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Digital Electronics: The Most Scoring Subject in GATE (ECE)

Hi Guys,

The next subject that in our subject-wise blog series is Digital Electronics. This subject accounts for fewer marks in GATE-EE as compared to GATE-ECE. The weightage generally fluctuates between 3-5 marks in GATE-EE and 8-10 marks in GATE-ECE.

The books that you can refer for the theory are Digital Circuits by A. Anand Kumar and Digital Design by Morris Mano. Now, I would be discussing the chapters that you have to study in detail along with the topics that have to be studied in each chapter.
Number Systems
This topic is only included in GATE-ECE syllabus and not in GATE-EE. In this chapter, you have to study about conversions between different number systems such as Binary, Decimal, Octal, and Hexa-decimal. Also, you can study about representation in 2’s complement form and signed binary number representation. Also, you can study about binary arithmetic such as Addition and Subtraction of Binary Numbers.
In this chapter, you also have to study about different binary codes such as BCD Code, XS-3 Code, and Gray Code.
Logic Gates
This topic forms the basics of Digital Electronics so you cannot skip this chapter as the basic understanding of logic gates will enable you to study abut Combinational and Sequential Circuits. So you have to start from the basics. practice truth tables of all gates and make logic circuits for boolean functions.
Try to minimize the number of gates to implement a given function. Refer Morris Mano for this part. Study about 3 input Ex-OR and Ex-NOR gate and also study NAND and NOR implementation of all gates. In this chapter, you can also study about Venn Diagram Representation of Boolean Function and also the implementation of Logic Functions by the use of switches.

Minimization Techniques
Boolean Algebraic rules are very simple and you can practice some problems based on it. There are some of the rules which are more important than others such as Transposition Theorem, Redundant Literal Rule, etc. So, do remember these rules very well. But a better method for minimization of logic functions is by the use of K-Map. All Digital Electronics revolves around it.
So first you need to clear basics of minterms and max terms. How things unfold if we loop zeroes in K-Map? We can invert a function using K-Map. So simply make foolproof command over it. You will require it in counters also. Quine Maccluskey method is not important. For practice go to Kanodia, Morris Mano and coaching booklets.
Combinational Circuit  
Basic concepts regarding adders, subtractors, and comparator should be studied. Remember basic logic expressions for Sum, Difference, carry and Borrow because sometimes based on logic circuit given, the functionality of the circuit is asked and if you know the logic expression then you can easily find the answer. Also, you need to study about logic circuits of Code Converters such as Binary to Gray Code Converter and Gray Code to Binary Converter. 
The most important topics in this chapter are Multiplexer and Decoder and here you need to understand how to implement any logic expression using Multiplexer and Decoder. Also, you need to understand how to implement higher-order device using a lower order device. the other topics such as Encoder and Demultiplexer are not that important and overview of these topics is sufficient.
Programmable Logic Design
This topic is only included in GATE ECE and here you need to study the concept of ROM and different types of ROM such as ROM, PROM, EPROM, and EEPROM. Then you need to study about the implementation of Logic Expressions using ROM. There are two more methods of implementation of logic expressions using PLA and PAL. You need to be very careful about the differences in ROM, PAL, and PLA.
Sequential Circuits
It is the most important part of Digital Electronics and though it may seem tough from outset these topics mainly require patience and if you are willing to put in effort into this chapter then you can get a good command over it.
The first topic that you need to gain a command over are Latches and Flip Flops such as SR- Latch and its implementation using NAND and NOR Gates and then different flip-flops such as SR Flip Flop, D-Flip Flop, JK Flip Flop, and T Flip Flop. You need to under the race around condition in JK Flip Flop and then the concept of Master Slave Flip Flop. Also, you must focus concept about Asynchronous Inputs such as Preset and Clear input. You need to remember the Characteristic Equation of Flip Flops, State Table and Excitation Table of all the flip-flops.
The next part is the application of Flip Flops which is Shift Registers and here you need to study about different types of Shift Registers such as SISO, SIPO, PISO, and PIPO. You need to understand the flow of data into Shift registers and how to find the data present at different clock cycles. You also need to remember the number of clock cycles required to load the data and obtain the data from a Shift Register.
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The most important part of this chapter is the concept of Counters and you need to understand the concept of Asynchronous Counters first of all that is how to design the up and down counter in Asynchronous Counters and how to change the MOD of the counter using Preset and Clear inputs. Then you can study the design of Synchronous Counter and calculating the MOD of the counter if the circuit is given,. To calculate the MOD it is helpful if you have a habit of designing the state table to count the number of states in the counter. In synchronous counters, you also need to study about Ring Counters and Johnson Counters.

Finite State Machines
This topic is only included in GATE ECE and in this topic, you need to study the concept of State Diagram and the concept of Mealy and Moore Machines. In this chapter, your major focus should be on learning the design procedure of State Diagram and how to interpret the working of FSM just by looking at a State Diagram. The design process of FSM is similar to Synchronous Counter Design but there are additional steps such as State Reduction and State Assignment because the states in FSM are not represented using binary codes, unlike counters.
Data Converters
This chapter is very small and does not have a high weightage in GATE exam so it can be learned very quickly. In this chapter, there are two parts ADC and DAC. The chapter begins with a study of DAC because DAC is used in some of the ADC as well. You first have to study the parameters of DAC such as Resolution or Step Size and then study about the two types of DAC i.e. Weighted Resistor type DAC and R-2R ladder network.
In ADC you need to study about the followings types of ADC: Counter Type ADC, Tracking Type ADC, Dual Slope Integrating Type ADC, Successive Approximation Type ADC and at last Flash Type ADC.
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Logic Family
This topic has now been removed from GATE syllabus of both EE and ECE but still, you need to understand about the CMOS implementation of logic Gates as that is a very trivial and easy topic and is included in GATE ECE Syllabus.
Semiconductor Memories
This topic is only included in GATE ECE syllabus and is a very small topic but you never know when can a question be asked from this topic. this chapter mainly focuses on Random Access Memory (RAM). There are basically two types of RAM: SRAM and DRAM and SRAM is simply a combination of two inverters and is very trivial but DRAM is important and questions have been seen in the past from this topic. Also, you need to have a basic understanding of MOSFET to solve problems based on DRAM.
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