1.10 (a) Function of AND, OR, NOT, NAND & NOR Logic gates and their input/output.

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1 Chapter 1.10 Logic Gates 1.10 (a) Function of AND, OR, NOT, NAND & NOR Logic gates and their input/output. Microprocessors are the central hardware that runs computers. There are several components that make a processor. The first is the transistor. Next, are logic gates where you put more than one transistor to work with others. The microprocessor works with binary arithmetic by using binary math to perform operations. When a microprocessor is designed, along with other design focus areas "Logic gate cell library (a library is collection of all low level logic functions like AND, OR and NOT etc.), which is used to implement the logic" is also deeply planned and developed. Logic gates carry out the instructions mathematical or otherwise that a processor performs, for example a logic gate performs a logical operation on one or more logic inputs and produces a single logic output. When you connect a variety of logic gates together, the results are circuits. The logic is called Boolean logic and is most commonly found in digital circuits. Following five logic gates are part of syllabus. 1. AND gate, 2. OR gate, 3. NOT gate, 4. NAND gate, and 5. NOR gate. AND gate: AND gate symbol The AND gate is a digital logic gate that behaves according to the table on your right. A HIGH output (1) results only if both the inputs to the AND gate are HIGH (1). If neither or only one input to the AND gate is HIGH, a LOW output results. INPUT OUTPUT A B A AND B (Q) Page 1 of 15

2 OR Gate: OR gate symbol The OR gate is a digital logic gate that behaves according to the table on your right. A HIGH output (1) results if one or both the inputs to the gate are HIGH (1). If neither input is HIGH, a LOW output (0) results. NOT gate (Inverter): INPUT A B OUTPUT A + B (Q) NOT gate symbol In digital logic, an inverter or NOT gate is a logic gate which implements logical negation. Not gate represents perfect switching behavior. NAND gate: INPUT OUTPUT A NOT A NAND gate symbol The Negated AND, NO AND or NAND gate is the opposite of the digital AND gate, and behaves in a manner that corresponds to the opposite of AND gate, as shown in the truth table on the right. A LOW output results only if both the inputs to the gate are HIGH. If one or both inputs are LOW, a HIGH output results. The NAND gate is significant because any Boolean function can be implemented by using a combination of NAND gates. INPUT A B OUTPUT A NAND B (Q) Page 2 of 15

3 NOR gate: NOR gate symbol The NOR gate is a digital logic gate that implements logical NOR - it behaves according to the truth table to the right. A HIGH output (1) results if both the inputs to the gate are LOW (0). If one or both input is HIGH (1), a LOW output (0) results. NOR is the result of the negation of the OR operator. INPUT A B OUTPUT A NOR B (Q) Page 3 of 15

4 1.10 (b) Calculate outcome from a set of logic gates. In this part 1.10 (b) we will explore the application of Boolean algebra in the design of electronic circuits. The basic elements of circuits are gates. Each type of gate implements a Boolean operation. Consider Boolean expression a(x)=x ; i.e., a(x) is the complement of x. Now a(0)=1 and a(1) =0. This Boolean operation, i.e., compliment can be implemented using a device called NOT gate or the Inverter. It can be expressed as below: NOT gate (INVERTER) Now consider Boolean expression a(x,y)=xy; i.e., a is the Boolean product of x & y. As we know that a(0,0)=0, a(0,1)=0, a(1,0)=0 and a(1,1)=1. This Boolean operation, i.e., product can be implemented using a device called AND gate. It can be expressed as below: AND gate Next consider Boolean expression a(x,y)=x+y; i.e., a is the Boolean sum of x & y. As we know that a(0,0)=0, a(0,1)=1, a(1,0)=1 and a(1,1)=1. This Boolean operation, i.e., sum can be implemented using a device called OR gate. It can be expressed as below: OR gate The circuits for expressions x y and xy are shown below in figures (a) and (b), respectively: Page 4 of 15

5 The circuits for expressions x +y and x+y are shown below in figures (a) and (b), respectively: The circuit for x y and x +y are shown below in figures (a) and (b), respectively: If a is xy +x y, then it circuit diagram is shown below, We now break xy and x y and include their own circuits in diagram above and create a new circuit with four gates. In order to simplify the circuit above we can split x and y inputs half way and use it for two or more gates. So the above diagram can be simplified and presented as: Page 5 of 15

6 Truth Table: A truth table is a mathematical table used in logic specifically in connection with Boolean algebra and Boolean functions to compute the functional values of logical expressions on each of their functional arguments, that is, on each combination of values taken by their logical variables. In particular, truth tables can be used to tell whether an expression is true for all valid input values, that is, logically valid. Practically, a truth table is composed of one column for each input variable (for example, A and B), and one final column for all of the possible results of the logical operation that the table is meant to represent (for example, A OR B). Each row of the truth table therefore contains one possible configuration of the input variables (for instance, A=true B=false), and the result of the operation for those values. Number of possible rows in a truth table is directly dependant on the number of inputs and can be easily find out by applying 2 n, where n is the number of inputs mentioned in truth table. Truth tables for logic gates are already shown above with their definitions. Here is a truth table giving definitions of the most commonly used 5 out of the 16 possible truth functions of 2 binary inputs (P,Q are thus Boolean variables): P Q P AND Q P OR Q NOT P NOT Q P NAND Q P NOR Q Note that total number of rows is, 2 2 = 4. Page 6 of 15

