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BT0064 Logic Design part -1

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1. Convert the following binary numbers to base 10.


a. 10101101 = 173


b. 110110.1 = 54.1


2. How AND gate can be realized  using NOR gate?



The NOR gate can also be classed as a "Universal" type gate. NOR gates can be used to produce any other type of logic gate function just like the NAND gate and by connecting them together in various combinations the three basic gate types of AND function can be formed using only NOR's, for example.




3. Simplify the following three-variable Boolean functions algebraically:



  1. f = ∑ 1,2,5,6


b. f = ∑  0,1,2,3


4. Draw the circuit for 3-to-8 line decoder.

 


5. Write a short note on J-K Master Slave Flip-Flop.

The Master-Slave Flip-Flop is basically two JK bistable flip-flops connected together in a series configuration with the outputs from Q and Q from the "Slave" flip-flop being fed back to the inputs of the "Master" with the outputs of the "Master" flip-flop being connected to the two inputs of the "Slave" flip-flop as shown below.
      1. Master-Slave JK Flip-Flops

The input signals J and K are connected to the "Master" flip-flop which "locks" the input while the clock (Clk) input is high at logic level "1". As the clock input of the "Slave" flip-flop is the inverse (complement) of the "Master" clock input, the outputs from the "Master" flip-flop are only "seen" by the "Slave" flip-flop when the clock input goes "LOW" to logic level "0". Therefore on the "High-to-Low" transition of the clock pulse the locked outputs of the "Master" flip-flop are fed through to the JK inputs of the "Slave" flip-flop making this type of flip-flop edge or pulse-triggered.
Then, the circuit accepts input data when the clock signal is "HIGH", and passes the data to the output on the falling-edge of the clock signal. In other words, the Master-Slave JK Flip-flop is a "Synchronous" device as it only passes data with the timing of the clock signal.


6. Convert the following decimal numbers to base 2:



a. 122 = (1111010)


b. 98 = (1100010)


7. List the fundamental logical gates.



While each logical element or condition must always have a logic value of either "0" or "1", we also need to have ways to combine different logical signals or conditions to provide a logical result.
For example, consider the logical statement: "If I move the switch on the wall up, the light will turn on." At first glance, this seems to be a correct statement. However, if we look at a few other factors, we realize that there's more to it than this. In this example, a more complete statement would be: "If I move the switch on the wall up and the light bulb is good and the power is on, the light will turn on."
If we look at these two statements as logical expressions and use logical terminology, we can reduce the first statement to:
Light = Switch
This means nothing more than that the light will follow the action of the switch, so that when the switch is up/on/true/1 the light will also be on/true/1. Conversely, if the switch is down/off/false/0 the light will also be off/false/0.Looking at the second version of the statement, we have a slightly more complex expression:Light = Switch and Bulb and PowerNormally, we use symbols rather than words to designate the and function that we're using to combine the separate variables of Switch, Bulb, and Power in this expression. The symbol normally used is a dot, which is the same symbol used for multiplication in some mathematical expressions. Using this symbol, our three-variable expression becomes:Light = Switch Bulb PowerWhen we deal with logical circuits (as in computers), we not only need to deal with logical functions; we also need some special symbols to denote these functions in a logical diagram. There are three fundamental logical operations, from which all other functions, no matter how complex, can be derived. These functions are named and, or, and not. Each of these has a specific symbol and a clearly-defined behavior, as follows:

The AND Gate
The AND gate implements the AND function. With the gate shown to the left, both inputs must have logic 1 signals applied to them in order for the output to be a logic 1. With either input at logic 0, the output will be held to logic 0.
If your browser supports the Javascript functions required for the demonstrations built into this page, you can click the buttons to the left of the AND gate drawing to change their assigned logic values, and the drawing will change to reflect the new input states. Other demonstrations on these pages will work the same way.
There is no limit to the number of inputs that may be applied to an AND function, so there is no functional limit to the number of inputs an AND gate may have. However, for practical reasons, commercial AND gates are most commonly manufactured with 2, 3, or 4 inputs. A standard Integrated Circuit (IC) package contains 14 or 16 pins, for practical size and handling. A standard 14-pin package can contain four 2-input gates, three 3-input gates, or two 4-input gates, and still have room for two pins for power supply connections.


