Introduction to Digital Systems: Modeling, Synthesis, and Simulation Using VHDL

5.7 PRACTICAL ASPECTS OF LOGIC GATES

5.7.1 Fan-in and Fan-out Effects

In this section we describe the physical aspects of logic gates, which include the fan-in, fan-out, noise margin, power dissipation, and propagation delays. The fan-in is the number of inputs of a logic gate. For examples, a two-input AND gate has a fan-in of 2 and a three-input NAND gate has a fan-in of 3. So a NOT gate always has a fan-in of 1, which implies that for any logic circuit the inputs cannot be increased beyond a finite number (i.e., fan-in). If the number of inputs is increased, the parasitic capacitance and thus the propagation delay is increased and the noise margin is lowered. Normally, the propagation delay increases as a quadratic function of the fan-in. The number of gates that each logic gate can drive while providing voltage levels in the guaranteed range is called the standard load or fan-out. The fan-out depends on the amount of electric current that a gate can source or sink while driving other gates. The effects of loading a logic gate output with more than its rated fan-out include the following:

• In the LOW state the output voltage V_OL may increase above V_OLmax.
• In the HIGH state the output voltage V_OH may decrease below V_OHmin.
• The operating temperature of the device may increase, thereby reducing the reliability of the device and eventually causing the device to fail.
• Output rise and fall times may increase beyond specifications.
• The propagation delay may rise above the value specified. ...