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OMAP-L138 Internal Pulls Strength

Juan Vega
Prodigy 45 points

Other Parts Discussed in Thread: OMAP-L138

Hi,

We are using the OMAP-L138BGWTMEP in one of our designs.  We have a signal that is an input to the OMAP, using GPIO (GPIO 0, bit 7 to be exact) and we don't have an external pull on that net.  Our questions is, can we rely on the OMAP internal pull to maintain a net (say 4 inches long) in a known state if the driver is tri-stated?  How much margin is there for noise?  Our concern comes because we don't want that pin being accidentally switched to some unwanted state, when the driver is tri-stated.  Please let me know.  Thanks!

Best Regards,

- J. Vega

  • 0
    TI__Expert 4760 points

     

    Vega,

    Normally we don’t rely on internal Pull-Ups if the pins are connected to some other device .This OMAP-L138 device includes internal pull-up and pull-down resistors that prevent floating input pins. These internal resistors are generally very weak and their use is intended for pins that are not connected on the board design. For pins that are connected, external pull-up and pull-down resistors are recommended

    Please refer section 10.11.5 Pull-Up/Pull-Down Disable in the Technical Reference manual SPRUH77A

    For your design I suggest you to go through the technical note to select the appropriate Pull-Up/Pull-down resistor that is t for open drain outputs but the theory remains the same.

    http://www.ti.com/lit/an/slva485/slva485.pdf

    Regards

    Antony

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  • 0 in reply to Antony Samsunil
    Prodigy 45 points

    Anthony,

    Thanks for the reply! ... Your response is based on the statement found on SPRUH77A.  How about the statement found in SPRS815B?

    Page 76, Section 3.3 Pullup/Pulldown Resistors

    "Proper board design should ensure that input pins to the device always be at a valid logic level and not
    floating. This may be achieved via pullup/pulldown resistors. The device features internal pullup (IPU) and
    internal pulldown (IPD) resistors on most pins to eliminate the need, unless otherwise noted, for external
    pullup/pulldown resistors."

    So my question now is this:

    If the intention of the internal pulls is to maintain unconnected input pins at certain known logic level, then why bother making this a user selectable option, allowing the user to select internal pullup or pulldown for the different pin groups?   Please refer to sections 11.5.22 and 11.5.23 in the Technical Reference manual SPRUH77A.

    Please let me know.  Thanks!

    Best Regards,

    - J. Vega



  • 0 in reply to Juan Vega
    TI__Expert 4760 points

    Vega,

    As per my statement from the TRM is the recommendation and leaves the options to the designer for your case I opt for an external Pull-up resistor due to the weak nature of the internal pull-up  

    Yes I agree we have the user selectable Internal Pull-Up and Pull-down options .That leaves the option to the designer to make the design tradeoff according to their need due to the weak nature of the internal Pull-up and Pull-down option we have it’s a PRECAUTION.

    Regards

    Antony

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  • 0 in reply to Antony Samsunil
    TI__Expert 4760 points

    Vega,

    Hope this post might help

    http://e2e.ti.com/support/dsp/omap_applications_processors/f/42/p/251504/881174.aspx

    Regards

    Antony

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