Logic Levels and Pulse Waveforms

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A digital system is a type of electronic system that utilizes the binary number system to work. In other words, a digital system is a two-state electronic system used to represent two binary digits 0 and 1, where 0 represents the low or "off" state and 1 represents the high or "on" state of the system.

In the field of digital electronics, different voltage levels are used to represent the two binary values, i.e., 0 and 1 in a digital signal. These voltage levels are known as logic levels.

In this chapter, we will learn the concept of logic levels and pulse waveforms.

What is a Logic Level?

In digital electronics, a voltage level that represents a specific binary value either 0 or 1 is called a logic level. Here, the binary value 0 represents the low voltage level while the binary value 1 represents the high value level.

Hence, the logic levels can be classified into the following two types −

• High Logic Level
• Low Logic Level

Lets discuss these two logic levels in detail.

High Logic Level

In the case of a digital system, the voltage level closer to the maximum voltage level that the system can handle without getting damaged is called high logic level.

The high logic level is represented by the binary digit "1". The voltage level for a high logic level depends on the technological standard used to design the system. Typically, the voltage value between 2 V and 5 V represents the high logic level or 1.

Low Logic Level

In a digital system, the low logic level is defined as the maximum voltage level for which the system will remain in the OFF state.

The low logic level is represented by the binary digit "0". Similar to the high logic level, the voltage level for a low logic level depends on the technology standard used to design the system. In actual practice, the voltage value between 0 V and 0.8 V represents the low logic level or logic 0.

In most practical digital system, the ground voltage is used to represent the low logic level.

Note − The voltage range between the voltage values 0.8 V and 2 V is known as the indeterminate logic range. If a digital signal lies between the value 0.8 and 2 V, the response of the system is not predictable.

What is a Pulse?

A pulse is a type of an electronic signal that can change suddenly between two possible states i.e., high state and low state.

The graph used to represent the transition of a pulse is called the pulse waveform. Pulses are very important in the operation of digital systems, communication systems, and many other electronics devices and circuits.

Depending on the switching characteristics, the pulses can be classified into the following two types −

• Positive Pulse − When a signal normally goes from low logic level to the high logic level and then returns to its normal low logic level, then it is called a positive pulse.
• Negative Pulse − When a signal normally goes from high logic level to the low logic level and then returns to its normal high logic level, then it is known as a negative pulse.

The pulse waveforms for positive and negative pulses are depicted in the following figure.

A pulse has two edges namely, a leading edge and a trailing edge.

In the case of a positive pulse, the edge going from low logic level to high logic level is called the leading edge, and the edge going from high logic level to low logic level is called the trailing edge.

In the case of a negative pulse, the edge going from high logic level to low logic level is called the leading edge, whereas the edge going from low logic level to high logic level is called the trailing edge.

The positive and negative pulse waveforms shown in the above figure are ideal pulse waveforms, because their leading and trailing edges change instantaneously i.e., in zero time. But in actual practice, the edges of pulses do not change instantaneously from low logic level to high logic level or from high logic level to low logic level.

The pulse waveforms that take a finite time to change from low logic level to high logic level and vice-versa are known as non-ideal pulse waveforms.

In the case of a non-ideal pulse waveform, the time taken by the pulse to go from low logic level to high logic level is called the rise time. The time taken by the pulse to go from the high logic level to the low logic level is called the fall time.

Types of Pulse Waveforms

The pulse waveforms used in digital systems are mainly classified into the following two types −

Periodic Waveforms

A pulse waveform that repeats itself at regular intervals of time is called a periodic waveform. The time taken to complete one cycle is called the time period of the periodic waveform.

Non-periodic Waveforms

A pulse waveform which does not repeat itself at regular intervals of time is termed as a non-periodic or aperiodic waveform.

Conclusion

In conclusion, "logic level" is a concept used in digital systems to represent the state of the system. There are two possible logic levels in the case of digital systems namely, high logic level and low logic level. The high logic level is represented by the binary 1 while the low logic level is represented by the binary 0.

The graphical representation of a digital signal or a pulse is termed as the "pulse waveform". Pulse waveforms are used to represent the transition of a pulse or digital signal or the states of a digital system. In this chapter, we have discussed the concept of logic levels and pulse waveforms. In the next chapter, we will learn about "components of a digital system".