Latches and Flip-Flops 1 - The SR Latch
Summary
TLDRThis video script delves into the world of computer memory, focusing on the SR latch, a fundamental building block that acts as a one-bit memory unit. It explores the latch's operation using NOR and NAND gates, detailing how it can store a state triggered by input pulses and maintain it until changed. The script also discusses the latch's truth tables, the concept of 'active high' and 'active low' configurations, and its practical applications, such as debouncing mechanical switches to prevent signal interference in digital circuits.
Takeaways
- 🌟 Latches and flip-flops are fundamental to computer memory, with the SR latch being a basic building block.
- 🔄 The SR latch functions as a one-bit memory, capable of holding one of two stable states until changed by an input pulse or power removal.
- 🔩 The SR latch is constructed using fundamental logic gates, such as the OR, AND, NOR, and NAND gates, which have specific behaviors defined by their truth tables.
- 🔄 The SR latch has two inputs, S (Set) and R (Reset), and can be built using either NOR or NAND gates, resulting in different control mechanisms.
- ↔️ In a NOR-based SR latch, the output is influenced by high logic levels, making it an active-high SR latch, whereas a NAND-based latch is controlled by low logic levels, making it active-low.
- 🔄 The SR latch can be in one of two states: Set (storing a '1') or Reset (storing a '0'), and it transitions between these states based on the input pulses applied to S and R.
- 🚫 Both S and R should not be high at the same time in an SR latch, as this would create an illegal state where the latch's next state cannot be determined.
- 🔄 The SR latch can be used to debounce mechanical switch signals, ensuring that only one signal is recognized despite multiple electrical signals generated during switch press.
- 🛠️ SR latches are used in control applications to monitor and react to changing conditions, such as in a burglar alarm system where the state of a door or window can trigger an alarm.
- 🔌 The SR latch is also the foundation for more complex memory circuits, highlighting its importance in the design of digital systems.
- 🔄 Understanding the behavior of SR latches, including their truth tables and forbidden states, is crucial for designing reliable digital circuits.
Q & A
What are latches and flip-flops in the context of computer memory?
-Latches and flip-flops are fundamental building blocks of computer memory. They are used to store and manipulate binary data, with latches being simpler and flip-flops being more complex and versatile.
What is an SR latch and what does it represent?
-An SR latch, also known as a Set-Reset latch, is a type of latch that can be thought of as a one-bit memory. It has two stable output states and is triggered by input pulses to set or reset its state.
How does the SR latch remember its state?
-The SR latch remembers its state through positive feedback within the circuit. It maintains the state until it is changed again by another input pulse or until power is removed.
What are the basic logic gates used in constructing an SR latch?
-The basic logic gates used in constructing an SR latch are NOR gates and NAND gates. These gates are combined in a specific way to create the latch's functionality.
What is the purpose of the NOR gate in an SR latch?
-In an SR latch constructed with NOR gates, the NOR gate is used to create a form of positive feedback through cross-coupling, which allows the latch to maintain its state until a new input pulse is received.
How does the output of an AND gate become the input for a NOT gate?
-By inverting the output of a regular AND gate with a NOT gate, the zeros in the output column of the truth table are swapped for ones, and the ones are swapped for zeros.
What is the significance of the 'Q' and 'not Q' outputs in an SR latch?
-The 'Q' output represents the current state of the latch, while 'not Q' is the inverse of 'Q'. These outputs are crucial for the latch to function as a memory element, storing and providing the state information.
Why is it important that S and R inputs of an SR latch are never left high?
-It is important that S and R inputs are never left high because the latch is controlled by pulses. Continuously setting either input high could lead to an invalid state, causing the latch to malfunction.
What is a race condition in the context of an SR latch?
-A race condition in an SR latch occurs when both inputs S and R are set high simultaneously, causing both outputs to be indeterminate. This is an illegal state for the latch and should be avoided.
How does an SR latch help in debouncing a mechanical switch?
-An SR latch helps in debouncing a mechanical switch by providing a stable single signal output even when the switch generates multiple electrical signals due to bounce. The latch ignores further set signals after it has been set, ensuring a clean signal for the controlling circuit.
What are the two types of SR latches based on the logic used for setting and resetting?
-There are two types of SR latches: the active-high SR latch, which is built from NOR gates and uses high logic levels for setting and resetting, and the active-low SR latch, which is built from NAND gates and uses low logic levels for the same purpose.
Outlines
📦 Building Blocks of Memory: SR Latch Basics
This paragraph introduces the concept of latches and flip-flops as fundamental components of computer memory. It specifically focuses on the Set-Reset (SR) latch, which acts as a one-bit memory capable of holding a stable output state triggered by input pulses. The SR latch is explained as a 'bistable' latch, maintaining its state until changed by another input or power removal. The paragraph also reviews basic logic gates like OR, AND, NOR, and NAND, which are essential for understanding the construction of SR latches. The NOR gate-based SR latch is described in detail, including its inputs (R and S), outputs (Q and not Q), and the positive feedback mechanism that enables its latching behavior. The explanation includes how the latch operates with different input pulses to set or reset its state.
