Sequential logic circuits pdf

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Modeling combinational logic as a process– All signals referenced in process must be in the sensitivity list. entity And_Good is port (a, b: in std_logic; c: out std_logic); end And_Good; architecture Synthesis_Good of And_Good is. begin. process ( a,b) — gate sensitive to events on signals a and/or b. begin
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
C. E. Stroud Sequential Logic Analysis (1/06) 1 Sequential Logic Analysis • Used to determine: ¾How a sequential logic circuit works ¾Given an initial state and and input sequence: • What will be the output sequence • What will be the final state ¾Logic simulation cannot always do this • Unless initial state can be set • The
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
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
Pipelined Logic using C2MOS φ φ In φ φ F φ φ G NORA CMOS (NO-RAce logic) Race free as long as all the logic functions F and G between the latches are non-inverting C 1 C 2 C 3 out V DD V DD V DD
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 be a function of the input AT SOME
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.
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.
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.
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