7 1.10 (c) Producing simple logic circuits from Boolean statements. Consider the following problem: If button A or button B are on and button C is off then the alarm X goes on We can convert this onto logic gate terminology (ON = 1 and OFF = 0): If (A = 1 OR B = 1) AND (C = NOT 1) then (X = 1) (Notice: rather than write 0 we use NOT 1) To draw the logic network, we do each part in brackets first i.e. A = 1 OR B = 1 is one gate then C = NOT 1 is the second gate. These are then joined together by the AND gate. Once the logic network is drawn we can then test it using a truth table. Remember the original problem we are looking for the output to be 1 when A or B is 1 and when C is 0. Thus we get the following logic network and truth table from the network. Looking at the values in the truth table, we will be able to clearly see that it matches up with the original problem which then gives us confidence that the logic network is correct. Page 7 of 15

8 Let us now consider a second problem: A steel rolling mill is to be controlled by a logic network made up of AND, OR and NOT gates only. The mill receives a stop signal (i.e. S = 1) depending on the following input bits: A stop signal (S = 1) occurs when: either Length, L > 100 metres and Velocity, V < 10 m/s or Temperature, T < 1000 C and Velocity, V > 10 m/s Draw a logic network and truth table to show all the possible situations when the stop signal could be received. The first thing to do is to try and turn the question into a series of logic gates and then the problem becomes much simplified. The first statement can be re-written as: (L = 1 AND V = NOT 1) since Length > 100 metres corresponds to a binary value of 1 and Velocity < 10 m/s corresponds to a binary value of 0 (i.e. NOT 1). The second statement can be re-written as (T = NOT 1 AND V = 1) since Temperature < 1000C corresponds to a binary value of 0 (i.e. NOT 1) and Velocity > 10 m/s corresponds to a binary value of 1 Both these statements are joined together by OR which gives us the logic statement: if (L = 1 AND V = NOT 1) OR (T = NOT 1 AND V = 1) then S = 1 We can now draw the logic network and truth table to give the solution to the original problem (input L has been put at the bottom of the diagram just to avoid crossing over of lines; it merely makes it look neater and less complex and isn t essential): Page 8 of 15

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10 Points to consider when studying or teaching Following information is just an opinion and is given for the better understanding of chapter 1.10 so that students are not under or over taught. Teachers must not limit their visualization just to these points. 1. Permanently refreshing memory should not be taught. 2. Circuits that are used to teach may only have two inputs to every gate, 3. Gates in any given circuit must be limited to 4 in number, 4. To teach gates both circles and actual gates shapes can be used. In any case gate name must be written inside the gate shape or circle, 5. Teaching gates using their shapes is a better idea, 6. Teaching static memory (SRAM) or dynamic memory (DRAM) is INAPPROPRIATE, 7. Teach students to read circuits and draw simple circuits to produce specific outcomes. 8. Students will be required to fill given truth tables after reading the given circuit, Page 10 of 15

11 Example Questions: In questions 1 to 6, draw each circuit using their proper symbols and produce truth tables. Remember that if there are TWO inputs then there will be four (2 2 ) possible outputs and if there are THREE inputs there will be eight (2 3 ) possible outputs. i.e: Page 11 of 15

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13 Questions 7 to 10 require both the logic network to be created and also the truth table. The truth table can be derived from the logic network, but also from the problem. This is a check that the logic network actually represents the original problem. (7) A computer will only operate if three switches P, S and T are correctly set. An output signal (X = 1) will occur if P and S are both ON or if P is OFF and S and T are ON. Design a logic network and draw the truth table for this network. (8) A traffic signal system will only operate if it receives an output signal (D = 1). This can only occur if: either (a) signal A is red (i.e. A = 0) or (b) signal A is green (i.e. A = 1) and signals B and C are both red (i.e. B and C are both 0) Design a logic network and draw a truth table for the above system. (9) A chemical plant gives out a warning signal (W = 1) when the process goes wrong. A logic network is used to provide input and to decide whether or not W = 1. Page 13 of 15

14 A warning signal (W = 1) will be generated if either (a) Chemical Rate < 10 m3/s or (b) Temperature > 87 C and Concentration > 2 moles or (c) Chemical rate = 10 m3/s and Temperature > 87 C Draw a logic network and truth table to show all the possible situations when the warning signal could be received. (10) A power station has a safety system based on three inputs to a logic network. A warning signal (S = 1) is produced when certain conditions occur based on these 3 inputs: A warning signal (S = 1) will be generated if: either (a) Temperature > 120C and Cooling Water < 100 l/hr or (b) Temperature < 120C and (Pressure > 10 bar or Cooling Water < 100 l/hr) Draw a logic network and truth table to show all the possible situations when the warning signal could be received. (11) (a) Two logic gates are the AND gate and the OR gate. Complete the truth tables for these two gates: Page 14 of 15

15 (b) Complete the truth table for the following logic circuit: (12) (a) (i) Complete the truth table for the following logic circuit: (ii) Which single logic gate has the SAME function as the above logic circuit? [1] Page 15 of 15