2. COMBINATIONAL CIRCUITS
Combinational Circuits are made of logic gates.
Doesn’t contain memory element , that’s why they cant
store any information.
Value of present output is determined by latest input.
Examples of combinational circuits are half adders, full
adders, sub tractors etc.
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4. SEQUENTIAL LOGIC CIRCUITS
Made up of combinational circuits and memory
elements.
These memory elements are devices capable of
storing ONE-BIT information.
Output depend on latest input and previous state.
Examples of sequential circuits are flip flops,
counters, shift registers
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6. TYPES OF SEQUENTIAL CIRCUITS
Sequential circuits are of two types:
1.SYNCHRONOUS CIRCUITS:
In synchronous sequential circuits, the state of the
device changes only at discrete times in response to
a clock Pulse.
2.ASYNCHRONOUS CIRCUITS:
Asynchronous circuit is not synchronized by a
clock signal; the outputs of the circuit change
directly in response to changes in Inputs.
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7. inputs X outputs Z
present state next state
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Synchronous Sequential Circuits
Combinational
Circuits
Flip-Flops
clock
Memory
Synchronous circuits employs a synchronizing signal called clock (a
periodic train of pulses; 0s and 1s)
A clock determines when computational activities occur
Other signals determines what changes will occur,i.e.determining the
output from the inputs whenever clock ticks.
8. SYNCHORONOUS CIRCUITS:-
inputs X outputs Z
present state next state
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Combinational
Circuits
Flip-Flops
clock
The storage elements (memory) used in clocked sequential circuits are
called flip-flops
Each flip-flop can store one bit of information 0,1
A circuit may use many flip-flops; together they define the circuit state
Flip-Flops (memory/state) update only with the clock
10. inputs X outputs Z
Example
D Q
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Problem with Latches
Problem: A latch is transparent; state keep
changing as long as the clock remains active
Due to this uncertainty, latches can not be
reliably used as storage elements.
Noise effects are more because the circuit is
enabled for complete ON/OFF time
C
Q
Q
Clock
Combinational
Circuits
Latches
clock
11. SEQUENTIAL CIRCUITS…
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bistable (2 stable states are stable).....
Bistable logic devices: latches (level) and flip-flops (edge).
Latches and flip-flops differ in the method used for changing
their state.
Latches are useful in asynchronous sequential circuits
Flip-Flips are built with latches
12. Difference between flip flop and latch
Flip flop and latch are two basic building blocks of
sequentional circuit but there is suitable difference
between the two is;
A flip flop continuously checks its inputs and
corresponding changes its output only at times
determined by clocking the signal.
Where as latch is a device which continuously checks
all its inputs and correspondingly changes its output,
independent of time determined by clocking signal.
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13. Difference between flip flop and latch
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A unique signal called “enable” is provided
with latch.
The output changes only when enable signal is
active.
No change in output take place when the
enable signal is inactive.
Flip flop are edge trigger, while latches are
level trigger.
14. FLIPFLOPS :--
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What is Flip flop?
Answer:
In digital circuits, the flip-flop, is a kind
of bistable multivibrator.
It is a Sequential Circuits / an electronic circuit which
has two stable states and thereby is capable of
serving as one bit of memory , bit 1 or bit 0.
15. CHARECTERSTICS OF FLIPFLOP…
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flip flop is built with a pair of latches.
The two latches are master and slave latches.
clk clk
16. CHARECTERSTICS ….(CONT’D)
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A master latch receives external inputs
A slave latch receives inputs from the master latch
Depending on the clock signal, only one latch is active at any
given time
If clk=1, the master latch is enabled and the inputs are latched
if clk=0, the master is disabled and the slave is activated to generate the
outputs
The above shown circuit acts as D Flipflop
17. CHARECTERSTICS….(CONTD)
level
positive (rising) edge negative (falling) edge
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Active only at transitions; i.e. either from 0 1 or 1 0
flip flop is one bit storage element.
flip flops are edge sensitive.
18. GRAPHIC SYMBOL OF FLIPFLOP
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A Flip Flop is designated by a rectangular block with inputs on
the left and outputs on the right (similar to latches)
The clock is designated with an arrowhead
A bubble designates a negative-edge triggered flip flops
19. CHARECTERSTICS …(CONT’D)
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ASYNCHRONOUS INPUTS:--
Some flip-flops have asynchronous inputs to set/reset
their states independently of the clock.
Preset or direct set, sets the flip-flop to 1
Clear or direct reset, set the flip-flop to 0
When power is turned on, a flip-flop state is unknown;
Direct inputs are useful to put in a known state
20. METASTABILITY:--
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Flip-flops are subject to a problem
called metastability
V1
V2
Stable point
Metastable point
Stable point
21. METASTABILITY:-
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metastability condition is occurred because by violating
setup and hold time violation.
Due to metastability state corruption of data will be
occur.
Setup time is the minimum amount of time the data signal should
be held steady before the clock event so that the data are
reliably sampled by the clock. This applies to synchronous input
signals to the flip-flop.
Hold time is the minimum amount of time the data signal should
be held steady after the clock event so that the data are reliably
sampled. This applies to synchronous input signals to the
flip-flop.