The OR Gate
The OR gate is sort of the reverse of the AND gate. The OR function, like its verbal counterpart, allows the output to be true (logic 1) if any one or more of its inputs are true. Verbally, we might say, "If it is raining OR if I turn on the sprinkler, the lawn will be wet." Note that the lawn will still be wet if the sprinkler is on and it is also raining. This is correctly reflected by the basic OR function.
In symbols, the OR function is designated with a plus sign (+). In logical diagrams, the symbol to the left designates the OR gate.
As with the AND function, the OR function can have any number of inputs. However, practical commercial OR gates are mostly limited to 2, 3, and 4 inputs, as with AND gates.



The NOT Gate, or Inverter
The inverter is a little different from AND and OR gates in that it always has exactly one input as well as one output. Whatever logical state is applied to the input, the opposite state will appear at the output.
The NOT function, as it is called, is necesasary in many applications and highly useful in others. A practical verbal application might be:
The door is NOT locked = You may enter
The NOT function is denoted by a horizontal bar over the value to be inverted, as shown in the figure to the left. In some cases a single quote mark (') may also be used for this purpose: 0' = 1 and 1' = 0. For greater clarity in some logical expressions, we will use the overbar most of the time.
In the inverter symbol, the triangle actually denotes only an amplifier, which in digital terms means that it "cleans up" the signal but does not change its logical sense. It is the circle at the output which denotes the logical inversion. The circle could have been placed at the input instead, and the logical meaning would still be the same.



The logic gates shown above are used in various combinations to perform tasks of any level of complexity. Some functions are so commonly used that they have been given symbols of their own, and are often packaged so as to provide that specific function directly. On the next page, we'll begin our coverage of these functions.
By combining thousands or millions of logic gates, it is possible to perform highly complex operations. The maximum number of logic gates on an integrated circuit is determined by the size of the chip divided by the size of the logic gates. Since transistors make up most of the logic gates in computer processors, smaller transistors mean more complex and faster processors.Minimize the following functions using the ‘don’t care’ terms for simplification wherever possible:
a. f(A,B,C) = ∑3,5 with don’t care term 0,7


b. f(A,B,C,D) = ∑1,2,3,4,5,6,7,8,9,10  with ‘don’t care’ terms 9, 12, 15


9.  Design full-adder using only NOR gates.



First of all, NAND and NOR gates have been proven to be logically complete. That means you can create every other logical gate with just NANDs or just NORs. Other gate functions are XOR, AND, OR, NOT, etc.

To clarify: The notation I am using is [NAND] represents the NAND gate. Having A,B means the two inputs are A and B. If there is just one input, then that input gets mapped to both inputs. The '|' is supposed to show a junction between two lines. '-' is a single line and '=' is a double line. The '. . .' is just empty space, tying to keep the diagram intact.

Some NAND designs we will need are:
AND: A,B--[NAND]==[NAND]-->out

OR: A--[NAND]--|==[NAND]--out
. . . .B--[NAND]--|

first NAND connects to top and bottom NANDs
last NAND has inputs taken from output of the middle two NANDs
. . . . . . . . . . . . . . . A--|[NAND]---|
XOR: A,B--[NAND]------| . . . . . . . |===[NAND]--out
. . . . . . . . . . . . . . . B--|[NAND]---|


The second part is what a full adder is. A full adder is a system that takes in three signals and outputs two. The three input signals are A, B, and Cin. The two outputs are S and Cout. The point of a full adder is that you can cascade them. The Cout of the first becomes the Cin of the second. That way you have a general one bit adder, but can make an N bit adder by stringing cascading them. So the first step in creating a full adder is writing out the truth table, a table where inputs get mapped to specific outputs. I am going to use '-' in place of spaces so the spacing stays the same after I post.

Input- - - - - - - - - - - - - - - -Output
A - - B - - Cin - - - - - - - - - S - - Cout
0 - - 0 - - 0 - - - - - - - - - - -0 - - 0
0 - - 0 - - 1 - - - - - - - - - - -1 - - 0
0 - - 1 - - 0 - - - - - - - - - - -1 - - 0
0 - - 1 - - 1 - - - - - - - - - - -0 - - 1
1 - - 0 - - 0 - - - - - - - - - - -1 - - 0
1 - - 0 - - 1 - - - - - - - - - - -0 - - 1
1 - - 1 - - 0 - - - - - - - - - - -0 - - 1
1 - - 1 - - 1 - - - - - - - - - - -1 - - 1

