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SN74LV4T125: what's the state of OE and A state when unconnected?

Howard Zou
Genius 12365 points
Part Number: SN74LV4T125
Other Parts Discussed in Thread: SN74LVC1G14

Hi,

my customer would like to know what's the state of enable pin OE and input pin A when left unconnected. Will it be high or low or undetermined? Do we have a schematic to show the input structure?

  • 0
    TI__Guru* 96681 points

    When a CMOS input is left unconnected, this is referred to as "leaving the input floating" and is very bad for the device.

    There's an FAQ on this topic here: [FAQ] How does a slow or floating input affect a CMOS device?

    It also includes a schematic for a typical CMOS input.

  • 0 in reply to Emrys Maier
    TI__Genius 12365 points
    Emrys,
    Thank you.
    In the article it's said"
    "For any CMOS device that does not have Schmitt-trigger inputs, the datasheet defines a minimum input transition rate that will prevent this from happening."
    1. How do I know if the device is CMOS, BiCMOS or bipolar?
    2. How do I know if the device have Schmitt-trigger input?
  • 0 in reply to Howard Zou
    TI__Guru* 96681 points

    Hi Howard,

    To answer your questions directly:

    1. How do I know if the device is CMOS, BiCMOS or bipolar?

    This will typically be stated explicitly in the datasheet.  For example, in the SN74LV4T125 datasheet, page 1:

    SN74LV4T125 is a low-voltage CMOS buffer gate...

    2. How do I know if the device have Schmitt-trigger input?

    This is a little bit harder to answer. Typically, a Schmitt-trigger input device will directly state that.  For example, the SN74LVC1G14's title is "SN74LVC1G14 Single Schmitt-Trigger Inverter"

    This is not always the case, however. It's best too look in the electrical characteristics table to find the input threshold specifications.  A standard CMOS device will only specify VIH and VIL, while a Schmitt-trigger input CMOS device will specify VT+, VT-, and ΔVT.