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

5.2 LOGIC SIGNALS

Logic variables can be used to represent such electronic signals as voltage, current, and frequency. There are a number of systems for representing binary information in physical systems, such as:

• A voltage signal with zero (0) corresponding to 0 Vand one (1) corresponding to 5 or 3 V.
• A sinusoidal signal with zero (0) corresponding to a frequency and one (1) corresponding to another frequency.
• A current signal with zero (0) corresponding to 4 mA and one (1) corresponding to 20 mA.
• For switches, open is indicated by 0 and closed is indicated by 1.

Now let us describe the logic levels using voltage signals. To represent voltage signals using logic levels, we first have to define threshold voltage. Any voltage below the threshold voltage represents one logic value, and voltages above this reference voltage represent the other logic value. We can assign either 1 or 0 to represent any level, depending on the logic system. There are two different types of logic systems: positive and negative. In a positive logic system, any voltage greater than the threshold voltage is represented by logic 1, and voltages below the reference level are represented by logic 0. The reverse notation is used for a negative logic system: Logic 1 and logic 0 are used to represent lower voltage levels and higher voltage levels, respectively.

Figure 5.1 Logic Levels for Positive Logic

Generally, ...