Spring 2011 ECE 331 - Digital System Design 2 Flip flop is a sequential circuit which generally samples its inputs and changes its outputs only at … Nearly all sequential logic today is clocked or synchronous logic. 6.004 Worksheet L10 – Sequential Circuits Problem 4.« Consider the following sequential logic circuit. They are, synchronous sequential circuits and asynchronous sequential circuits. It consists of one input IN, a 2-bit register that stores the current state, and some combinational logic that determines the state (next value to load into the register) based on the current state and the input IN. We give an example that shows that the new architecture can reduce power dissipation for a larger class of sequential circuits than the previously developed architecture. 2. Q.1. * In sequential circuits, the \state" of the circuit is crucial in determining the output values. Synchronous sequential logic. The detector should keep checking for the appropriate sequence and should not reset to the initial state after it has recognized the sequence. The major difference between combinational and sequential logic circuit is that the combinational logic circuit consists of only logic gates while the sequential logic circuits consist of logic gates and memory elements.. Another crucial factor which differentiates combinational and sequential logic circuit is the state of output elements. sequential logic circuits and allows the precomputation logic to be a function of a subset or all of the input variables. Note that there are don’t care entries whenever There are fewer states than possible state vectors. Q 1 Q 0 N.S. Autumn 2003 CSE370 -VI -Sequentai lLogci 1 Sequential logic Sequential circuits simple circuits with feedback latches edge-triggered flip-flops Timing methodologies cascading flip-flops for proper operation clock skew Asynchronous inputs metastability and synchronization Basic registers shift registers simple counters Hardware description languages and sequential logic Sequential circuits * The digital circuits we have seen so far (gates, multiplexer, demultiplexer, encoders, decoders) are combinatorial in nature, i.e., the output(s) depends only on the present values of the inputs and not on their past values. The basic memory element in sequential logic is the flip-flop. • Synchronous – where flip-flops are used to implement the states, and a clock signal is used to ... Derivation of logic expressions for the sequential circuit in Figure 6.6. w 00 01 11 10 0 1 0 y 2 y 1 Y 1 wy y 2 = w 00 01 11 10 0 1 d d y 2 y 1 Y 2 wy 1 y 2 wy y 2 = + d d 0 0 1 0 Types of digital logic circuits are combinational logic circuits and sequential logic circuits. Ð255 ! L5: 6.111 Spring 2004 Introductory Digital Systems Laboratory 4 The Sequential alwaysBlock Edge-triggered circuits are described using a sequential alwaysblock module combinational(a, b, … Introduce several structural and behavioral models for synchronous sequential circuits. The sequential Circuits are designed using the combinational circuits along with a memory devices known as Flip-Flops. 5 ENEE 359a Lecture/s 12-15 Sequential Logic Bruce Jacob University of Maryland ECE Dept. For sequential logic, the sensitivity list may not include all the signals that are read inside the alwaysblock For edge-triggered sequential logic use: always @(posedge signal1, negedge signal2, …) The positive edge or negative edge of each signal can be specified in the sensitivity list Lecture 11: Sequential Circuits S Q R 2 Overview Last lecture: Boolean logic and combinational circuits.! ENEE 245: Digital Circuits & Systems Lab — Lab 2 Additional Designs!e following are additional designs, several of which are used in high-speed I/O circuits. – Assign statements to combinational logic – Case statement to multiplexers • Behavioral (always @) blocks – Synthesis to combinational and sequential logic – Depends on sensitivity list • No sensitivity to both edges of the clock, changing sensitivity lists, etc. “State Elements” A Q Figure 1. Combinational and Sequential Circuits Design 2.1. Sequential logic devices have some sort of feedback, where the output of some logic device is fed back to the input of a logic device. SLIDE 38 UNIVERSITY OF MARYLAND Some Issues Feed-through and race conditions D Q Q Clk CLK D The values stored in memory elements define the state of a sequential component. A simple memory circuit constructed from a OR gate is shown on Figure 1. 2. Analysis and Design of Sequential Logic Circuits 2 Example 1: Design a sequential circuit that produces “1” on its output if it detects the sequence “101” on it’s input. But sequential circuit has memory so output can vary based on input. The active edge of the clock acts as Since memory is finite, therefore, the sequence size must always be finite, which means that the sequential logic can contain only a finite number of states. So sequential circuits are sometimes called finite-state machines. They typically have an input (or inputs) that can cause the state to change. The basic classification of sequential circuits is event-driven, pulse driven, and clock-driven. The output BJT switches between its cut-off and saturation regions to create logic 1 and 0, respectively. Ans. Sequential Logic Circuits (Circuits with memory): In this type of logic circuits outputs depend on the current inputs and previous inputs. 