In the last video we built a JK
flip-flop that didn't work all that well
with we use an RC circuit to detect the
rising edge of the clock but because the
JK flip-flop toggled so quickly we
encountered this racing phenomenon where
we toggle many times for each clock
pulse and we found it was really
difficult to fix you know almost
impossible to fix on the breadboard to
get that clock pulse to be so short you
know the pulse that we're generating the
RC circuits from the rising edge of the
clock to get that to be so short that
was only toggle once we found out was it
almost impossible to to get that working
for bus the on the breadboard so what
can we do
well there's another way to build a JK
flip-flop and that's this circuit here
which is called a master-slave JK
flip-flop and so what you'll see if you
look at this is you see there to
essentially 2 SR latches built into
this, there is one here and then one here and
you can think of this is having you know
you're you're set in your reset and this
case will be the Q compliments and
this will be the Q output and then over
here you know this will be the reset and
the set and this is our Q output in
our compliments Q. So these are
two SR latches which you know you can
look at my previous video on those, but then
outside of that you have these AND gates
that are essentially gating the clock
or gating the SR latches based on the
clock so for the moment let's, let's
ignore the feedbacks we'll get to that
in a minute but if you look at just
these two inputs of the AND gate the J
input and the clock or in this case
the K input and the clock
basically we're saying is we're saying
this SR latch is not going to be active
unless the clock is high
when the clock is high then you know J
will pass through and set or K will pass
through and reset anytime the clock is high
So this is just sort of like a you know an
SR latch with an Enable you want to think
of the clock in that way and then same
thing if you look at that the slave over
here and on the salve side you've got an SR
latch as well, the inputs are
coming from the master but then instead
of the clock input coming into the AND
gates you've got the inverted clock
input and so the slave is only going to
be active when the clock is low and so
the interesting piece of this is that
you're never going to have a situation
where both of these SR latches are
active at the same time because the
clock is either going to be high which
case this was active or the clock is
going to be low which case this one is
active and so if you want to step this
this flip-flop you want to get the Q
output to be high then you know you
bring J high and then you toggle the
clock high and low and so with J high
when the clock goes high then it's going
to set the first latch and so the Q
output of that first latch is going to
go high then when the clock goes low
that enables the second latch and that Q
output gets fed through and sets the
second latch which causes this Q output
to go high so you can see setting the J
input toggling the clock high then low
causes the Q to get set and then the
same way with K if you set K high and
then you toggle the clock high then
low that it resets
this latch and then the clock goes low
and reset this latch which resets the
output so it sets Q to 0 so effectively kind
of a two-step process with the clock
has to go high then low now what about
that JK flip-flop so we should have that
property where J&K are both high than
the output should toggle each time the clock
pulses so in that case when J & K are both
high when the clock goes high the first
latch is either going to set or reset
depending on what the current output is
so the current output is that Q is high
then he's gonna come out here and this
the AND gate is going to turn on its
going to reset but if the current output
is low and then a compliment output is
high then this is going to come around
at the top AND gate is going to turn on
its going to set so essentially this
first latch will output on this Q the
opposite of whatever currently being
output over here if both JK are set and
the clock is high but its fine if the
clock stays high because the second
latch is not going to be enabled the
clock is high then when we invert it this
is going to be a LOW these AND gates are
going to be off this latch is not going
to be active and so this output will
stay stable so it doesn't matter if the
clock state high for for whatever period
of time the first latch will switch but the
second one won't until the clock goes
low once the clock goes low then that
second latch enables and the output of
the first latch gets fed into the second
latch so whether that was setting it or
resetting it and then when that second
latch either sets or resets that's when
the final output changes and of course
at that point the output changes it
doesn't matter what's getting fed into
the first stage over here because your
clock is low at this point and so these AND
gates you both going to be OFF so
nothing is going to change here
So this circuit essentially has the
exact same behavior as the first circuit
that we're looking at except we don't
have this issue of of trying to make
sure this pulse is so narrow that we
don't have this racing where you know
the outlet toggles and then a toggles
again and toggles again toggles again
over and over because we're not we're
not trying to use it on RC circuits to
create a pulse out of our clock where
you know using the fact that the clock
goes high to set one latch and when the
