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CS1104 – Computer Organization Aaron Tan Tuck Choy School of Computing National University of Singapore

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CS Lecture 12: Sequential Logic: Design with Flip-flops 2 Lecture 12: Sequential Logic Design with Flip-flops Introduction Introduction Flip-flop Characteristic Tables Flip-flop Characteristic Tables Sequential Circuit Analysis Sequential Circuit Analysis Flip-flop Input Functions Flip-flop Input Functions Analysis: Example #2 Analysis: Example #2 Analysis: Example #3 Analysis: Example #3 Flip-flop Excitation Tables Flip-flop Excitation Tables

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CS Lecture 12: Sequential Logic: Design with Flip-flops 3 Lecture 12: Sequential Logic Design with Flip-flops Sequential Circuit Design Sequential Circuit Design Design: Example #1 Design: Example #1 Design: Example #2 Design: Example #2 Design: Example #3 Design: Example #3 Design of Synchronous Counters Design of Synchronous Counters

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CS Models of Sequential Circuits4 Introduction Sequential circuits has an extra dimension – time. Combinational circuit output depends only on the present inputs Sequential circuit output depends on the history of past inputs as well More powerful than combinational circuit, able to model situations that cannot be modeled by combinational circuits Building blocks of synchronous sequential logic circuits: gates and flip-flops. Flip-flops make up the memory M while the gates form one or more combinational subcircuits C 1, C 2, …, C q.

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CS Flip-flop Characteristic Tables5 Each type of flip-flop has its own behavior. The characteristic tables for the various types of flip- flops are shown below: JK Flip-flopSR Flip-flop D Flip-flopT Flip-flop

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CS Sequential Circuit Analysis6 Given a sequential circuit diagram, analyze its behaviour by deriving its state table and hence its state diagram. Requires state equations to be derived for the flip-flop inputs, as well as output functions for the circuit outputs other than the flip-flops (if any). We use A(t) and A(t+1) to represent the present state and next state, respectively, of a flip-flop represented by A. Alternatively, we could simply use A and A + for the present state and next state respectively.

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CS Sequential Circuit Analysis7 Example (using D flip-flops): State equations: A + = A.x + B.x B + = A'.x Output function: y = (A + B).x' A A' B B' y x CP D Q Q' D Q Figure 1.

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CS Sequential Circuit Analysis8 From the state equations and output function, we derive the state table, consisting of all possible binary combinations of present states and inputs. State table Similar to truth table. Inputs and present state on the left side. Outputs and next state on the right side. m flip-flops and n inputs 2 m+n rows.

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CS Sequential Circuit Analysis9 State table for the circuit of Figure 1: State equations: A + = A.x + B.x B + = A'.x Output function: y = (A + B).x'

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CS Sequential Circuit Analysis10 Sequential Circuit Analysis Alternate form of state table:

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CS Sequential Circuit Analysis11 Sequential Circuit Analysis From the state table, we can draw the state diagram. State diagram Each state is denoted by a circle. Each arrow (between two circles) denotes a transition of the sequential circuit (a row in state table). A label of the form a/b is attached to each arrow where a denotes the inputs while b denotes the outputs of the circuit in that transition. Each combination of the flip-flop values represents a state. Hence, m flip-flops up to 2 m states.

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CS Sequential Circuit Analysis12 Sequential Circuit Analysis State diagram of the circuit of Figure 1: /0 0/1 0/0 1/00/1

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CS Flip-flop Input Functions13 Flip-flop Input Functions The outputs of a sequential circuit are functions of the present states of the flip-flops and the inputs. These are described algebraically by the circuit output functions. In Figure 1: y = (A + B).x' The part of the circuit that generates inputs to the flip-flops are described algebraically by the flip-flop input functions (or flip-flop input equations). The flip-flop input functions determine the next state generation. From the flip-flop input functions and the characteristic tables of the flip-flops, we obtain the next states of the flip-flops.

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CS Flip-flop Input Functions14 Flip-flop Input Functions Example: circuit with a JK flip-flop. We use 2 letters to denote each flip-flop input: the first letter denotes the input of the flip-flop (J or K for JK flip-flop, S or R for SR flip-flop, D for D flip-flop, T for T flip-flop) and the second letter denotes the name of the flip-flop. A B C' x ByBy CP J Q Q' K B' C x' JA = B.C'.x + B'.C.x’ KA = B + y

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CS Flip-flop Input Functions15 Flip-flop Input Functions In Figure 1, we obtain the following state equations by observing that Q + = DQ for a D flip-flop: A + = A.x + B.x (since DA = A.x + B.x) B + = A'.x (since DB = A'.x) A A' B B' y x CP D Q Q' D Q Figure 1.

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CS Analysis: Example #216 Analysis: Example #2 Given Figure 2, a sequential circuit with two JK flip- flops A and B, and one input x. Obtain the flip-flop input functions from the circuit: JA = BJB = x' KA = B.x'KB = A'.x + A.x' = A x A B x CP J Q Q' K J Q K Figure 2.

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CS Analysis: Example #217 Analysis: Example #2 Flip-flop input functions: JA = BJB = x' KA = B.x'JB = A'.x + A.x' = A x Fill the state table using the above functions, knowing the characteristics of the flip-flops used

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CS Analysis: Example #218 Analysis: Example #2 Draw the state diagram from the state table

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CS Analysis: Example #319 Analysis: Example #3 Derive the state table and state diagram of the following circuit. Flip-flop input functions: JA = BJB = KB = (A x)' = A.x + A'.x' KA = B' J Q Q' K J Q K A x CP B y Figure 3.