16 = 4,080 inputs " 24080 rows in truth table! ¢ Sequential Circuits: §Give sequence to events which allows ordering of operations §Need a special circuit to store the current state of circuit ¢ Controlling when to change state §ASYNCHROUNOUS As soon as circuit finishes, it moves to next state design of sequential circuits has now been reduced to the design of some combinational circuits. 3 Introduction • Logic circuits for digital systems may be – Combinational – Sequential • A combinational circuit consists of logic gates whose outputs at any time are determined by the current input values, i.e., it has no memory elements • A sequential circuit consists of logic gates whose outputs at any time are determined by the current input values as well as the past input The “state-holding circuit” we developed retains logic data using the hysteresis of a series circuit composed of a RHET and multiple resistors (Fig. In this memory device, if A and Q are initially at logic 0, then Q remains at logic 0. These are defined as digital circuit whose output is dependent not only on the present input value but also on the past history of its input. Onward Similar to combinational logic, we'll start with the building blocks of sequential logic: the flip-flop in its various forms. Sequential logic There are two types of sequential circuits. In principle, can build TOY computer with a combinational circuit. Combinational Logic A combinational system (device) is a digital system in which the value of the output at any instant depends only on the value of the input at that same instant (and not on previous values). circuits to add A, B (ALUs) •Sequential Logic (SL) –Circuits that “remember” or store information –a.k.a. Basic abstraction = controlled switch.! Yes, Sequential circuits deliver output based on the past state inputs and current input where past information is stored in a memory element. Carnegie Mellon 3 Introduction ¢ Outputs of sequential logic depend on current andprior input values –it has memory. This type of circuits uses previous input, output, clock and a memory element. P.S. Type of Circuits •Synchronous Digital Systems consist of two basic types of circuits: •Combinational Logic (CL) –Output is a function of the inputs only, not the history of its execution –e.g. Sequential logic is a type of logic circuit whose output is characterized by not only with the present input but also on the history of the input. Course material: https://github.com/DrWaleedAYousef/Teaching Example Sequential Circuits (cont’d) • Counters ∗ Easy to build using JK flip-flops » Use the JK = 11 to toggle ∗ Binary counters » Simple design – B bits can count from 0 to 2B−1 » Ripple counter – Increased delay as in ripple-carry adders – Delay proportional to the number of bits » Synchronous counters Sequential circuits, on the other hand, do have state. 19a).Because the RHT-1 collector is connected to the base, the RHET-1 operates as a negative differential-conductance diode. • Sequential logic differs from combinational logic in several ways: – Its outputs depend not only on logic inputs but also the internal state of the logic. Next, we designed basic components for sequential logic circuits. 3. Sequential circuit components: Circuit, State Diagram, State Table Sequential circuit components Flip-flop(s) Clock Logic gates Input Output Sequential circuits can be: ! Ðno simple pattern Ðeach circuit element used at most once High-Level Specification of … In a synchronous sequential circuit the sequential elements (the flip-flops) use a basic clock for the state transition. 1 Lecture #7: Flip-Flops, The Foundation of Sequential Logic Sequential Logic and Clocked Circuits • From combinational logic , we move on to sequential logic . What are the three main classifications of sequential logic circuits? Demonstrate by example how to analyze synchronous sequential Sequential Logic Circuits (Lecture #21) The slides included herein were taken from the materials accompanying Fundamentals of Logic Design, 6 th Edition, by Roth and Kinney, and were used with permission from Cengage Learning. 1 Elec 326 1 Sequential Circuit Analysis Sequential Circuit Analysis Objectives This section introduces synchronous sequential circuits with the following goals: Give a precise definition of synchronous sequential circuits. CS 150 - Fall 2005 – Lec #7: Sequential Implementation – 1 Sequential Logic Implementation Models for representing sequential circuits Abstraction of sequential elements Finite state machines and their state diagrams Inputs/outputs Mealy, Moore, and synchronous Mealy machines Finite state machine design procedure (Q 1 *Q 0 • Sequential Logic (Outputs depend on inputs and past events – has clock) – Implemented with LUTs and Flip-Flops – Will always have a clock edge – Can only be described using an Always Statement • Sensitivity list must only include clock edge and any async signals • … What is sequential circuit? Block diagram Flip Flop. Sequential Logic Circuits • Unlike combinational logic circuits, the output of sequential logic circuits not only depends on current inputs but also on the past sequence of inputs. SR or JK flip-flops are used zSince truth tables with don’t care entries frequently result in circuit simplifications, this favors JK flip-flops. In a synchronous circuit, an electronic oscillator called a clock (or clock generator) generates a sequence of repetitive pulses called the clock signal which is distributed to all the memory elements in the circuit.