted to VCC

    A 1.14 to 1.32V level on the nPORRST pin is between VIL and VIH so we can't say whether or not nPORRST will be asserted or deasserted.


    >> If nPORRST is shorted to VCCIO then the device may not get a valid power on reset and therefore may not startup in a known state. 

    If the rails are out of range - there is some on-chip protection in the form of the VMON.  If the device you are asking about has the VMON it would be described along with the spec's on min and max trip points, in the device datasheet.

    For example in SPNS215a you'll find the VMON specs in table 5-1.   It's not precise enough to use in place of an external supervisor but it will provide some gross protection.


    You should also look at the 'Power Sequencing and Power on Reset' section of the datasheet for information about this critical time.

  • 0 in reply to Anthony F. Seely
    Prodigy 30 points
    I'm using TMS5700432
    It has VMON
    If the nPORRST is shorted to one of the VCC rails, is the VMON circuitry downstream enough from the pin to be capable of still triggering a reset?
  • 0 in reply to Donald Hughes
    TI__Guru 68830 points
    Donald,

    It is, and in fact this helps during the initial parts of the power on ramp.
    But the VMON isn't very precise and has a wide spec range. So it's more about keeping the device in a known state when the rails are grossly out of range.

    For example, in the TMS570LS0432 datasheet that the VMON may release nPORRST internally anywhere between 0.75V and 1.13V on the core rail and 1.85 to 2.9V on the IO rail. The typical values are given as 0.9 and 2.4 and so you can say that typically - the VMON will release reset internally slightly before the device is within the recommended operating ranges of the voltage rails. Which means VMON alone isn't enough for a proper startup.