“To construct Gated SR-Latch using NOR Gate & To Verify its Different States”
“To construct Gated SR-Latch using NOR Gate & To
Verify its Different States”
1. Objective:
The main objective is to design and verify the different states of
i.
Gated SR
Latch using NOR Gate
2. Components:
i.
Bread
Board
ii.
Connecting
Wires
iii.
2 LED’s
iv.
Battery
v.
IC for
NOR Gate
3. Introduction:
3.1.Bread Board:
A
bread board is an electronic board into which the electrical components like
diodes and resistors are placed. The sockets of bread board are sturdy and
rugged. Diodes or other electrical components should be placed without making
any damage to the board.
3.2.Integrated Circuits:
A circuit of transistors, resistors, and capacitors constructed on a single semiconductor wafer or chip, in which the components areinterconnected to perform a given function. IC’s consume very little current, generate comparatively
little heat, and are far more shock-proof and reliable than the older
discrete-component circuits.
An integrated circuit consists of a number of circuit components (e.g. transistors, diodes, resistors
etc.) and them inter connections in a single small package to perform a
complete electronic function. These components are formed and connected within
a small chip of semiconductor material. IC’s can be used as advanced logic
gates.
3.3.Light Emitting Diodes:
A light-emitting diode (LED) is a semiconductor device that emits visible
light when an electric current passes through it. LED’s
convert electrical energy to light energy. In electronics, polarity indicates whether a circuit component is symmetric or not.
Being diodes, will only allow current to flow in one direction. And when
there’s no current-flow, there’s no light. The positive side of the LED is
called the “anode” and is marked by having a longer
“lead,” or leg. The other, negative side of the LED is called the “cathode.”
3.4.SR-Latch:
A latch
is a fundamental component of data storage. A single latch can hold 1-bit of data,
increase that number by many orders of
magnitude and can create kilo, mega, giga, even terabytes of memory. Of course,
like most digital circuits, latches are made out of digital logic gates.
There are two controllable inputs: reset (R) and set (S), which produce the two
outputs: Q and Q(“Q-not”). That’s where the SR latch
get’s its name it’s a set/reset latch.
The SR latch comes with a
rule, which cannot ever be broken: Q must always be the
opposite of Q. These outputs are called complements. Here we made SR-Latch using following way:
3.4.1. SR-Latch
using NOR gate:
To create an S-R latch, we can wire two NOR gates in
such a way that the output of one feeds back to the input of another, and vice
versa,
The Q and Q not outputs are supposed to be in opposite
states. Here making both the S and R inputs equal to 1 results in both Q and Q
not being 0. For this reason, having both S and R equal to 1 is called an invalid or illegal state for the S-R Latch using NOR Gate.
Otherwise, making S=1 and R=0 “sets” the Latch so that Q=1 and Q not =0.
Conversely, making R=1 and S=0 “resets” the Latch in the opposite state. When S
and R are both equal to 0, the Latch’s outputs 7in their prior states. This is
the No Change State. By definition, a condition of Q=1 and Q not=0 is set.
A condition of Q=0 and Q not=1 is reset. These terms are universal in
describing the output states of any Latch circuit.
4. Gated SR Latch:
It is sometimes useful in logic circuits to have a
multivibrator which changes state only when certain conditions are met,
regardless of its S and R input states. The conditional input is called the enable,
and is symbolized by the letter E.
To accomplish this we need to have an extra input pin
called clock for enabling or disabling the latch. When disabled, the previous
states remain unchanged. Circuit that uses this type of mechanism are called
gated latches. In this case the circuit is called Gated SR Latch. This latch is
active high, that is when clock signal goes high the Gated SR Latch behaves as
a normal SR Latch. And when the clock signal goes low, whatever the logic at
inputs maybe, the output will remain the same unless and until the clock signal
goes high again.
5. Circuit Diagram:
6. Experimentation:
1. First of all, suitable IC for NOR gate is
chosen, i.e. IC 7402.
2. Inserted IC on the bread board.
3. As there are 14 pins on IC, 7th
pin is grounded and a positive potential is applied to the 14th pin
via a battery of potential less than 5V.
4. First Input S of first NOR is given the input
with battery and its second input is connected with the first input of second
NOR gate. Which is then connected to the negative terminal of the battery so
that Enable is on 0.
5. Second Input R of second NOR is connected
with the battery.
6. Output of first NOR gate is given to the
first input R’ of third NOR gate while output of second Nor is given to the
second input S’ of fourth NOR gate.
7. Second input of third NOR gate is connected
with the output Q’ of fourth NOR & the first input of fourth NOR Is
connected to the output of third NOR.
8. First LED is inserted on bread board with its
positive terminal connected to the output
of third NOR gate and negative signal is
given to the other terminal. Similarly Second LED is inserted on bread board
with its positive terminal connected to the output of fourth NOR gate and
negative signal is given to the other terminal.
9. Then one by one checked all the five possible
combination of S and R so as to verify the different states of Gated SR-Latch. Table
is verified and drawn.
7. Observations & Calculations:
The different states for SR-Latch using NOR is given below:
S
|
R
|
S'
|
R'
|
Q
|
Q'
|
State
|
0
|
1
|
0
|
1
|
0
|
1
|
Set
|
1
|
1
|
0
|
0
|
0
|
1
|
No
Change
|
1
|
0
|
1
|
0
|
1
|
0
|
Reset
|
1
|
1
|
0
|
0
|
1
|
0
|
No
Change
|
0
|
0
|
1
|
1
|
0
|
0
|
Invalid
|
8. Results & Discussion:
We have designed Gated SR-Latch using NOR Gates and the different states
are verified. We can deduce some main points from this. In the case of NOR gate
SR-Latch invalid state occur when both S & R are 0. In NOR gate
configuration 1,1 correspond to No Change State.
9. Conclusion:
Through this experiment we have designed Gated SR-Latch using NOR Gate
and its states are verified.
10. References:
I.
http://logic.ly/lessons/nor-gate/
VI.
http://elprojects.blogspot.com/2011/01/gated-sr-latch-using-nor-gates.html
Comments
Post a Comment