Fundamental Mode Model
- An asynchronous system uses feedback to produce memory elements as does the synchronous state machine.
- The asynchronous machine generally uses gates rather than flip-flops.
- The figure demonstrates a simple asynchronous circuit.
- X and Y are the system inputs while Z is the system output.
- The signal Z is fed back, however, to a gate input and in this way helps determine its own value.
- When an X or Y change dictates a change in Z, this change occurs. And only after the cumulative propagation delay time through the gates.
- It is characteristic of asynchronous circuits that the feedback variables along with system inputs determine the values of these same feedback variables.
- An idealized model has proposed to reflect this behavior. The above circuit took for the discussion.
- The gates are considered to have no delay in this model, while the delay element has an output that follows its input after a delay of Δt.
- The variable at the input of the delay element is called the excitation variable. While the feedback variable appears at the output of the delay element.
- In order to characterize the behavior of a circuit. We plot a map of excitation variable as a function of gate inputs.
- It is important to realize that the value Z takes on will also the value assumed by z after a delay of Δt.
- Thus, the information depicted on the map represents a dynamic situation.
- This can demonstrated by supposing the system inputs X = Y = 1 and z = 1, which leads to Z = 1. This called a stable state since z = Z.
- If X then changed to 0, the output Z changes to 0 as indicated by the map location corresponding to X = 0, Y = 1, and z = 1.
- This condition will persist for only Δt since z will assume a value of 0 at this time. Also, moving the system to the X = 0, Y = 1, and z = 0 location.
- The location 011 is a transient state, while 010 is a stable state.
- The stable states normally identified on the map by drawing a circle around the excitation variable such as in the figure.