Asynchronous Sequential Circuit Design

SSC versus ASC

Asynchronous Sequential Machine Modes

Prepared By

AJIT SARAF

SSC versus ASCSynchronous Sequential Circuits Asynchronous Sequential Circuits

Change of state occurs only in response to a synchronizing clock pulse.

State of the circuit can change immediately when an input change occurs.

All the flip-flops are clocked simultaneously by a common clock pulse.

It does not use clock.

Input changes are assumed to occur between clock pulses.The circuit must be in the stable state before next clock pulse arrives.

Input changes should occur only when the circuit is in a stable state.

The speed of operation depends on the maximum allowed clock frequency.

ASCs do not require clock pulses and they can change state with the input change. ASCs are faster than SSCs.

SSC versus ASCSynchronous Sequential Circuits Asynchronous Sequential Circuits

Memory elements are clocked flip-flops.

Memory elements are either unclocked flip-flops (latches) or gate circuits with feedback producing the effect of latch operation.

Any number of inputs can change simultaneously (during the absence of the clock).

Only one input is allowed to change at a time in the case of the level inputs and only one pulse input is allowed to be present in the case of the pulse inputs.

Asynchronous Sequential Machine Modes

Two modes of operation of asynchronous sequential

machines depending upon the type of input signals.

Fundamental Mode

Pulse Mode

Fundamental Mode

All of the input signals are considered to be levels.

Input signals will be changed only when the circuit is

in a stable state.

Only one variable can change at a given time.

Pulse Mode The inputs are pulses rather than levels.

The width of the input pulses is critical to the circuit

operation.

Input pulse must be long enough for the circuit to

respond to the input.

But it must not be so long as to be present even after

new state is reached.

Otherwise state of the circuit may make another

transition.