• State TI Thinks Resolved
  • Locked Locked
  • Replies 4 replies
  • Answers 3 answers
  • Subscribers 19 subscribers
  • Views 308 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

SN74AHCT08Q-Q1: SN74AHCT08Q-Q1: floating connections

Tobias Fernandes
Prodigy 90 points
Part Number: SN74AHCT08Q-Q1

Hello Folks,

I have one question about the output state of logic gates (AND/OR/Schmitt-Trigger/...) in case of fault:

1. if GND-Pin is open (not connected to GND), all other pins are connected.

2. if GND-Pin is connected, but the power supply Pin is open (the component is unpowered).

3. if GND-Pin und Power Supply Pin are connected, and VCC is shorted to the GND.

4-  Considering one AND gate logic where both pins are HIGH, then one of the input pin is open (Floating), what will happen with the output? it goes to Low / High / Oscillation?

Thanks in Advance!

  • 0
    TI__Mastermind 42670 points

    Hi Tobias,

    This table should help with most of these.

    For question 4, with one input tied high it effectively makes it a buffer. So if the input is unknown, the output is also unknown. Its advised not to leave inputs floating see this FAQ:https://e2e.ti.com/support/logic/f/151/t/737694

  • 0
    Guru 244360 points

    1. If GND is open, then the chip's internal ground might still be pulled low through the ESD diode at any input or output pin, if that pin has a low voltage applied.

    2. If VCC is open, then the chip's internal VCC might still be pulled high through the ESD diode at any output pin, if that pin has a high voltage applied.

    3. If VCC = 0 V, then the chip is powered off. The inputs are overvoltage tolerant, i.e., it is allowed to apply voltages to any input.

    4. A floating input pin might damage the device. See [FAQ] How does a slow or floating input affect a CMOS device?

  • 0 in reply to Clemens Ladisch
    Prodigy 90 points

    Hello Folks,

    For the second point, I still have one question:

    "2. If VCC is open, then the chip's internal VCC might still be pulled high through the ESD diode at any output pin, if that pin has a high voltage applied."

    In the case of VCC is open and there are not pull up to VCC at the ouput pins (no voltage applied). What could happen?

    More over for the third point:

    "3. If VCC = 0 V, then the chip is powered off. The inputs are overvoltage tolerant, i.e., it is allowed to apply voltages to any input."

    In this case the chip is powered off (VCC=0) and both inputs are set HIGH. what will happen at the output (HIGH, LOW, high impedance)?

  • 0 in reply to Tobias Fernandes
    Guru 244360 points

    When there is no voltage applied to an output, then the chip cannot be powered through this output.

    When VCC = 0 V, then any voltage larger than about 0.5 V at an output causes a current to flow into VCC.