🔄 SR Latch Operation and Truth Tables
This paragraph delves deeper into the operation of the SR latch, emphasizing the importance of input pulses to control the latch's state. It clarifies that inputs S and R should not be left high continuously, as the latch is designed to be triggered by pulses. The paragraph presents the truth table for the SR latch, illustrating the possible states of the latch depending on the inputs. It also discusses the forbidden state when both inputs are high simultaneously, which leads to an indeterminate state. Additionally, the paragraph compares the active-high SR latch built from NOR gates with the active-low SR latch built from NAND gates, highlighting the differences in their behavior and control logic.
🛠 Applications and Debouncing with SR Latches
The final paragraph discusses practical applications of the SR latch, particularly in scenarios where a single, clean signal is required from a mechanical switch that may generate multiple signals due to 'switch bounce.' It describes how an integrated circuit with an SR latch can be used to debounce such signals, ensuring that only one signal is processed even if the switch is pressed multiple times in quick succession. The paragraph also touches on other potential uses of SR latches, such as in control systems that need to monitor and react to changing conditions. Furthermore, it emphasizes the SR latch's role as a foundational element in the development of more complex memory circuits.
Mindmap
Keywords
💡Latches
💡Flip-Flops
💡SR Latch
💡Stable Output States
💡Input Pulse
💡Logic Gates
💡Truth Table
💡Positive Feedback
💡Debounce
💡Active High
💡Active Low
💡Race Condition
Highlights
Latches and flip-flops are fundamental building blocks of computer memory.
The SR latch acts as a one-bit memory with stable output states triggered by input pulses.
An SR latch remembers its state until changed by another input pulse or power removal.
Fundamental logic gates such as OR, AND, NOR, and NAND are essential for constructing SR latches.
The NOR gate version of an SR latch uses crosscoupling to create positive feedback.
SR latches require power to function, though power connections are not shown in diagrams.
The SR latch has two inputs, R and S, and provides an output Q and its inverse.
Applying a pulse to input R resets the latch, changing the output state.
A set pulse at input S forces the latch into a set state, storing a one.
The latch's state is latched and remains until another valid input pulse is applied.
An SR latch has a unique truth table reflecting its behavior with S and R inputs.
Simultaneously setting both S and R inputs creates an invalid state for the SR latch.
The SR latch is controlled by pulses, not continuous high or low signals.
An SR latch built from NAND gates is known as an active low SR latch.
NAND gate-based SR latches have a different truth table and behavior compared to NOR gate versions.
An SR latch can be used to debounce mechanical switch signals in digital circuits.
The SR latch serves in control applications and as a building block for more complex memory circuits.
Transcripts
latches and flip-flops are the building
blocks of computer memory in this
particular video we'll focus on the
so-called Sr latch later we'll see how
this circuit can be
enhanced the set reset latch or Sr latch
for short can be thought of as a one bit
memory it can be put into one of two
stable output States triggered by an
input
pulse the circuit remembers the this
state until it's changed Again by
another input pulse or until the power's
removed for this reason the circuit is
known as a bable
latch before we consider the
construction of Sr latches let's remind
ourselves of some fundamental logic
gates this is an or gate and this is the
truth table that describes Its Behavior
any combination of inputs A and B
results in a one at output p except when
both inputs are zero in which case the
output is
zero this is an and gate and this is the
truth table that describes Its Behavior
any combination of inputs A and B
results in a zero at output P except
when both inputs are one in which case
the output is
one if we modify the output of a regular
or gate by inverting it with a not gate
then we can swap the zero for a one and
the ones for zeros in the output column
of the truth
table in a similar fashion we can invert
the output of a regular and gate then we
can swap the zeros for ones and the one
for a zero in the output column of this
tooth
table each of these gate combinations
has its own name and its own symbol they
are are known as the Norgate and the
nand
gate the Norgate only produces an output
of one if both of the inputs are zero
the nand gate only produces an output of
zero if both of the inputs are
one the SR latch can be built using one
of these two basic building
blocks let's start by considering an Sr
latch built from nor
gates in this Norgate version of an Sr
latch two nor gates are connected
together in such a way as the output of
each nor gate is one of the inputs of
the other this crosscoupling of two
gates results in a form of positive
feedback Sr latches like all electronic
circuits require power to work the power
connections aren't shown on this
diagram the SR lat has two inputs R and
S and the output Q the SR latch also
makes the inverse of the output
available on this diagram you can see
not q a q with a bar above
it the starting State here is that S and
R are both low that is both inputs are
zero Q is high that is the output is one
and not Q the inverse of this is
zero both of the inputs of the top n
gate are zero so the output of the top
gate is one this is exactly what you
would expect from a
Norgate the inputs of the lower Norgate
are one and zero so the output of the
Lower Gate is
zero because Q is one the latch is
currently storing
one now we apply a pulse to input R to
reset the
latch this changes the the output of the
