UNIT II SYNCHRONOUS SEQUENTIAL LOGIC
Introduction to Sequential Circuits – Flip–Flops – operation and excitation tables,
Triggering of FF, Analysis and design of clocked sequential circuits – Design –
Moore/Mealy models, state minimization, state assignment, circuit
implementation – Registers – Counters
Sequential Circuit
Sequential circuits are digital circuits that store and use previous state information to
determine their next state. They are commonly used in digital systems to implement state
machines, timers, counters, and memory elements and are essential components in digital
systems design. Sequential circuits are commonly used in digital systems to implement state
machines, timers, counters, and memory elements. The memory elements in sequential
circuits can be implemented using flip-flops, which are circuits that store binary values and
maintain their state even when the inputs change.
Sequential circuit is a combinational logic circuit that consists of inputs variable (X), logic
gates (Computational circuit), and output variable (Z).
A combinational circuit produces an output based on input variables only, but a sequential
circuit produces an output based on current input and previous output variables. That
means sequential circuits include memory elements that are capable of storing binary
information. That binary information defines the state of the sequential circuit at that time.
A latch capable of storing one bit of information.
,As shown in the figure, there are two types of input to the combinational logic:
1. External inputs which are not controlled by the circuit.
2. Internal inputs, which are a function of a previous output state.
Secondary inputs are state variables produced by the storage elements, whereas secondary
outputs are excitations for the storage elements.
Types of Sequential Circuits
There are two types of sequential circuits
Asynchronous Sequential Circuit
These circuits do not use a clock signal but uses the pulses of the inputs. These circuits
are faster than synchronous sequential circuits because there is clock pulse and change their
state immediately when there is a change in the input signal. We use asynchronous sequential
circuits when speed of operation is important and independent of internal clock pulse.
But these circuits are more difficult to design and their output is uncertain.
Synchronous Sequential Circuit
These circuits use clock signal and level inputs (or pulsed) (with restrictions on pulse width
and circuit propagation). The output pulse is the same duration as the clock pulse for the
clocked sequential circuits. Since they wait for the next clock pulse to arrive to perform the
,next operation, so these circuits are bit slower compared to asynchronous. Level output
changes state at the start of an input pulse and remains in that until the next input or clock
pulse.
We use synchronous sequential circuit in synchronous counters, flip flops, and in the design
of MOORE-MEALY state management machines. We use sequential circuits to design
Counters, Registers, RAM, MOORE/MEALY Machine and other state retaining machines.
Clock Signal and Triggering
Clock signal is a kind of control signal that allows the elements of synchronous circuits to be
in phase or phenomena that occur in circuits. It is derived from the square wave that has a
high and a low level, it helps in measuring the sequential changes in the circuit states. The
clock signal also makes a pulse simultaneously on all the circuit parts that are needed for the
proper work of synchronous sequential circuits.
Types of Triggering
In Sequential circuits, triggering denotes the way, in terms of which the state changes take
place. There are two main types of triggering
Level Triggering
Level triggering happens when the change of state is from the level of the clock signal is high
or low. The circuit depends on the level of the clock signal rather than the rising or the falling
edge of it. There are two types of level triggering:
1. Positive Level Triggering: The circuit changes state when it is high time in the
clock cycle i.e., when the clock signal is high.
2. Negative Level Triggering: The circuit changes state when the clock signal is in
the low state.
, Edge Triggering
Edge triggering occurs when the state change is initiated by the transition (rising or falling
edge) of the clock signal. The circuit responds to the clock signal’s edges rather than its levels.
There are two types of edge triggering:
1. Positive Edge Triggering: The circuit changes state on the rising edge (transition
from low to high) of the clock signal.
2. Negative Edge Triggering: The circuit changes state on the falling edge
(transition from high to low) of the clock signal.
Edge Triggering
Advantages of Sequential Circuits
1. Memory: Sequential circuits have the ability to store binary values, which makes
them ideal for applications that require memory elements, such as timers and
counters.
2. Timing: Sequential circuits are commonly used to implement timing and
synchronization in digital systems, making them essential for real-time control
applications.
