Latches and Flip-Flops 2 - The Gated SR Latch
Summary
TLDRThis script explains the concept of a gated SR latch, an advanced version of the basic SR latch, which can only change state when enabled. It uses the analogy of an air conditioning system to illustrate how each room's cooling unit can be independently controlled by its own SR latch. The video describes how to convert a regular SR latch into a gated one by adding AND or NAND gates, creating an additional 'enable' input. It also introduces the symbol for a gated SR latch and uses timing diagrams to demonstrate its behavior, highlighting its utility in various applications.
Takeaways
- π A gated SR latch is an SR latch with an additional 'enable' functionality, allowing it to change state only when enabled.
- π’ The concept is illustrated with an air conditioning system, where each room's cooling unit is controlled by an SR latch with set/reset signals from sensors.
- π An SR latch is level-sensitive, meaning it responds to the level of the input signal (high or low) rather than the duration of the pulse.
- π οΈ To create a gated SR latch, AND gates are added to an SR latch built from NOR gates, or additional NAND gates are added for one built from NAND gates.
- π The addition of gates changes the latch from active low to active high, depending on the type of gates used (NOR or NAND).
- π« It's invalid for both S and R inputs to be high at the same time in an SR latch.
- π The third input 'E' in a gated SR latch is crucial as it enables or disables the latching effect.
- π The behavior of a latch can be visualized using a timing diagram, which shows changes in voltage over time for each input and the output.
- π The normal states of S and R are reversed when transforming an active low latch into an active high latch using additional gates.
- π The term 'steering gates' refers to the additional gates that create the gated functionality in an SR latch.
- ποΈ A transparent latch allows input to affect the output unconditionally, whereas a gated SR latch is transparent only when enabled.
Q & A
What is a gated set/reset latch?
-A gated set/reset latch is an SR latch that can only change state when it is enabled. It incorporates an additional input, 'E', which acts as an enable signal to control the latching effect.
How can a gated SR latch be used in an air conditioning system?
-In an air conditioning system, each room could have its own cooling unit controlled by an SR latch. The set and reset signals might come from temperature or humidity sensors, and a central control panel could enable or disable these units on a room-by-room basis.
What is an active high SR latch?
-An active high SR latch is one that requires a high pulse (1) at input S to set the latch and a high pulse at input R to reset it, resulting in a high output at Q.
What does it mean for an SR latch to be level sensitive?
-A level-sensitive SR latch responds to a valid change in either S or R, regardless of the duration of the input pulse. It is the level (high or low) that matters, not the duration of the input.
How can an SR latch be made into a gated SR latch?
-A regular SR latch can be turned into a gated SR latch by connecting a pair of AND gates in series with the inputs of an OR-based SR latch, or by adding another pair of NAND gates to a NAND-based SR latch.
What is the purpose of the steering gates in a gated SR latch?
-Steering gates, which are the additional AND or NAND gates in a gated SR latch, control the transparency of the latch. They determine whether the latch will respond to changes in S or R inputs only when enabled.
What is the difference between a transparent and a gated SR latch?
-A transparent SR latch is one where a valid input will affect the output unconditionally. A gated SR latch, on the other hand, is transparent only when it is enabled by the additional input E.
What symbol is used to represent a gated SR latch in diagrams?
-A gated SR latch has its own symbol that simplifies diagrams, showing inputs SE and R, the output Q, and its inverse, to focus on what the latch does rather than the internal components.
How can the behavior of a latch be described using a timing diagram?
-The behavior of a latch can be described using a timing diagram that shows changes in voltage against time for each input (S, R, E) and the output Q, allowing visualization of the circuit's operation over time.
How does the enable input 'E' affect the behavior of a gated SR latch?
-The enable input 'E' determines whether the gated SR latch will respond to changes in S or R. If 'E' is high, the latch is enabled and will change state based on S and R inputs; if 'E' is low, the latch is disabled and will not respond to S or R inputs.
What is the significance of the transition time in a digital circuit?
-The transition time, which is the time it takes for the voltage to change from low to high or vice versa, is significant in digital circuits. Although it is assumed to be instantaneous in the script for simplicity, in reality, it takes a few nanoseconds and can affect circuit performance.
