2009م - 1444هـ
09 – Asynchronous Sequential Logic



BY :M. Morris Mano



Chapter 9

Asynchronous Sequential Logic

9.1 INTRODUCTION

A sequential circuit is specified by a time sequence of inputs, outputs, and internal states, In

synchronous sequential circuits, the change of internal state occurs in response to the synchronized

clock puIses, Asynchronous sequential circuits do not use clock pulses. The change

of internal state occurs when them is a change in the input variables. The memory elements in

synchronous sequential circuits are clocked flip-flops, The memory elements in asynchronous

sequential circuits are either unclocked flip-flops or time-delay elements. The memory capability

of a time-delay device hpends on the finite amount of time it takes for the signal to

propagate through digital gates. An asynchronous sequential circuit quite often resembles a

combinational circuit with feedback.

The design of asynchronous sequential circuits is more difficult than that of synchronous circuits

because of the timing problems involved in the feedback path. In a properly designed

synchronous system, timing problems are eliminated by triggering all flip-flops with the pulse

edge. The change from one state to the next occurs during the short lime of the pulse transition.

Since the asynchronous circuit does not use a clock, the state of the system is allowed to

change immediately after the input changes. Care must be taken to ensure that each new state

keeps the circuit in a stable condition even though a feedback palh exists.

Asynchronous sequentid circuits are useful in a variety of applications. They are used when

speed of operation is important, especially in those cases where he digital system must respond

quickly without having to wait for a clock pulse. They are more economical to use in

small independent systems that require only a few components, as it may not be practical to

go to the expense of providing a circuit for generating clock pulses. Asynchronous circuits are

useful in applications where the input signals to the system may change at any time, independently

of an internal dock. The communication between two units, each having its own



An asynchronous circuit, or self-timed circuit, is a sequential digital logic circuit which is not governed by a clock circuit or global clock signal. Instead it often uses signals that indicate completion of instructions and operations, specified by simple data transfer protocols. This type of circuit is contrasted with synchronous circuits, in which changes to the signal values in the circuit are triggered by repetitive pulses called a clock signal. Most digital devices today use synchronous circuits. However asynchronous circuits have the potential to be faster, and may also have advantages in lower power consumption, lower electromagnetic interference, and better modularity in large systems. Asynchronous circuits are an active area of research in digital logic design.