top gate and then this is fed back into
the Lower Gate the lower Gate's output
also changes and this is fed back into
the top gate the pulse that was applied
to reset the SR latch is then removed
and R is zero
again but the output at Q is now zero so
the latch is now storing a
zero in order to store a one again a
pulse must be applied to input s which
will set the lat
again notice how the various changes are
propagated around the
circuit the set pulse is then
removed and the circuit is now latched
into a set State it's storing a one
again notice that if another set pulse
is applied it has no
effect applying a set pulse at s will
always for Force the latch into a set
State regardless of the previous state
of the latch similarly applying a reset
pulse will always Force the latch into a
reset state it should be noted that S
and R are never left high that is
neither is ever set continuously to the
value one the latch is controlled by
pulses
only this gives us an unusual looking
truth table
when both S and R are set to zero Q may
be one or it may be zero depending on
the previous state of the circuit let's
examine the truth table as this Sr latch
is reset
again the reset pulse is applied s is
zero R is one output Q is zero and its
inverse not Q is one the SR latch is
storing a
zero the reset pulse is removed both S
and R are zero again output Q remains at
zero and its inverse remains at one the
SR latch is still storing a
zero a set pulse is applied s is one and
R is zero the output Q becomes one and
its inverse becomes
zero the set pulse is removed s is zero
and R is zero output Q is still one and
not Q is of course
zero now the only circumstance we
haven't considered is when both inputs S
and R are set to one at the same
time if this were to occur we'd be
telling the SR latch to set the value of
Q to both one and zero
simultaneously in reality Q would become
zero and not Q would also become zero
this would sort itself out if one of the
inputs fell to zero before the other for
example if r fell to zero first with s
still at one then Q would become one
again if however both inputs were at one
and both fell to zero at the same time
we'd have what's known as a race
condition between the two gates they'd
be racing each other to feed back their
new output and it's impossible to know
which one would win hence if both puts
are high the next state of the latch
can't be determined this is not a state
that the latch should ever be in it's
illegal it's
invalid most of the time inputs S and R
should both be at zero and only
momentarily will one or the other input
become one and at any time while it's
operating the SR latch should either be
in a set state or a reset state with q
and not Q the opposite of each other
one final piece of terminology this type
of Sr latch is said to be an active High
Sr latch because the normal condition
for S andr is low and a high pulse at
one of these inputs is required to bring
about a
change now let's consider an Sr latch
built from nand Gates here's a reminder
of the nandate truth table only when
both inputs are high is the output
low the wiring of this SL latch is the
same but notice that input s is at the
top now and R is at the
bottom here's a truth table for this
variation of the SR latch it's a little
different from the truth table we've
just seen because this latch behaves
differently the difference is that R and
S are kept High most of the time here we
can see that output Q is zero so the
latch is storing a zero when input s is
made low momentarily that is when s
becomes zero and R is still one the
output at Q becomes one the latch is now
storing a one and S can be returned to
its normal high
State when R is set low momentarily the
output at Q is changed to
zero R can then return to its normal
high value and the latch is now storing
a zero
again an Sr latch built from nand Gates
like this is more explicitly known as an
active low Sr
latch an Sr latch based on nand Gates
also has a forbidden state that is when
both S and R are simultaneously
zero this would result in an illegal
state in which both q and its complement
are one
to summarize then an Sr latch can be
built from nor Gates or from nand Gates
both types of latch do the same job but
they're just controlled in a slightly
different way the Norgate based Sr latch
is set or reset with high logic that is
it is an active High Sr latch the nand
gate Sr latch on the other hand is set
or reset with low logic that is it's an
active low Sr
latch let's consider a particular
application of an Sr
latch when a mechanical switch is
pressed it may actually generate several
electrical signals in a tiny fraction of
a second when only one signal is
required lots of onoff signals like this
could then cause problems with the
circuit that this switch is supposed to
be
controlling the effect is known as
switch
bounce
an integrated circuit with an Sr latch
on it can be purchased commercially to
allow clean interfacing between a
mechanical switch and the digital
circuit it's
controlling here we can see a switch
that will connect input s which is
currently High to Earth making it low
and thereby changing the output of this
nand based Sr latch from low to
high this Sr latch will ignore any
further set signals after it's already
been set
so it's serving to debounce the signal
from the mechanical
switch you can imagine other systems
which might make good use of this
debouncing effect for example a burglar
alarm may be triggered when a window or
a door is opened but we don't want the
alarm Bell to stop ringing if the
burglar shuts the
door on its own an Sr lch has a few uses
mainly in control applications where we
need to monitor some condition that
might change or change back again and
react
accordingly but more importantly as
you'll see later the simple Sr lch is
the building block of sophisticated
memory
circuits
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