Page 166 - DCAP108_DIGITAL_CIRCUITS_AND_LOGIC_DESIGNS

Unit 10: Clocked Sequential Circuits

Nearly, all digital electronic devices (microprocessors, digital clocks, mobile phones, cordless Notes
telephones, electronic calculators, etc.) are designed as clocked sequential systems. Notable
exceptions include digital asynchronous logic systems.
In particular, nearly all computers are designed as clocked sequential systems. Notable exceptions
include analogue computers and clock less CPUs.
Typically each bit of the “state” is contained in its own flip-flop. Combinational logic decodes the
state into the output signals. More combinational logic encodes the current state and the inputs
into the next-state signals. The next-state signals are latched into the flip-flops under the control
of the global clock signal (a wire connected to every flip-flop).

10.1 Sequential Circuits

Digital electronics is classified into combinational logic and sequential logic. Combinational logic
output depends on the inputs levels, whereas sequential logic output depends on stored levels
and also the input levels.
Figure 10.1: Combinational Logic and Sequential Logic

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 a function of a
previous output states are.
The internal inputs and outputs are referred to as “secondary” in the course notes. Secondary
inputs are state variables produced by the storage elements, where as secondary outputs are
excitations for the storage elements.
The memory elements are devices capable of storing binary info. The binary info stored in the
memory elements at any given time defines the state of the sequential circuit. The input and the
present state of the memory element determine the output. Memory elements next state is also a
function of external inputs and present state. A sequential circuit is specified by a time sequence
of inputs, outputs, and internal states.
There are two types of sequential circuits. Their classification depends on the timing of their signals:

• Synchronous sequential circuits
• Asynchronous sequential circuits
Asynchronous Sequential Circuit
This is a system whose outputs depend upon the order in which its input variables change and
can be affected at any instant of time.
Gate-type asynchronous systems are basically combinational circuits with feedback paths. Because
of the feedback among logic gates, the system may, at times, become unstable. Consequently they
are not often used.

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