Latches and Flip-Flops 6 - The JK Flip Flop
Summary
TLDRThe video script delves into the versatility of the JK flip-flop, a fundamental building block in digital electronics used in various applications like shift registers and counters. It contrasts the JK flip-flop with the SR latch, highlighting the JK's ability to avoid unpredictable behavior with invalid input combinations. The script explains the construction of JK latches from both NOR and NAND gates and their transition to flip-flops with the introduction of an enabling input and clock signal. It also demonstrates how the JK flip-flop operates as a toggle device when J and K inputs are combined.
Takeaways
- đ The JK flip-flop is a versatile component used in various applications such as shift registers, ripple counters, event detectors, and frequency dividers.
- đ It is known as the universal programmable flip-flop because it can mimic the behavior of other flip-flop types by manipulating its inputs.
- đ« The JK flip-flop overcomes the limitation of the set-reset latch, which can exhibit unpredictable behavior with certain input combinations.
- đ© The set-reset latch built from NOR gates has an active-high configuration, while the one built from NAND gates is active-low.
- đ In a set-reset latch, an invalid input combination of both S and R being high can lead to a race condition and unpredictable output states.
- đ The JK latch, a precursor to the JK flip-flop, can be built from both NOR and NAND gates, eliminating the invalid state issue of the set-reset latch.
- đ A JK latch built from NOR gates will oscillate between states when both J and K inputs are high, while a NAND-based JK latch behaves similarly.
- đ Adding an enabling input (E) to the JK latch allows it to respond to J and K inputs only when E is high, effectively gating the latch.
- âČïž The JK flip-flop operates on the rising edge of a clock signal, toggling its state when both J and K are high during this transition.
- đ A modified truth table can describe the behavior of the JK flip-flop, showing how Q changes on the rising edge of the clock based on J and K inputs.
- đ By connecting J and K together and labeling it as 'T' for toggle, the JK flip-flop can be adapted to create a T flip-flop that toggles state on the rising edge of the clock when T is high.
Q & A
What is a JK flip-flop and why is it considered versatile?
-A JK flip-flop is a type of bistable multivibrator that can be used as a shift register, ripple counter, event detector, frequency divider, and more. It is considered versatile because it can be manipulated to behave like other types of flip-flops, overcoming limitations of set-reset latches.
What is the main advantage of a JK flip-flop over a set-reset latch?
-The main advantage of a JK flip-flop over a set-reset latch is that it eliminates the issue of unpredictable behavior when both set and reset inputs are high simultaneously, which is an invalid state for a set-reset latch.
How does an SR latch built from NOR gates work?
-An SR latch built from NOR gates operates with active high inputs. The output Q is set to 1 when a high pulse is applied at S, and reset to 0 when a high pulse is applied at R. The latch holds the last value when both inputs are low.
What is the problem with an SR latch when both S and R inputs are high?
-When both S and R inputs of an SR latch are high, it creates an invalid state where the latch is told to store both 1 and 0 at the same time. This results in Q and not Q both becoming 0, leading to unpredictable behavior.
How does an SR latch built from NAND gates differ from one built from NOR gates?
-An SR latch built from NAND gates is active low, meaning S and R should be kept high and a low pulse at either S or R will set or reset the latch, respectively. It avoids the same invalid state issue as the NOR-based latch when both inputs are high.
What is a JK latch and how does it differ from a JK flip-flop?
-A JK latch is a modified version of an SR latch with added AND gates at the inputs, labeled J and K for set and reset. It differs from a JK flip-flop in that it does not have a clock signal and can oscillate between states when both J and K are high.
How does a JK flip-flop solve the invalid inputs problem of an SR latch?
-A JK flip-flop solves the invalid inputs problem by using AND gates at the inputs, which ensures that when both J and K are high, the flip-flop toggles between states rather than resulting in an invalid state.
What is the purpose of the enabling input 'E' in a gated JK latch?
-The enabling input 'E' in a gated JK latch is used to control when the latch responds to the J and K inputs. The latch only updates its state when 'E' is high and the clock signal is on the rising edge.
How can a JK flip-flop be adapted to create a toggle flip-flop?
-A JK flip-flop can be adapted to create a toggle flip-flop by connecting J and K together to form a single toggle input. This results in the flip-flop changing state only when the toggle input is high during the rising edge of the clock signal.