Outlines
π Understanding Gated SR Latches
This paragraph introduces the concept of a gated set/reset latch, which is a type of SR latch that can change state only when enabled. It uses the analogy of an air conditioning system to explain how each room's cooling unit can be independently controlled by its own SR latch. The set and reset signals could be derived from temperature or humidity sensors, with a central control panel managing the enabling or disabling of these units. The paragraph explains the difference between an active high and active low SR latch, the importance of not applying high pulses to both S and R simultaneously, and the concept of level sensitivity. It then describes how to enhance a basic SR latch by adding AND or NAND gates to create a gated SR latch, which introduces a third input E for enabling or disabling the latching effect. The paragraph also discusses the idea of transparency in latches and introduces the symbol for a gated SR latch, emphasizing the use of timing diagrams to illustrate the behavior of the latch.
π Visualizing Gated SR Latch Operation
The second paragraph delves into the visualization of a gated SR latch's operation through timing diagrams. It explains how to stack multiple charts for each input (S, R, D) and the output (Q) along a common time axis to observe the circuit's behavior. The paragraph describes the normal latching behavior when the enable input E is high and how the latch behaves like a simple SR latch without steering gates in this case. It then discusses the effects of varying the E input, showing how the latch responds when E is high and becomes non-responsive when E is low. The summary highlights the transformation of an SR latch built from NOR gates into a gated SR latch by adding AND gates, and similarly, the conversion of an SR latch built from NAND gates into a gated SR latch by adding additional NAND gates, which also changes it from active low to active high. The paragraph concludes by emphasizing the gated SR latch's additional input E, which must be high for the latch to respond to changes in S or R, and the use of its own symbol to simplify circuit diagrams.
Mindmap
Keywords
π‘Gated Set/Reset Latch
π‘SR Latch
π‘Active High
π‘Level Sensitive
π‘Steering Gates
π‘Transparent Latch
π‘NAND Gates
π‘Enable Input (E)
π‘Timing Diagram
π‘Binary Value
Highlights
A gated set/reset latch is an SR latch with an additional 'enable' functionality.
The enable feature allows for central control over individual units, such as in a building's air conditioning system.
An SR latch is level-sensitive, responding to the level of the input signal rather than the duration.
An SR latch can be built using NOR gates, requiring a high pulse at input S to set and at R to reset.
It is invalid for both S and R to be high simultaneously in an SR latch.
Gated SR latches can be created by adding AND gates to an SR latch, enabling control over the latching effect.
An SR latch built with NAND gates is active low, requiring a low pulse to set or reset.
Adding NAND gates to a NAND-based SR latch changes it from active low to active high.
The additional gates in a gated SR latch are sometimes called steering gates.
A gated SR latch is transparent only when enabled, unlike a non-gated latch which is always transparent.
A gated SR latch has its own symbol for simplified diagram representation.
The behavior of a latch can be visualized using a timing diagram showing voltage changes over time.
A level-sensitive gated SR latch's time intervals can vary greatly depending on the application.
The voltage transition in a digital circuit is assumed to be instantaneous for simplification.
The classic square wave representation is used to describe the behavior of a latch in a timing diagram.
Stacking timing diagrams allows for a comprehensive visualization of a latch's operation over time.
The enable input E must be high for a gated SR latch to respond to changes in S or R.
The behavior of a gated SR latch is demonstrated through its response to variations in the enable signal E.
A gated SR latch maintains its state when disabled, not responding to input changes.