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CS Analysis: Example #320 Analysis: Example #3 Flip-flop input functions: JA = BJB = KB = (A x)' = A.x + A'.x' KA = B' State table:

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CS Analysis: Example #321 Analysis: Example #3 State diagram: /1 1/0 0/1 1/1 0/0 1/0

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CS Flip-flop Excitation Tables22 Flip-flop Excitation Tables Analysis: Starting from a circuit diagram, derive the state table or state diagram. Design: Starting from a set of specifications (in the form of state equations, state table, or state diagram), derive the logic circuit. Characteristic tables are used in analysis. Excitation tables are used in design.

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CS Flip-flop Excitation Tables23 Flip-flop Excitation Tables Excitation tables: given the required transition from present state to next state, determine the flip-flop input(s). JK Flip-flopSR Flip-flopD Flip-flopT Flip-flop

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CS Sequential Circuit Design24 Sequential Circuit Design Design procedure: Start with circuit specifications – description of circuit behaviour. Derive the state table. Perform state reduction if necessary. Perform state assignment. Determine number of flip-flops and label them. Choose the type of flip-flop to be used. Derive circuit excitation and output tables from the state table. Derive circuit output functions and flip-flop input functions. Draw the logic diagram.

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CS Design: Example #125 Design: Example #1 Given the following state diagram, design the sequential circuit using JK flip-flops

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CS Design: Example #126 Design: Example #1 Circuit state/excitation table, using JK flip-flops X 1 X 0 X X 0 X 1 0 X 1 X X 1 X 0 0 X 1 X X 0 X 1 JK Flip-flop’s excitation table.

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CS Design: Example #127 Design: Example #1 Block diagram. Combinational circuit A' A B B' x KAJAKBJB BB’AA’ External input(s) CP External output(s) (none) J QQ' KJ Q K What are to go in here?

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CS Design: Example #128 Design: Example #1 From state table, get flip-flop input functions. A B x A Bx XXXX 1 JA = B.x'JB = x A B x A Bx X 1XX X1 KA = B.x A B x A Bx X 1 XXX KB = (A x)' A B x A Bx X1 X1X X

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CS Design: Example #129 Design: Example #1 Flip-flop input functions. JA = B.x'JB = x KA = B.xKB = (A x)' Logic diagram: x B A CP J QQ' KJ Q K

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CS Design: Example #230 Design: Example #2 Design, using D flip-flops, the circuit based on the state table below. (Exercise: Design it using JK flip- flops.)

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CS Design: Example #231 Design: Example #2 Determine expressions for flip-flop inputs and the circuit output y. DA(A, B, x) = m(2,4,5,6) DB(A, B, x) = m(1,3,5,6) y(A, B, x) = m(1,5) DA = A.B' + B.x' A B x A Bx DB = A'.x + B'.x + A.B.x' A B x A Bx A B x A Bx 1 1 y = B'.x

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CS Design: Example #232 Design: Example #2 From derived expressions, draw logic diagram: DA = A.B' + B.x' DB = A'.x + B'.x + A.B.x' y = B'.x D Q Q' D Q A A' B B' y CP x

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CS Design: Example #333 Design: Example #3 Design with unused states. Given theseDerive these Unused state 000:

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CS Design: Example #334 Design: Example #3 From state table, obtain expressions for flip-flop inputs. A C x AB Cx 11 B X X XX X XX X X SA = B.x A C x AB Cx 1 B X X X X X XX X X X RA = C.x' B A C x AB Cx 1X X X X XXX SB = A'.B'.x RB = B.C + B.x' A C x AB Cx B X X X 1 X XXX 1 XXXX 1

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CS Design: Example #335 Design: Example #3 From state table, obtain expressions for flip-flop inputs (cont’d). A C x AB Cx 1 1 B X X X X X XX X X SC = x' A C x AB Cx 1 B X X 1X X XX 1 X X RC = x y = A.x B A C x AB Cx 1 X X X XXX 1

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CS Design: Example #336 Design: Example #3 From derived expressions, draw logic diagram: SA = B.xSB = A'.B'.xSC = x' RA = C.x'RB = B.C + B.x'RC = x y = A.x A A' B B' y CP x S Q Q' R S Q R S Q R C

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CS Design of Synchronous Counters37 Design of Synchronous Counters Counter: a sequential circuit that cycles through a sequence of states. Binary counter: follows the binary sequence. An n-bit binary counter (with n flip-flops) counts from 0 to 2 n -1. Example 1: A 3-bit binary counter (using T flip-flops)

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CS Design of Synchronous Counters38 Design of Synchronous Counters 3-bit binary counter (cont’d). TA 2 = A 1.A 0 A2A2 A1A1 A0A0 1 1 TA 1 = A 0 TA 0 = 1 A2A2 A1A1 A0A A2A2 A1A1 A0A

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CS Design of Synchronous Counters39 Design of Synchronous Counters 3-bit binary counter (cont’d). TA 2 = A 1.A 0 TA 1 = A 0 TA 0 = 1 1 A2A2 CP T Q T Q T Q A1A1 A0A0

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CS Design of Synchronous Counters40 Design of Synchronous Counters Example 2: Counter with non-binary sequence: 000 001 010 100 101 110 and back to JA = BJB = CJC = B' KA = BKB = 1KC = 1

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CS Design of Synchronous Counters41 Design of Synchronous Counters Counter with non-binary sequence (cont’d). JA = BJB = CJC = B' KA = BKB = 1KC = 1 CP A 1 J QQ' KJ Q KJ Q K BC

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CS Summary42 Summary Sequential circuits have memory and they are more powerful than combinational circuits. Analyzing sequential circuits Flip-flop characteristic table State Table State diagram Designing sequential circuits Flip-flop excitation table State assignment Circuit output function Flip-flop input function

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