What is the significance of the rising edge detector in a JK flip-flop?
-The rising edge detector in a JK flip-flop ensures that the flip-flop only reacts to the inputs during the rising edge of the clock pulse. This prevents the flip-flop from oscillating uncontrollably when J and K are both high.
How is the toggling behavior of a JK flip-flop described in its truth table?
-The toggling behavior of a JK flip-flop is described in its modified truth table, where a rising edge of the clock pulse causes the output Q to change to its opposite state when both J and K are high, while leaving Q unchanged otherwise.
Outlines
đ Understanding the JK Flip-Flop and Its Versatility
The JK flip-flop is a highly versatile component in digital electronics, serving various functions such as shift registers, ripple counters, event detectors, and frequency dividers. It is known as the universal programmable flip-flop due to its ability to mimic other flip-flop types through input manipulation. Unlike the set-reset latch, which can exhibit unpredictable behavior with certain input combinations, the JK flip-flop overcomes this limitation. The script explains the set-reset latch's operation and its issues with invalid input combinations, which can lead to a race condition and unpredictable output states. The JK flip-flop is then introduced as a solution to these problems, highlighting its similarity to the set-reset latch but with improved input handling.
đ Building and Operating the JK Latch
This section delves into the construction of a JK latch, starting with an active-high SR latch made from NOR gates, and then adding AND gates to create the JK functionality. The JK latch demonstrates the fundamental characteristic of the JK flip-flop: the ability to set and reset the output and to toggle between states when both inputs are high. The script also covers building a JK latch from an active-low SR latch using NAND gates, which results in a functionally identical device to the NOR-based version. The importance of eliminating the invalid state where both Q and not Q could be high simultaneously is emphasized, showcasing the reliability and functionality of the JK latch in various configurations.
đ Clock Signal Integration in the JK Flip-Flop
The integration of a clock signal is crucial for transforming a JK latch into a JK flip-flop. The script uses a timing diagram to illustrate the flip-flop's behavior under different input conditions and clock signals. It explains how the flip-flop only reacts to inputs during the rising edge of the clock pulse, effectively toggling the output Q between states. A modified truth table is introduced to describe this toggling behavior, emphasizing that the JK flip-flop will only change state when both J and K inputs are high during a rising clock edge. The script concludes by showing how the JK flip-flop can be adapted to create a toggle flip-flop, where J and K are connected to form a single toggle input.
Mindmap
Keywords
đĄFlip-flop
đĄJK Flip-flop
đĄShift Registers
đĄRipple Counters
đĄEvent Detectors
đĄFrequency Divider
đĄSet-Reset Latch
đĄPropagation Delays
đĄOscillator
đĄRising Edge Detector
đĄTruth Table
đĄToggle Flip-flop
Highlights
The JK flip-flop is a versatile device used in various applications such as shift registers, counters, and frequency dividers.
It is known as the universal programmable flip-flop due to its ability to mimic other types of flip-flops by manipulating inputs.
The JK flip-flop overcomes the limitation of unpredictable behavior with invalid input combinations, unlike the set-reset latch.
An SR latch built from NOR gates is susceptible to invalid states when both set and reset inputs are high.
The output of an SR latch can become unpredictable in a race condition when both inputs return to zero simultaneously.
An SR latch built from NAND gates also faces issues with invalid input combinations, leading to unstable outputs.
The JK latch, a precursor to the JK flip-flop, eliminates the invalid state issue by introducing new inputs J and K.
A pulse at K in a JK latch will reset the latch, similar to the reset function in an SR latch.
When both J and K inputs are high, the JK latch oscillates, which is a key feature for its operation as a flip-flop.
The JK flip-flop can be built from both NOR and NAND gate-based SR latches, with functionally identical results.
Adding an enabling input to a JK latch, similar to an SR latch, controls its response to J and K inputs.
A clock signal at the enabling input turns a gated JK latch into a functional JK flip-flop.
The JK flip-flop's behavior can be visualized through a timing diagram, showing how it responds to clock signals and input changes.
The JK flip-flop only reacts to inputs on the rising edge of the clock pulse, preventing uncontrollable oscillation.
