Types of Asynchronous Circuits

Types of Asynchronous Circuits

Dec.-03, 06, 12, 15

• Fig. 7.2.1 shows the block diagram of an asynchronous sequential circuit. It consists of a combinational circuit and delay elements connected to form feedback loops. As shown in the Fig. 7.2.1, there are n input variables, m output variables, and k internal states. The delay elements provide a short term memory for the sequential circuit. The present state and next state variables in asynchronous sequential circuits are called secondary variables and excitation variables, respectively.

• When an input variable changes in value, the secondary variables, i.e. y₁ y₂, … y_r do not change instantaneously. Certain amount of time is required for the input signal to propagate from the input terminals through the combinational circuit and the delay elements. The combinational circuit generates Y excitation variables which gives the next state of the circuit. The excitation variables are propagated through delay elements to become the new present state for the secondary variables, i.e. y₁ y₂, … y_r In the steady state condition excitation and secondary variables are same, but during transition they are different. In other words we can say that, for a given value of input variables, the system is stable if the circuit reaches a steady state condition with y_i = Y_i. for i = 1,2,-”,r; otherwise the circuit is in a continuous transition and is said to be unstable.

• To ensure proper operation, it is necessary for asynchronous sequential circuit to attain a stable state before the input is changed to a new value. Because of unequal delays in the wires and gate circuits, it is impossible to have two or more input variables change at exactly same instant. Therefore, simultaneous changes of two or more input variables are usually avoided. In other words, we can say that only one input variable is allowed to change at any one time and the time between two  input changes is kept longer than the time it takes the circuit to reach a stable state.

• According to how input variables are to be considered, there are two types of asynchronous circuits :

* Fundamental mode circuits and

* Pulse mode circuits.

Fundamental mode circuit

It assumes that :

• Input changes should be spaced in time by at least At, the time needed for the circuit to settle into a stable state following an input change. That is, the input variables should change only when the circuit is stable.

• Only one input variable can change at a given instant of time and

• Inputs are levels and not pulses.

• Delay lines are used as memory elements.

Pulse mode circuit

It assumes that :

• The input variables are pulses instead of levels.

• The width of the pulses is long enough for the circuit to respond to the input.

• The pulse width must not be so long that it is still present after the new state is reached.

• Pulses should not occur simultaneously on two or more input lines.

• Flip-flops are commonly used as a memory elements.

• Memory element transitions are initiated only by input pulses.

• Input variables are used only in the uncomplemented or the complemented forms, but not both.

Review Questions

1. What is a fundamental mode asynchronous sequential circuit ?

2. Illustrate pulse mode asynchronous circuit.

AU : Dec.-06, Marks 8

3. Sketch the block diagram of an asynchronous sequential circuit.

4. Compare pulsed mode and fundamental mode asynchronous circuit.