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The output of a CMOS logic IC does not stabilize in the High or Low state. What could be the cause of this?

Major causes include the following:

  1. The input signal voltage might be outside the specified VIL and VIH ranges. Ensure that the input signal meets the VIL and VIH specifications.
  2. An unused input pin might be left open. Unused input pins must be connected to either VCC or GND.
  3. A slowly changing input signal might exceed the specified input rise and fall times. Ensure that the input signal meets the input rise and fall times specified in the datasheet, or use a Schmitt-trigger input type.
  4. The open-drain output type requires a resistor for output stabilization.
  5. The power supply or GND might not be stable. In this case, stabilize the power supply, for example, by using a bypass capacitor. For power supply stabilization, please refer to “Is a decoupling capacitor required between VCC and GND?” in the Frequently Asked Questions (FAQ) page.
  6. An input signal might have noise. Use a lowpass filter to filter out noise from the input signal. If the use of a lowpass filter makes it impossible to meet the input rise and fall times specifications, use a Schmitt-trigger input type.
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