A modified truth table for the JK flip-flop illustrates its toggle effect and input conditions for state changes.
The JK flip-flop can be adapted to create a T flip-flop, which toggles state only when the input is high at the rising edge of the clock.
Transcripts
when it comes to flip-flops the JK flip
flop is one of the most versatile
it's widely used in shift registers
Ripple counters event detectors
frequency dividers and more
the JK flip-flop is often referred to as
the universal programmable flip-flop
because by simply manipulating its
inputs it can be made to behave like
other types of flip-flop
in essence the JK flip-flop is very
similar in operation to the set reset
latch but it overcomes a serious
limitation of the set reset latch namely
that an invalid combination of inputs
can result in unpredictable Behavior
let's remind ourselves of this
limitation here's an Sr latch built from
nor Gates
the inputs S and R are normally kept low
and a high pulse is applied at one or
the other in order to set or reset it
the latch is said to be active High
in this case the value of the output Q
is 1 so the latch is currently storing a
one
the value of not Q should always be the
opposite of Q not Q is currently zero
a short pulse is applied at R to reset
the latch s is still zero
trace the highs and lows through the
cross-connected nor Gates and you'll see
that the output at Q changes to zero
when the reset pulse is removed R
becomes zero again but the outputted Q
is still zero the latch is now holding
on to the zero
when a set pulse is applied at s the
output at Q changes to 1. the latch is
now storing a one
however we have a problem when both S
and R are made one at the same time with
this combination of inputs we're
effectively telling the latch that we
want to store one and zero
simultaneously which of course is
nonsense the output at Q can only have
one value
in reality the outputted Q will become
zero but not Q will also become zero
eventually when both inputs fall back to
zero the resting state of the latch will
depend on which input Falls to zero
first if R Falls to zero first Q will
become one if s Falls to zero first Q
will become zero
but if both inputs fall to zero at
exactly the same time then we'll have a
race condition the nor Gates will be
racing to feed each other their new
output
one of them will eventually win because
of imperfections in the circuitry or
external factors like temperature but
it's impossible to know which one if
both inputs become High Then Fall to
zero at the same time the next state of
the latch is impossible to predict
this is a state that the latch should
never be in it's invalid
most of the time inputs S and R should
be both resting at zero and only
momentarily should one or the other
become one
at any point in time the output at Q
should be one or zero and not Q should
always be the opposite of Q
here's an Sr latch built from nand Gates
this version of an Sr latch is active
low S and R should be kept High most of
the time and the drop in voltage at s
that is a low pulse at s will set it a
low pulse at R will reset it
at the moment the outputted Q of this
latch is zero so the latch is currently
storing a zero
when s becomes zero momentarily the
output at Q becomes one when s returns
to its normal high state q is still one
the latch is now storing a one
when R is made low for a moment then 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
if both S and R become zero at the same
time we have a similar problem to the
one that we saw with an Sr latch built
from nor Gates both q and not Q become
one
if S and R return to their normal high
State at the same time it's impossible
to predict the final value of Q with a
nand-based Sr latch the input
combination s equals zero and R equals
zero is invalid
here are the two methods of building an
Sr latch side by side
so how does a JK flip-flop solve the
invalid inputs problem
well before we think about this let's
take a look at something that we will
call for want of a better name a JK
latch
here's our simple active High Sr latch
based on nor Gates
we'll add a pair of and Gates
immediately after the inputs
then we'll feed the value of output Q
into the top and gate and the value of
not Q into the bottom and gate
finally we'll re-label the inputs J and
K J to set K to reset
this is a JK latch it's not a flip-flop
yet but it does illustrate a fundamental
characteristic of the JK flip-flop
a pulse at K will reset the latch
and the output at Q will become zero
just like an Sr latch
a pulse at J will set the latch
making the outputted Q equal to 1.
if the latch is already storing a one
and another set pulse is applied J
it will have no effect whatsoever
but if both inputs are made high at the
same time
the value of Q will become zero and if
both inputs remain high the latch will
switch back to the opposite State the
outputted Q is 1 again
keeping both inputs high will cause the
latch to switch repeatedly from one
state to the other the output of the
latch is now oscillating between 1 and
0.
