数字电路教学课件：CHAPTER5

1、Sequential CircuitChapter 6OverviewlThe output, O, is a function of only the present state, St.lE = f( I, St )lSt+1 = f( St, E )lO = g( St )Combinational transform (f)Memory MCombinational logic (g)Input (I)(E)CLKState variables(S)Excitation variablesOutput (O)lThe output, O, is a function of both t

2、he present state, St, and the input variables, I.lE = f( I, St )lSt+1 = f( St, E )lO = g( I, St )Combinational transform (f)Memory MCombinational logic (g)Input (I)(E)CLKState variables(S)Excitation variablesOutput (O)Universal modelSystem output VariablesO0OmErExcitation VariablesE0System Input Var

3、iablesI0InS0SxState VariablesMemoryFlip-flopsCLOCKVariablelInput variablelOutput variablelState variablelExcitation variable lExcitation variable are the input to memory (flip-flops)VariablelStatelState is defined by the content of memory. lWhen memory is realized with flip-flops, the machine state

4、is defined by the Q output.lEach state of a sequential machine must be unique and unambiguous.lState variables and states are related byl2x=ylx= number of State variables ly= number of StatesPresent state, next statelPresent statelThe status of all state variables, at some time, t , before the next

5、clock edge, represent a condition called present statelNext statelThe status of all state variables, at some time, t+1 , represent a condition called next statelA state diagram is a graphic rendition of a sequential circuit, where individual states are represented by a circle with an identifying sym

6、bol located inside.lChange from state to state are indicated by directed arcs. Input conditions that cause state changes to occur and the resulting output signals are written adjacent to the directed arc.lState diagram graphically represent the relationships between states. They show the input-outpu

7、t variable combinations as the transitions from one state to the next.DACB0/01/00/01/10/00/01/01/0Input conditions that cause state changes to occurthe resulting outputlIn Mealy machine, the output is a function of both the present state and the input variables.n An input-output statement is written

8、 adjacent to each directed arc. D2Y2Y2D1Y1Y1XZCP10/100/011/001/00 1 100011lMoore machine notation, where the output is depend only on the present state, represent the output variable inside the state circle.lA state diagram is somewhat analogous to the programmers flowchart. The states can be viewed

9、 as tasks or operations and the directed arcs, indicating input conditions, as conditional statements.lThe addition of one input variable double the number of possible next state transitions. lWith large numbers of input variables, it is convenient to indicate only the asserted conditions that cause

10、 state or output variable changes. The unspecified input variable combinational are assumed to leave the state machine in its present state.lState table are tabular forms of diagrams. The purpose of the state table is to indicate the state transitions.lThe present state column list all of the possib

11、le state in the machine. A series of next state columns exist, one from each combination. takes the state table one step further. lThe state diagram represent states using symbol or names, whereas the transition table assign specific state variable values to each state. 01 01D2Y2Y2D1Y1Y1XZCP01Univer

12、sal modelSystem output VariablesO0OmErExcitation VariablesE0System Input VariablesI0InS0SxState VariablesMemoryFlip-flopsCLOCKSynchronous sequential circuitBasic conceptlSynchronous sequential circuit memory, usually consisting of flip-flops, update circuit states information. lAll flip-flops share

13、a common clock pulse input.lThe clock input is not a binary value representing the time of day, but rather a ”synchronous” train of pulses. Synchronous memory changes its data only at certain time intervals .lThe flip-flops can change state only on a clock pulse edge.lThe output of memory (flip-flop

14、s) defines the state of a sequential machine. It implies that the state variables are the flip-flop outputs. The next-state equation is the characteristics equation of the flip-flops used in circuit.lExcitation variables are input to memory (flip-flops). When flip-flops are used for the system memor

15、y, the excitation variables are the inputs (J-K, R-S, D, T) to the flip-flops. Excitation variables are generated by the input combinational logic operating on the state variables and input variables.Basic conceptlMealy model machinelE = f( I, St )lSt+1 = f( St, E ) flip-flop characteristics equationlO = g( I, St )Describe a SSCCombinational transform (f)Memory MCombinational logic (g)Input (I)(E)CLKState variables(S)Excitation variablesO