What is the main reason why a CMOS logic IC is unstable?

There are various causes for CMOS logic IC output instability, such as input signals and power supply noise.

The following can be expected to cause CMOS logic IC to be unstable:

  1. The input signal voltage might be outside the specified low-level input voltage (VIL) or high-level input voltage (VIH) range. Ensure that the input signal meets the VIL and VIH specifications.
  2. An unused input pin might be left open. Since an open input pin is in the High-Z state, a false output might appear because of the influence of a surrounding electric field, affecting other pins. To prevent this, connect unused input pins to VCC or GND.
  3. A slow transition rate signal exceeding the specified rise or fall time may have been input. In this case, satisfy the specified input rise and fall times or use an IC with a Schmitt-trigger input.
  4. A pull-up resistor is not connected to the open-drain output of an IC.
  5. VCC or GND might not be stable. In this case, stabilize VCC, for example, by using a bypass capacitor.
  6. An input signal might have noise. Use a low-pass filter to filter out noise from the input signal. If the use of a low-pass filter makes it impossible to meet the rise and fall time specifications, use an IC with a Schmitt-trigger input.
  7. The output signal might be about to enter into oscillation because of its feedback to the input. Be sure to keep the input signal trace away from the output signal trace on a printed circuit board. If you cannot help but run these traces in parallel, insert a guard track between them.

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