Transcripts
a gated set/reset latch is an SR latch
that can only change state while it's
enabled for example imagine an air
conditioning system in the building
each room could have its own cooling
unit controlled independently by its own
SR latch the set and reset signals might
come from a temperature sensor or a
humidity sensor in the room if these
were gated SR latches then a central
control panel could be used to enable or
disable the switching on and off of
these units on a room-by-room basis
here's an uncoated SR latch remember an
SR latch built from nor gates like this
one is an active high SR latch this
means it requires a high pulse that's a
1 to be applied at input s in order to
get a high output at Q in other words to
set the latch alternatively to reset the
latch and make the output at Q 0 it
requires a high pulse to be applied to
input R it's invalid for both s and R to
be made high at the same time an SR
latch is said to be level sensitive this
means it will respond to a valid change
in either S or R regardless of the
duration of the input pulse it's the
level high or low that matters not how
long it's applied for to build a gated
SR latch
we can make some simple enhancements to
an SR latch by connecting a pair of and
gates in series with the inputs of an or
based SR latch we've created a third
input e which can be used to enable or
disable the latching effect a regular
and gate will only have a high output if
both inputs are high so only when e is
set to one with a one from s or a one
from our get through to our cross
connected nor gates here's an SR latch
built from NAND gates this is a
of sensitive active low sr latch in
other words both S&R are normally high
and it requires a low pulse that's a
zero to be applied to s in order to set
it it also requires a low pulse to be
applied to R in order to reset it by
connecting an extra pair of NAND gates
to the latch like this not only have we
created a third input e we now have a
new circuit in which the normal states
of s and r are zero and high pulses are
required to set or reset the latch in
other words our NAND based latch has
been changed from an active low latch
into an active high latch these
additional gates on a basic SR latch are
sometimes referred to as steering gates
an SR latch without steering gates is
said to be transparent a valid input
will affect the output unconditionally a
gated SR latch on the other hand is
transparent only when it's enabled it's
convenient to give a gated SR latch its
own symbol because we can now focus on
what the latch does rather than what's
going on inside it the inputs SE and r
are still shown and so is the output q
and its inverse not cute
we can illustrate what's going on at any
one of the inputs or the output on a
chart showing changes in voltage against
time at any given moment the voltage
will be either low zero volts to all
intents and purposes and representing
the binary value zero or high at about
five volts representing the binary value
one now of course everything can happen
very quickly in a digital circuit each
time interval on this chart could be in
the order of a microsecond that's a
millionth of a second
having said that for a level sensitive
gated SR latch each time interval might
be one second or one minute or even an
hour it really depends on the
application for the purposes of this
discussion it takes no time for the
voltage to change from low to high or
from high to low in reality this
transition takes a few nanoseconds the
significance of which we'll ignore for
now but come back to in a later video
let's proceed on the assumption that
switching from low to high or vice-versa
is instantaneous this gives us the
classic square wave that you can see
here we now have a convenient way to
describe the behavior of a latch by
stacking several charts together one for
each input SR D and one for the output Q
with a common time axis we can visualize
this circuit in action
at the time indicated by the vertical
yellow line the output Q is low s and R
are also both low but E is high so the
latch is enabled in fact in this diagram
the latch is always enabled so it's
going to behave exactly like a simple SR
latch without steering gates as time
passes if s goes high
so does Q because it's a latch when s
drops too low again Q stays high
if s goes high while Q is high it has no
effect Q is already high when R goes
high Q goes low when s goes high again
so does Q this is normal
latching behavior again we see our going
high
so the latches reset again
the latch is already reset so another
pulse it R has no effect now let's
examine what happens when e varies you
can see that some of the time he is high
and some of the time he is low we begin
with an output of zero but Q and because
he is low the latch is disabled s goes
high but it has no effect on the output
of the latch now he is high the latch is
enabled so when s goes high
so does Q and when there stops too low
again the latch stays high until such
time as R goes high and then Q drops to
zero again now he is zero the latch is
no longer enabled so if s goes high it
has no effect on queue
he is high again the latch is enabled
and so it responds when s goes high the
latch is set again he goes to zero the
latch is disabled R goes high but Q
doesn't drop it remains in its high
state to summarize then an SR latch
built from nor gates can be turned into
a gated SR latch by adding a pair of and
gates to it an SR latch built from NAND
gates can be turned into a gated SR
latch by adding another pair of NAND
gates to it this also has the effect of
changing the SR latch from an active low
to an active high latch a gated SR latch
has an additional input e which must be
high before the latch will respond to
any changes in S or R the gated SR latch
has its own symbol to simplify diagrams
and the behavior of a latch can be
described by means of a timing diagram
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