What Is a Synchronous Circuit?

Sequential circuit is a circuit commonly used in computers and other electronic systems. It is two different types of circuits from the combination circuit. Sequential circuits are divided into two categories: synchronous sequential logic circuits and asynchronous sequential logic circuits. There is a common clock signal in the synchronous sequential logic circuit. Each memory element in the circuit is controlled by it. Only when the clock signal arrives, the state of the memory element can change, so that the output of the sequential circuit changes. With a clock signal, the state of the memory element and the circuit output state can only be changed once.

The most common in sequential logic circuits are registers, shift registers, counters, sequential pulse generators, and sequence signal generators. The design method of sequential logic circuits is based on the method of combinatorial logic circuits. When designing a sequential logic circuit, the designer is required to find a logic circuit that implements this logic function according to the specific logic problem given. The resulting design results are kept simple.

When small-scale integrated circuits are used for design, the simplest standard of the circuit is that the number of flip-flops and gates used is the smallest, and the number of input terminals of the flip-flops and gates is also the smallest. The simplest standard of the circuit is that the number of integrated circuits used is the least, the types are the least, and the connections between them are also the least.

When designing a synchronous sequential logic circuit, generally proceed as follows:

1. Logic abstraction, draw the state transition diagram or state transition table of the circuit;

A. Define the logic, that is, determine the input variables, output variables, and the number of states of the circuit. Usually, the cause of the event is used as the input variable, and the result of the event is used as the output variable.

B. Define the meaning of the input and output logic states and the state of each circuit, and number the circuit states sequentially;

C. List the state transition table of the circuit or draw the state transition diagram of the circuit according to the intent;

2. State reduction

If two circuit states have the same output under the same input and transition to the same secondary state, then these two states are called equivalent states. Obviously the equivalent state can be merged into one. The fewer states the circuit has, the simpler the circuit can be designed.

3 State allocation

First, determine the number n of flip-flops. Because 13 flip-flops have 2 "state combinations, in order to obtain the M states required by the sequential circuit, you must use

. Secondly, the corresponding flip-flop state combination must be specified for each circuit state. Each combination of triggers is a set of binary codes. If the designed circuit has three states, we can choose two flip-flops as the core of the circuit, because the output states of the two flip-flops are four two-digit binary codes: 00, 01, 10, 11 and the circuit only The three states can be replaced by any three of them. If the coding scheme is selected properly, the design result can be very simple; otherwise, if the coding scheme is not selected properly, the designed circuit will be much more complicated.

4 Selected trigger type

Find the state equation, driving equation and output equation of the circuit

And the initial state of the circuit

The input variables together satisfy a certain logical function relationship. According to the state transition diagram and the selected state code, the type of flip-flop, it is necessary to write out the state equation, driving equation and output equation of the circuit.

All the storage elements in the synchronous sequential circuit are under the unified control of the clock pulse CP, and the flip-flop is used as the storage element. Almost all sequential logic is "synchronous logic": there is a "clock" signal, and all internal memory ('internal state') will only change at the edges of the clock. The most basic storage element in sequential logic is

• A flip-flop is a basic unit circuit with memory and constituting sequential logic. A flip-flop can "store" a single binary number: "0" or "1" ^[1] .

  1. A trigger has two stable states:

  1. "0" status: Q = 0, = 1;

  2. "1" status: Q = 1, = 0.

  2. The trigger (FF) should have the following functions:

  1. Before the new data is input (no trigger signal), the trigger remains in the original state (original data).

  2. When triggered by an input signal, it can transition from one state to another. That is: FF can "receive", "hold" and "output" digital information.

  3. Classification of triggers (FF):

  1. Rising edge trigger

  2. Falling edge trigger

  3. High potential trigger

  4. Low potential trigger

  5. Potential trigger:

  6. Edge trigger:

  7. Basic RS trigger

  8. Synchronous Trigger (Clock CP-FF)

  9. Master-slave trigger

  10. Maintenance-blocking trigger (WZ-FF)

  11. CMOS edge trigger

  12. RS trigger (set to 0, set to 1)

  13. JK trigger (multi-function trigger)

  14. D trigger (delayed trigger)

  15. T trigger (flip-flop)

  16. From function points:

  17. From the structure points:

  18. From trigger mode points: