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UNIT-II -DIGITAL SYSTEM DESIGN

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UNIT-II -DIGITAL SYSTEM DESIGN

1. 1. Dr.Y.Narasimha Murthy., Ph.D yayavaram@yahoo.com UNIT II : SEQUENTIAL CIRCUIT DESIGN Introduction: The various stages involved in the design of sequential circuits include, the derivation of state tables, state table reduction , state assignment , and derivation of flip-flop input equations. The state table is derived by determining the relationship between the input and output sequences or sometimes can be derived directly from the state graph also. After deriving the state table ,it is reduced to a minimum number of states. First, duplicate rows are eliminated by row matching method and, then using an implication table. If the reduced table has m states (2n–1 < m ≤ 2n), n flip-flops are required. Assign a unique combination of flip-flop states to correspond to each state in the reduced table. Form the transition table by substituting the assigned flip-flop states for each state in the reduced state table. The resulting transition table specifies the next states of the flip-flops, and the output in terms of the present states of the flip-flops and the input. Plot next-state maps and input maps for each flip-flop and derive the flip-flop input equations. (Depending on the type of gates to be used, either determine the sum-of-products form from the 1’s on the map or the product-of-sums form from the 0’s on the map.) Derive the output functions. Realize the flip-flop input equations and the output equations using the available logic gates. Check the design by signal tracing, computer simulation, or laboratory testing. Design of Iterative Circuits : An iterative circuit consists of a number of identical cells interconnected in a regular manner. Certain arithmetic operations, like binary addition are implemented with an iterative circuit because the same operation is performed on each pair of input bits. The regular structure of an iterative circuit makes it easier to fabricate in integrated circuit form than circuits with less regular structures. The simplest form of an iterative circuit consists of a linear array of combinational cells with signals between cells traveling in only one direction.Each cell is a combinational circuit with one or more primary inputs (xi) and possibly one or more primary outputs (zi). In addition, each cell 1