Basic Concepts. INTRODUCTION TO LOGIC. Logic is the science of the correctness or incorrectness of reasoning, or the study of the evaluation of arguments. A statement is a declarative sentence, or part of a sentence, that can be true or false. Jun 19, 2019 Some logic gates like NOT gate or Inverter has only one input and one output. The inputs of the logic gates are designed to receive only binary data (only low 0 or high 1) by receiving the voltage input. The low logic level represents Zero volts and high logic level represents 3.

In this tutorial, we will have a brief introduction to Logic Gates. We will see the different Logic Levels in TTL Circuits, CMOS Circuits, some simple Logic Gates like OR, NOT, NAND, AND, etc.

Introduction to Logic Gates

Logic gates are the heart of digital electronics. A gate is an electronic device which is used to compute a function on a two valued signal. Logic gates are the basic building block of digital circuits.

Basically, all logic gates have one output and two inputs. Some logic gates like NOT gate or Inverter has only one input and one output. The inputs of the logic gates are designed to receive only binary data (only low 0 or high 1) by receiving the voltage input.

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The low logic level represents Zero volts and high logic level represents 3 or 5 volts positive supply voltage.

We can connect any number of logic gates to design a required digital circuit. Practically, we implement the large number of logic gates in ICs, by which we can save the physical space occupied by the large number of logic gates. We can also perform complicated operations at high speeds by using integrated circuits (IC).

By combining logic gates, we can design many specific circuits like flip flops, latches, multiplexers, shift registers etc.

Digital logic levels

A logic level is defined as a specific state or voltage of a signal. We know that 0 and 1 are the two states of logic gates. The logic levels 0 and 1 are known as LOW and HIGH respectively. In digital electronics, these binary logic levels play a crucial role in data storage, data transfer.

Generally these logic levels can be understand as ON and OFF states. As we said earlier, the logic levels are introduced to the logic gate by the supply voltage. If the supply voltage to the logic gate is 0 volts, it refers to Low logic level or OFF state.

Similarly, if the supply voltage to the logic gate is 5 volts or 3.3 volts (for modern ICs), it refers to High logic level or ON state. The manufacturers will follow the TTL or Transistor – Transistor Logic as standard voltage level, while designing the ICs.

What is Active High and Active Low?

We see the active high input and active low input pins in ICs and micro controllers. Do you know what they really mean? They just describe us how the pin is getting activated.

This means the Active low pin must be connected to low logic level or Ground. In the same way, the Active high pin must be connected to high logic level or to 5 volts or 3.3 Volts.

Let’s understand about this in a simple way. When we see the enable pin CE in a shift register IC, without any line (bar) on it, we connect it to active low input i.e. to ground 0 volts. Otherwise, if we see the enable pin with a line on it as (CE) ̅, we connect it to the active high input i.e. to 3.3 or 5 volts supply, in order to enable the pin.

TTL Logic Levels

TTL logic level is the standard logic level for majority of the logic devices. TTL means Transistor – Transistor Logic. Transistors are electrically controlled switches. The voltage levels of logic families are

VOH – min. output voltage level for HIGH signal

VOL – max. Output voltage level for LOW signal

VIH – min. input voltage level of a device to be considered for HIGH signal

VIL – max. Input voltage level of a device to be considered for LOW signal

If we observe the TTL logic levels, we can observe that the minimum high voltage level for output is 2.7 volts. This means, when the device is driving HIGH, the voltage should be at least 2.7 volts.

Similarly, the minimum high voltage level for input is 2 volts. So the voltages greater than 2 volts will considered as logic 1, to a TTL device. The voltages between 0.8 volts and 2 volts are known as noise margin.

In the same way, the maximum LOW voltage level for output is 0.4 volts. This means, when the device is driving HIGH, the voltage should be less than 0.4 volts. Similarly, the maximum low voltage level for input is 0.8 volts.

So the voltages less than 0 volts will consider as logic 0, to a TTL device. So when the logic device is supplied with voltages between 0.8 V and 2 V, then logic level of the device will change between High and Low. This situation is called ‘Floating’.

Another procedure for input and output tolerances of TTL device is

CMOS Logic Levels

CMOS logic devices are also known as 3.3 volt devices, because the CMOS devices will have the maximum voltage level as 3.3 V. This is an advanced technology that will run the devices on low power supply (3.3 V instead of 5 V).

Mostly we use 5V devices (TTL compatible) to design logic gates, so these CMOS devices are used to interface with TTL devices. A CMOS device can interface with any TTL device and they do not require any additional components.

For example, the minimum value for a high logic level (1) of a CMOS device is 2.4 V. So this device can be interpreted with TTL device which has minimum input voltage for logic (V¬IH) 1 as 2 V.

But, before interfacing the TTL to CMOS devices (3.3 V and 5 V) we need to check that the 3.3 V devices are 5 V tolerant or not. Because many of the will permanently cause the chip damage when we supply voltages above 3.6 V. We can use a voltage divider circuit or logic level shifters to control the 5 V voltage signals.

Noise Margin

The noise margin of a logic level is defined as the voltage gap between the maximum low voltage of high input (VIL max) and maximum voltage of the low input (VIL min) of a logic gate. The noise margin is also defined as the amount by which the voltage signal exceeds the threshold level for the exact high or exact low.

Let’s clearly understand this with an example. When a logic circuit is altering between 0 volts and 1.2 volts, with any voltage below 0.2 volts is considered as LOW i.e. 0. And any voltage above 1 volt is considered as HIGH i.e. 1.

CMOS logic devices have higher noise level or noise margin than TTL logic devices because their minimum output voltage for high logic (VOH min) is closer to the supply voltage and maximum output voltage for low logic (VOL max) is nearly 0. So the noise level is the maximum amount of noise that a logic circuit can withstand.

If we apply a voltage of certain the noise level, we do not certainly know whether the circuit will respond or not. Noise level is the unwanted voltage level, caused by external interference such as supply voltage fluctuations and other conductors in circuit.

The noise level that a circuit can tolerate is called as “Noise immunity” or “Noise margin”. For TTL devices the tolerance range of output voltages is higher than that of input voltages.

Simple Diode Logic Gates

Diodes can act like switches, so these are used in digital logic operations and switching. For low and high impedance states a diode will work in forward bias and reverse bias.

The diode will conduct only in one direction (Forward bias) and it remains closed in reverse bias condition. So it behaves like a switch. Now let’s see some simple diode logic gates, which are constructed by using only Diodes and resistors.

OR Gate

The simple OR gate designed by two diodes is shown in the below figure. Inputs are given to this circuit by the two diodes. In this, the logic HIGH (1) is represented by +5 Volts and logic LOW (0) is represented by 0 Volts or Ground.

In the circuit below, the two inputs are left unconnected, so the output is 0 i.e. logic low.

If any one of the two inputs is connected to +5 volts, then the diode becomes forward biased and it will conduct .Thus the output is logic HIGH i.e. 1.

If voltage of +5 V is connecte

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