Digital electronics question with solution

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Definition: Digital Electronics is the sub-branch of electronics that deals with digital signals for processing and controlling various systems and sub-systems. In various applications like sensors and actuators, usage of digital electronics is increasing extensively.

Digital electronics is entirely the field in which digital signals is used. Digital signals are discretization of analog signals. A signal carries information. In digital signals, the values in a particular band is same i.e constant.

These above are question

Bidirectional counters, also known as Up/Down counters, are capable of counting in either direction through any given count sequence and they can be reversed at any point within their count sequence by using an additional control input

The circuit above is of a simple 3-bit Up/Down synchronous counter using JK flip-flops configured to operate as toggle or T-type flip-flops giving a maximum count of zero (000) to seven (111) and back to zero again. Then the 3-Bit counter advances upward in sequence (0,1,2,3,4,5,6,7) or downwards in reverse sequence (7,6,5,4,3,2,1,0).

Generally most bidirectional counter chips can be made to change their count direction either up or down at any point within their counting sequence. This is achieved by using an additional input pin which determines the direction of the count, either Up or Down and the timing diagram gives an example of the counters operation as this Up/Down input changes state.

Nowadays, both up and down counters are incorporated into single IC that is fully programmable to count in both an “Up” and a “Down” direction from any preset value producing a complete Bidirectional Counter chip.

The term CMOS stands for “Complementary Metal Oxide Semiconductor”. This is one of the most popular technology in the computer chip design industry and it is broadly used today to form integrated circuit in numerous and varied applications

CMOS Working Principle

In CMOS technology, both N-type and P-type transistors are used to design logic functions. The same signal which turns ON a transistor of one type is used to turn OFF a transistor of the other type. This characteristic allows the design of logic devices using only simple switches, without the need for a pull-up resistor.

In CMOS logic gates a collection of n-type MOSFETs is arranged in a pull-down network between the output and the low voltage power supply rail (Vss or quite often ground). Instead of the load resistor of NMOS logic gates, CMOS logic gates have a collection of p-type MOSFETs in a pull-up network between the output and the higher-voltage rail (often named Vdd).

Thus, if both a p-type and n-type transistor have their gates connected to the same input, the p-type MOSFET will be ON when the n-type MOSFET is OFF, and vice-versa. The networks are arranged such that one is ON and the other OFF for any input pattern as shown in the figure below.

CMOS offers relatively high speed, low power dissipation, high noise margins in both states, and will operate over a wide range of source and input voltages (provided the source voltage is fixed). Furthermore, for a better understanding of the Complementary Metal Oxide Semiconductor working principle, we need to discuss in brief CMOS logic gates as explained below.

SIPO shift register sensitive to positive edge of the clock pulse. Here the data word which is to be stored (Data in) is fed serially at the input of the first flip flop (D1 of FF1). It is also seen that the inputs of all other flip-flops (except the first flip-flop FF1) are driven by the outputs of the preceding ones say for example, the input of FF2 is driven by the output of FF1. In this kind of shift register, the data stored within the register is obtained as a parallel-output data word (Data out) at the individual output pins of the flip-flops (Q1 to Qn).

In general, the register contents are cleared by applying high on the clear pins of all the flip-flops at the initial stage. After this, the first bit, B1 of the input data word is fed at the D1 pin of FF1.

This bit (B1) will enter into FF1, get stored and thereby appears at its output Q1 on the appearance of first leading edge of the clock. Further at the second clock tick, the bit B1 right-shifts and gets stored into FF2 while appearing at its output pin Q2 while a new bit, B2 enters into FF1.

Similarly at each clock tick the data within the register moves towards right by a single bit while a new bit of the input word enters into the register. Meanwhile one can extract the bits stored within the register in parallel-fashion at the individual flip-flop outputs.

Combinational Circuit –

1. In this output depends only upon present input.
2. Speed is fast.
3. It is designed easy.
4. There is no feedback between input and output.
5. This is time independent.
6. Elementary building blocks: Logic gates
7. Used for arithmetic as well as boolean operations.
8. Combinational circuits don’t have capability to store any state.
9. As combinational circuits don’t have clock, they don’t require triggering.
10. These circuits do not have any memory element.
11. It is easy to use and handle.

Examples – Encoder, Decoder, Multiplexer, Demultiplexer

Sequential Circuit –

1. In this output depends upon present as well as past input.
2. Speed is slow.
3. It is designed tough as compared to combinational circuits.
4. There exists a feedback path between input and output.
5. This is time dependent.
6. Elementary building blocks: Flip-flops
7. Mainly used for storing data.
8. Sequential circuits have capability to store any state or to retain earlier state.
9. As sequential circuits are clock dependent they need triggering.
10. These circuits have memory element.
11. It is not easy to use and handle.

Examples – Flip-flops, Counters