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# Note for Microprocessor and Microcontroller - MPMC By Abhishek Singh

• Microprocessor and Microcontroller - MPMC
• Note
• Kalinga Institute of Industrial Technology - KIIT
• Electrical Engineering
• 12 Topics
• 47563 Views
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1 Review of Digital Electronics 1.1 Gate Types and Truth Tables The basic logic gates are AND, OR, NAND, NOR, XOR, INV, and BUF. The last two are not standard terms; they stand for “inverter” and “buffer”, respectively. Truth Tables are an easy way to represent a combinational logic output by tabulating all possible inputs. The symbols for these gates and their corresponding Boolean expressions and truth tables are given below. Figure 1: Basic Logic gates 1

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1.2 Boolean Algebra and K-Maps Boolean algebra can be used to formalize the combinations of binary logic states. Using the definition of Boolean addition, multiplication and inversion, we can define all the logic gates algebraically. For designing any combinational circuit, we use Boolean algebra. However, for any arbitrary circuit the boolean expression might be lengthy and cumbersome which might lead to inefficient implementation. Thus, the need of logic minimization. One method is through the use of Karnaugh Maps or K-Maps. For a boolean function of n variables, x1 , x2 , . . . xn , a product term in which each of the n variables appears once (in either its complemented or uncomplemented form) is called a minterm. The addition or “OR”-ing of minterms give the Sum of Products. For a boolean function of n variables, x1 , x2 , . . . xn , a sum term in which each of the n variables appears once (in either its complemented or uncomplemented form) is called a maxterm. The multiplication or “AND”-ing of maxterms give the Product of Sums. 1.3 Multiplexer A multiplexer (MUX) is a device which passes one of several data inputs to one output. Generally there are 2n data inputs and n control lines which determine which input is steered to the output. Hence, a MUX can take many data bits and put them, one at a time, on a single output data line in a particular sequence. This is an example of transforming parallel data to serial data. By adding gate-level circuitry to MUX inputs, any arbitrary combinational function can be realised with a 2:1 MUX. Also, any n variable combinational function can be implemented with a 2n : 1 MUX, 2n−1 : 1 MUX and so on. 1.4 Decoder Decoder (DEC) is basically, a combinational type logic circuit that converts the binary code data at its input into an equivalent decimal code at its output. Generally there are n inputs and 2n outputs. Depending on the input, the decoder activates only one of the 2

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2n outputs. Therefore, whichever output line is “HIGH” identifies the binary code present at the input, in other words it “de-codes” the binary input and these types of binary decoders are commonly used as Address Decoders in microprocessor memory applications. 1.5 Priority Encoder An encoder is a combinational logic circuit that accepts an active level on one of its inputs (inputs represents digits, such as decimal, octal and so on) and converts it to a coded output. An encoder has 2n input lines, only one of which is activated at a given time and produces an n-bit output code, depending on which input is activated. However, if more than one input are active simultaneously, the output is unpredictable. This ambiguity is resolved if priority is established so that only one input is encoded, no matter how many inputs are active at a given point of time. The priority encoder includes a priority function. The operation of the priority encoder is such that if two or more inputs are active at the same time, the input having the highest priority will take precedence. 1.6 Latches and Flip-Flops Latches and flip-flops are the basic elements for storing information. One latch or flipflop can store one bit of information. The main difference between latches and flip-flops is that for latches, their outputs are constantly affected by their inputs as long as the enable signal is asserted. In other words, when they are enabled, their content changes immediately when their inputs change. Flip-flops, on the other hand, have their content change only either at the rising or falling edge of the enable signal. This enable signal is usually the controlling clock signal. After the rising or falling edge of the clock, the flip-flop content remains constant even if the input changes. 3

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2 Tristate concept and Bus structure 2.1 Tristate logic In digital logic, we have two states: HIGH and LOW. Let us consider the switch connected between input (A) and output (Y) as shown below. Now let us switch it ON and apply 5V (HIGH) to the input. Since the switch is ON, input goes to the output and we get 5V as output. Similarly, when we apply 0V (LOW) as the input, we get 0V as output. However, when the switch in OFF, we cannot say that the output is at 0V (LOW). This new state, when the input is not connected to the output, is called a High-Impedance State (Z). Thus, any logic device which has the provision to effectively remove its influence from the circuit (usually through an ENABLE input) exhibits a tristate logic. For example, let us take a NOT gate as shown. The gate works as usual when the EN input is HIGH, but when EN is set to LOW, irrespective of the input, the output is at High-impedance state. 4