It can be seen that S = [A XOR B XOR Cin]
It can be seen that C = [(A AND B) OR (A AND Cin) or (B AND Cin)]
or equivelently (and more efficiently) C = [(A AND B) or (C AND (A XOR B))]


10. Draw and explain the working of JK, S-R, and D flip flops.

      In The J-K Flip-Flop
This diagram, P stands for "Preset," C stands for "Clear" and Clk stands for "Clock." The logic table looks like this:
P
C
Clk

J
K
Q
Q'
1
1
1-to-0

1
0
1
0
1
1
1-to-0

0
1
0
1
1
1
1-to-0


1
1

Toggles
1
0
X

X
X
0
1
0
1
X

X
X
1
0

Here is what the table is saying: First, Preset and Clear override J, K and Clk completely. So if Preset goes to 0, then Q goes to 1; and if Clear goes to 0, then Q goes to 0 no matter what J, K and Clk are doing. However, if both Preset and Clear are 1, then J, K and Clk can operate. The 1-to-0 notation means that when the clock changes from a 1 to a 0, the value of J and K are remembered if they are opposites. At the low-going edge of the clock (the transition from 1 to 0), J and K are stored. However, if both J and K happen to be 1 at the low-going edge, then Q simply toggles. That is, Q changes from its current state to the opposite state.
You might be asking yourself right now, "What in the world is that good for?" It turns out that the concept of "edge triggering" is very useful. The fact that J-K flip-flop only "latches" the J-K inputs on a transition from 1 to 0 makes it much more useful as a memory device. J-K flip-flops are also extremely useful in counters (which are used extensively  when creating a digital clock). Here is an example of a 4-bit counter using J-K flip-flops:

The outputs for this circuit are A, B, C and D, and they represent a 4-bit binary number. Into the clock input of the left-most flip-flop comes a signal changing from 1 to 0 and back to 1 repeatedly (an oscillating signal). The counter will count the low-going edges it sees in this signal. That is, every time the incoming signal changes from 1 to 0, the 4-bit number represented by A, B, C and D will increment by 1. So the count will go from 0 to 15 and then cycle back to 0. You can add as many bits as you like to this counter and count anything you like. For example, if you put a magnetic switch on a door, the counter will count the number of times the door is opened and closed. If you put an optical sensor on a road, the counter could count the number of cars that drive by.
Another use of a J-K flip-flop is to create an edge-triggered latch, as shown here:

In this arrangement, the value on D is "latched" when the clock edge goes from low to high. Latches are extremely important in the design of things like central processing units (CPUs) and peripherals in computers.
    1. SR Flip-Flop

An SR Flip-Flop can be considered as a basic one-bit memory device that has two inputs, one which will "SET" the device and another which will "RESET" the device back to its original state and an output Q that will be either at a logic level "1" or logic "0" depending upon this Set/Reset condition. A basic NAND Gate SR flip flop circuit provides feedback from its outputs to its inputs and is commonly used in memory circuits to store data bits. The term "Flip-flop" relates to the actual operation of the device, as it can be "Flipped" into one logic state or "Flopped" back into another.
The simplest way to make any basic one-bit Set/Reset SR flip-flop is to connect together a pair of cross-coupled 2-input NAND Gates to form a Set-Reset Bistable or a SR NAND Gate Latch, so that there is feedback from each output to one of the other NAND Gate inputs. This device consists of two inputs, one called the Reset, R and the other called the Set, S with two corresponding outputs Q and its inverse or complement Q as shown below.
      1. The SR NAND Gate Latch



D flip flops
The edge-triggered D flip-flop is easily derived from its RS counterpart. The only requirement is to replace the R input with an inverted version of the S input, which thereby becomes D. This is only needed in the master latch section; the slave remains unchanged.
One essential point about the D flip-flop is that when the clock input falls to logic 0 and the outputs can change state, the Q output always takes on the state of the D input at the moment of the clock edge. This was not true of the RS and JK flip-flops. The RS master section would repeatedly change states to match the input signals while the clock line is logic 1, and the Q output would reflect whichever input most recently received an active signal. The JK master section would receive and hold an input to tell it to change state, and never change that state until the next cycle of the clock. This behavior is not possible with a D flip-flop.


The edge-triggered D NAND flip-flop is shown below.





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