Chapter 3. - Sequential Circuits
The figure above shows a theoretical view of how sequential circuits
are made up from combinational logic and some storage elements.
Two Main Types of Sequential Circuits
There are two types of sequential circuit, synchronous
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.
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.