Chapter 3. - Sequential Circuits

State Diagrams


Combinational circuits and systems produce an output based on input variables only. Sequential circuits use current input variables and previous input variables by storeing the information and putting back into the circuit on the next clock (activation) cycle.

The figure above shows a theoretical view of how sequential circuits are made up from combinational logic and some storage elements.
There are two types of input to the combinational logic; External inputs which come from outside the circuit design and are not controlled by the circuit; Internal inputs which are a function of a previous output states.
The internal inputs and outputs are refered to as "secondaries" in the course notes. Secondary inputs are state variables produced by the storage elements, where as secondary outputs are excitations for the storage elements.

Two Main Types of Sequential Circuits

There are two types of sequential circuit, synchronous and asynchronous.
Synchronous types use pulsed or level inputs and a clock input to drive the circuit (with restrictions on pulse width and circuit propagation).

Asynchronous sequential circuits do not use a clock signal as synchronous circuits do. Instead the circuit is driven by the pulses of the inputs. You will not need to know anymore about asynchronous circuits for this course.

A pulsed output (as used in the block diagrams above) is an output that lasts for the duration of a particular input pulse but can be less in some cases. For the clocked sequential circuits, the output pulse is the same duration as the clock pulse.
A level output refers to an output that changes state at the start of an input pulse or clock pulse and remains in that state until the next input or clock pulse.

Important Sequential Logic Design Issue

An important thing to note with sequential circuits is that duration of the activating pulse should be low enough so that the secondary inputs do not change state in same activating pulse. Allowing the clock pulse to be too long would result in incorrect circuit function, as there will be two different secondary input values for one clock cycle and therefore lead to potentially two state changes in one clock.
It would be benefical to have our storage elements to be edge triggered, as this would mean that the clock pulse can be as long as we would like it to be and the circuit would behave in the same way. This is why flip-flops are used as they are edge triggered storage devices.

State Diagrams