SCHS246A August   1998  – August 2024 CD74AC251

PRODUCTION DATA  

  1.   1
  2. Features
  3. Description
  4. Pin Configuration and Functions
  5. Specifications
    1. 4.1 Absolute Maximum Ratings
    2. 4.2 Recommended Operating Conditions
    3. 4.3 Thermal Information
    4. 4.4 Electrical Characteristics
    5. 4.5 Switching Characteristics
  6. Parameter Measurement Information
  7. Detailed Description
    1. 6.1 Functional Block Diagram
    2. 6.2 Device Functional Modes
  8. Application and Implementation
    1. 7.1 Power Supply Recommendations
    2. 7.2 Layout
      1. 7.2.1 Layout Guidelines
  9. Device and Documentation Support
    1. 8.1 Documentation Support (Analog)
      1. 8.1.1 Related Links
    2. 8.2 Receiving Notification of Documentation Updates
    3. 8.3 Support Resources
    4. 8.4 Trademarks
    5. 8.5 Electrostatic Discharge Caution
    6. 8.6 Glossary
  10. Revision History
  11. 10Mechanical, Packaging, and Orderable Information

Layout Guidelines

When using multiple-bit logic devices, inputs should never float.

In many cases, functions or parts of functions of digital logic devices are unused, for example, when only two inputs of a triple-input AND gate are used or only 3 of the 4 buffer gates are used. Such input pins should not be left unconnected because the undefined voltages at the outside connections result in undefined operational states. Layout Diagram specifies the rules that must be observed under all circumstances. All unused inputs of digital logic devices must be connected to a high or low bias to prevent them from floating. The logic level that should be applied to any particular unused input depends on the function of the device. Generally they will be tied to GND or VCC, whichever makes more sense or is most convenient. It is generally acceptable to float outputs, unless the part is a transceiver. If the transceiver has an output enable pin, it will disable the output section of the part when asserted. This will not disable the input section of the I/Os, so they cannot float when disabled.