Sequential logic circuits pdf

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Section 6.6 − Analysis of Sequential Logic Page 4 of 6 Example 6.3: Input-based or Mealy-type sequential circuit. The output values are dependent on the input values as well as its present state. Derive the next state, the output tables, and the state diagram for the (modulo-4 counter) sequential circuit represented by the following schematic. the design of asynchronous sequential circuits! Not practical for use in synchronous sequential circuits! Avoid to use latches as possible in synchronous sequential circuits to avoid design problems 5-8 SR Latch! A circuit with two cross-coupled NOR gates or two cross-coupled NAND gates! Two useful states:! S=1, R=0 " set state (Q will become design combinational logic circuits • Combinational logic circuits do not have an internal stored state, i.e., they have no memory. Consequently the output is solely a function of the current inputs. • Later, we will study circuits having a stored internal state, i.e., sequential logic circuits. Sequential Logic Basic Definition • Combinational circuits' output is a function of the circuit inputs and a delay time - Examples: NAND, NOR, XOR, adder, multiplier • Sequential circuits' output is a function of the circuit inputs, previous circuit state, and a delay time - Examples: Latches, flip-flops, FSMs, pipelined Sequential circuits, on the other hand, do have state. They typically have an input (or inputs) that can cause the state to change. Onward Similar to combinational logic, we'll start with the building blocks of sequential logic: the flip-flop in its various forms. Once we go over the basics we'll look at some ways we can use them in larger Logic Design ( Part 5) Sequential Logic Devices & Sequential Circuits 1 2 Combinational vs. Sequential •Combinational Circuit •always gives the same output for a given set of inputs §ex: adder always generates sum and carry, regardless of previous inputs •Sequential Circuit •stores information Sequential Circuits • The design of a clocked sequential circuit starts from a set of specifications and ends with a logic diagram (Analysis reversed!) • Building blocks: flip-flops, combinational logic • Need to choose type and number of flip-flops • Need to design combinational logic together with This is followed by the most basic building blocks of sequential-circuit design-latches, flip-flops, counters and shift registers. 5.1 Sequential Circuit documentation standards. • State-machine layout. Within a logic diagram, a collection of flip-flops and combinational logic that forms a state machine should be drawn in a logical format on the Sequential logic Flip-flops from latches Timing HDLs and simple sequential logic Verilog and flip-flops Blocking vs. non-blocking assignments Divide circuit into combinational logic and state Localize the feedback loops and make it easy to break cycles Implementation of storage elements leads to various forms A.Thus far, we have been discussing COMBINATORIAL logic circuits. These have the property that the output at any time is a boolean function of the inputs (with some propagation delay when inputs change). B.Complete computer systems also require circuits that have MEMORY - that is, circuits whose output can b