Introduction to Sequential Circuits – Flip–Flops – operation and excitation tables,
Triggering of FF, Analysis and design of clocked sequential circuits – Design –
Moore/Mealy models, state minimization, state assignment, circuit
implementation – Registers – Counters
Sequential Circuit
Sequential circuits are digital circuits that store and use previous state information to
determine their next state. They are commonly used in digital systems to implement state
machines, timers, counters, and memory elements and are essential components in digital
systems design. Sequential circuits are commonly used in digital systems to implement state
machines, timers, counters, and memory elements. The memory elements in sequential
circuits can be implemented using flip-flops, which are circuits that store binary values and
maintain their state even when the inputs change.
Sequential circuit is a combinational logic circuit that consists of inputs variable (X), logic
gates (Computational circuit), and output variable (Z).
A combinational circuit produces an output based on input variables only, but a sequential
circuit produces an output based on current input and previous output variables. That
means sequential circuits include memory elements that are capable of storing binary
information. That binary information defines the state of the sequential circuit at that time.
A latch capable of storing one bit of information.
,As shown in the figure, there are two types of input to the combinational logic:
1. External inputs which are not controlled by the circuit.
2. Internal inputs, which are a function of a previous output state.
Secondary inputs are state variables produced by the storage elements, whereas secondary
outputs are excitations for the storage elements.
Types of Sequential Circuits
There are two types of sequential circuits
Asynchronous Sequential Circuit
These circuits do not use a clock signal but uses the pulses of the inputs. These circuits
are faster than synchronous sequential circuits because there is clock pulse and change their
state immediately when there is a change in the input signal. We use asynchronous sequential
circuits when speed of operation is important and independent of internal clock pulse.
But these circuits are more difficult to design and their output is uncertain.
Synchronous Sequential Circuit
These circuits use clock signal and level inputs (or pulsed) (with restrictions on pulse width
and circuit propagation). The output pulse is the same duration as the clock pulse for the
clocked sequential circuits. Since they wait for the next clock pulse to arrive to perform the
,next operation, so these circuits are bit slower compared to asynchronous. Level output
changes state at the start of an input pulse and remains in that until the next input or clock
pulse.
We use synchronous sequential circuit in synchronous counters, flip flops, and in the design
of MOORE-MEALY state management machines. We use sequential circuits to design
Counters, Registers, RAM, MOORE/MEALY Machine and other state retaining machines.
Clock Signal and Triggering
Clock signal is a kind of control signal that allows the elements of synchronous circuits to be
in phase or phenomena that occur in circuits. It is derived from the square wave that has a
high and a low level, it helps in measuring the sequential changes in the circuit states. The
clock signal also makes a pulse simultaneously on all the circuit parts that are needed for the
proper work of synchronous sequential circuits.
Types of Triggering
In Sequential circuits, triggering denotes the way, in terms of which the state changes take
place. There are two main types of triggering
Level Triggering
Level triggering happens when the change of state is from the level of the clock signal is high
or low. The circuit depends on the level of the clock signal rather than the rising or the falling
edge of it. There are two types of level triggering:
1. Positive Level Triggering: The circuit changes state when it is high time in the
clock cycle i.e., when the clock signal is high.
2. Negative Level Triggering: The circuit changes state when the clock signal is in
the low state.
, Edge Triggering
Edge triggering occurs when the state change is initiated by the transition (rising or falling
edge) of the clock signal. The circuit responds to the clock signal’s edges rather than its levels.
There are two types of edge triggering:
1. Positive Edge Triggering: The circuit changes state on the rising edge (transition
from low to high) of the clock signal.
2. Negative Edge Triggering: The circuit changes state on the falling edge
(transition from high to low) of the clock signal.
Edge Triggering
Advantages of Sequential Circuits
1. Memory: Sequential circuits have the ability to store binary values, which makes
them ideal for applications that require memory elements, such as timers and
counters.
2. Timing: Sequential circuits are commonly used to implement timing and
synchronization in digital systems, making them essential for real-time control
applications.