the speed of oscillation depends on
propagation delays within the logic
gates and the connecting circuitry but
it is of course very quick
this so-called JK latch is not
particularly useful unless you wanted to
build an oscillator like this and to be
honest there are more efficient ways to
build an oscillator
nevertheless we've eliminated the
possibility of Q and not Q having the
same value at the same time
and we're a step closer to the JK
flip-flop
before we do see how to create something
very useful let's try to build a JK
latch from an active low Sr latch
consisting of nand gates it will help us
to understand the possibilities
we'll add a pair of nand gates
immediately after the inputs
then we'll feed the value of output Q
into the bottom nand gate and the value
of not Q into the top nand gate
as before we'll re-label set and reset
as J and K
and again we have a JK latch but this
time we've changed the way the
nand-based SR latch works it's now
active High instead of active low
this JK latch is now functionally
identical to the JK latch that we built
from nor Gates
there's a zero at output Q so the latch
is currently storing zero
to set the latch we apply a high pulse
to J
now there's a one at Q
to reset the latch we apply a high pulse
to K and when K falls back to zero Q is
zero again
when both inputs are high then just like
the nor based JK latch the nand-based
latch oscillates between the two states
again we've built an oscillator
but more importantly we've eliminated
the possibility of Q and not Q having
the same value at the same time
here are our two JK latches side by side
remember they are functionally identical
they do the same thing
in the second video of this series we
saw how it was possible to add an
enabling input to a simple Sr latch this
involved a pair of steering Gates
we can do something similar here
the and Gates of the nor based latch on
the left now have three inputs each
all of the inputs of a three input and
gate must be one in order to get a one
out so the new input e must be high in
order for the latch to respond to
anything from j or k
the nand gates of the nand-based latch
on the right also have three inputs each
in order for the output of a three input
nand gate to drop to zero all of the
inputs must be one
so again e must be high in order for the
nand-based latch to respond to any
changes in j or k
the switching behavior of these latches
hasn't changed at all they simply have
to be enabled before they'll do anything
to turn a gated JK latch into a JK
flip-flop all we need to do is supply an
appropriate clock signal at the enabling
input
let's take a closer look at how this
works we can focus on either one of
these designs now they do the same thing
a timing diagram is a convenient way to
examine the behavior of a JK flip-flop
the top chart colored green represents
the clock signal the bottom red chart is
the value of output Q which you can see
starts off high
the values of inputs J and K are the
purple charts in between
in this scenario when input K goes High
there's no change at Q because the clock
is low
the flip-flop is momentarily disabled
but when the clock does go high the
reset signal gets through the and gate
and Q Falls to zero
Q then remains low regardless of what
the clock is doing and what's going on
at input K as long as J stays low but
when J is high K is low and the clock is
high the value of Q becomes 1 again
when inputs J and K and the clock are
all high at the same time the value of Q
begins to oscillate rather
uncontrollably
to take advantage of this oscillation we
need a flip-flop that will only react to
the inputs while the clock signal is
changing from low to high in other words
on the rising edge of each pulse
in the fourth video of this series I
showed you how to build a rising Edge
detector using an and gate and a not
gate this relies on the fact that a not
gate doesn't invert its input
immediately for a very brief period a
matter of nanoseconds the output of the
not gate is the same as its input so for
only a very brief period is the output
of the and Gate High we've effectively
shortened each clock pulse to just a few
nanoseconds
here's a new timing diagram only the
rising edge of each clock pulse has any
effect so that's all you can see on the
top chart
notice that when J and K are both High a
clock pulse will cause the flip-flop to
toggle from one state to another
way to describe this behavior is with a
modified truth table there's a column
for the rising edge of each clock pulse
the column labeled Q next indicates what
Q will become depending on the inputs
if both J and K are 0 Q remains
unchanged
across in the clock column means
anything other than the rising Edge and
crosses in columns J and K mean whatever
the values they'll have no effect Q
won't change
if however J and K are both one a rising
Edge will cause Q to change to its
opposite state
this is the definitive toggle effect of
the JK flip-flop
finally I want to quickly mention how
the JK flip flop can be easily adapted
to create a new device
by simply connecting together J and K to
make one input we now have a device that
will only toggle from one state to the
other when the input is high at the
rising edge of the clock
we can re-label the input t for toggle
we now have a type flip flop
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