What is the difference between synchronous and asynchronous sequential logic circuits

Synchronous sequential logic circuitry.

Differences with asynchronous sequential logic circuits:

1. The clock signal is different.

There is a common clock signal in the synchronous sequential logic circuit, and each memory element in the circuit is controlled by it unifiedly, and only when the clock signal arrives, the state of the memory element can change, so that the output of the sequential circuit changes, and every time a clock signal comes, the state of the memory element and the circuit output state can be changed once.

Since asynchronous circuits do not have a uniform clock, the moment of state change is unstable, and usually the input signal changes only when the circuit is in a steady state.

2. Triggers.

whether the state has changed.

Almost all of the timing logic in a synchronous timing circuit is "synchronous logic", with a "clock" signal and all internal memory ('Internal state') will only change at the edge of the clock.

When the clock valid signal of a flip-flop arrives, the flip-flop state changes according to the equation of state, and when the non-clock active signal arrives, the flip-flop state will remain unchanged.

The synchronization logic is triggered by a single clock, and the state of the circuit does not change until the edge of the clock.

Asynchronous logic is not triggered by a clock, and changes in the input signal cause a change in the state of the circuit.

The general steps for designing a synchronization counter are:

1. Analyze the design requirements, determine the number of triggers and special types;

2. Select the state code genus;

3. Finding the equation of state and driving the equation;

4. Draw logic diagrams according to the driving equations;

5. Check whether it can start automatically.

Design a three-variable ABC voting machine, where A has veto power and BC does not have veto power.

Four Steps:

1. Observe the circuit structure: synchronous or asynchronous, Moore or Milley 2. Write logic equations: output equations, excitation equations, equations of state, clock equations 3, column state microphones, draw state diagrams or sequence diagrams.

4. Explain the function.

Analyze the logic function requirements and draw a symbol state transition diagram.

Make a state simplification.

Determine the number of triggers, assign states, draw a state transition diagram, select the type of triggers, find the equations of the driving signal and circuit output of each trigger, and check whether the circuit can start; If not, it will be modified.

Draw a logic diagram and implement the circuit.

First, the core logic is different.

1. The core logic of asynchronous circuit circuits is combination circuits, such as asynchronous FIFO RAM read and write signals, address decoding signals and other circuits.

2. The core logic of the circuit is realized by a variety of flip-flops, so it is relatively easy to use the asynchronous reset set end of the register to make the whole circuit have a definite initial state.

Second, the output of the circuit is different.

1. The output of the asynchronous circuit does not depend on a certain clock, that is, it is not generated by the clock signal driving the trigger.

2. The synchronization of the whole circuit is driven by the clock edge.

Third, the characteristics are different.

1. Asynchronous circuits are very prone to burrs and are susceptible to environmental influences, which is not conducive to the transplantation of devices.

2. Synchronous circuit: The synchronous timing circuit with the flip-flop as the main body can well avoid the influence of burrs and make the design more reliable; The synchronous timing vertical shed circuit is conducive to device transplantation, because the influence of the environment and device process on the synchronous circuit can be almost ignored. The synchronization circuit can easily organize the assembly line and increase the operating rate of the chip.

The general steps for designing a synchronization counter are:

1. Analyze the design requirements and determine the number and type of triggers;

2. Select the status code;

3. Finding the equation of state and driving the equation;

4. Draw logic diagrams according to the driving equations;

5. Check whether it can start automatically.