clock goes low to set the next latch
based on the first latch in that way our
feedback you know can't get back around
to the input until you have a full clock
cycle and so this way this will only
toggle once per clock cycle and we don't
have to deal with any of the you know
really stringent timing requirements you
know where we're looking at this you
know 40 milliseconds propagation time
and so forth so go ahead built the this
circuit master-slave JK flip-flop and
just like last time i've got two inputs
here this is our J&K input and again
they're you know tide low to this
pulldown resistor but then when you push
the button that goes high. So those
two inputs go into this is again are
our three input AND gates so both of the
input go into those 2 AND gates and
then this this white wire is connecting
our clock and so will connect our clock
into that and then of course with the
third input is the feedback that comes
back around that these two green wires
here and the output of these AND gates are
the two blue wires to go up to our to
our first two NOR gates which are the
top two NOR gates on this says 74LS02
and then you know the outputs with other
connected back to the inputs you too
yellow wires in the outputs of those
come to these first two LEDs so you two
LEDs here give us that you know a way to
see what's going on at this point here
so this is our you know our Q
compliment for the first stage and this
is our Q for the first stage then those
outputs go over here to another to
another I guess that this is a 74LS08
it's a two input AND gates and also
connected to to those AND gates is our
clock except this time the clock is coming
so this is original clock it's going over to
74LS04 inverter and they're
coming out of that inverter and going
into the AND gates here and then the
outputs of AND gates go back to our
74LS02 are NOR gates and here we are using
the bottom two NOR gates for these and
so that is
yeah that's these blue wires the wires
are .. , No the blue wires are here
coming out of our AND gate going into
our NOR gates right that's those two
blue wires and then of course we've got
these two yellow wires 2 short yellow
wires are the cross overs here between
our inputs outputs and the NOR gates and then
the outputs and NOR gates that these two yellow
wires that go over to our two LEDs so
this is our Q our final Q output and this
is our Q complement the output so let's
hook it up to Power and see what
happens
I'm gonna hook into power our clock and
got running over here and so the first
thing you see is of course looks like Q
compliment is on on our first stage and
then of course Q complement is on on
our second stage and if I hook a clock up here
OK nothing happens
but i should be able to set and reset it
so I hit J that should set it so Q
should go high and in fact it does
of course you see Q goes high first on our
first stage and then on our second stage
and then if I reset it
it resets. The other thing that I'll show
you is like put our clock into manual
mode so here I can just manually
activate the clock that way what you'll see if
I go ahead and try to see if I set thi
so I turn on the J input , and of course
nothing's happening to the clock not
doing anything when the clock goes high
you see the first stage changes and then
it's when the clock goes low the second
state changes and now you know I set the
output by inputting a 1 to our J input and
toggling clock high then low so there's a
JK flip-flop it should toggle so if i
get a 1 both inputs and actually just
let the clock run you see with each
clock pulse each time the clock goes
high the first one toggle each time the
clock goes low the second one toggles
and so if you just look at the
bottom-left LED that's our Q output of
our flip-flops and you see it's just
going high-low high-low it's toggling
which is exactly what we'd expect
and of course because we don't have any
of the you know crazy timing
requirements here this is going to work
very consistently not gonna have any of
the you know flakiness issues that I was
having with with the JK flip-flop that
was using his RC circuit to try to
detect the rising edge of the clock and
in fact because it's much more reliable
this is this master slave arrangement
for building a JK flip-flop is
typically the way that JK flip-flops are
built-in in practice. So now that you
understand how the JK flip-flop works in
future videos rather than building you
know JK flip-flop from all these
components we go ahead and use a
prepackaged JK flip-flop in this case
74LS76 which is a dual master slave
JK flip-flop it's got 2 you know that's
what makes it a duel is two of them and
course master slave JK flip-flop you you
know what that means that's what we just
talked about and so if you look at at
the flip-flop here you see it's got you
know course the J input and the K
here it is the K input and the wire
goes all the way around there but
you've got your J input your K input
your Q output your complimentary Q
outputs that's what it means
complimentary output as both Q and the
inverted Q outputs and this will also
has a Preset in a Clear and so the
preset and clear
are these two other inputs that
basically just force it to either set or
reset so rather than it kind of bypasses
the clocking mechanism to you can just
force it to reset reset those kinda
another little feature that these have
and then the other thing is the
clock input here is says its inverted it
has a little bubble here and that's
actually absolutely correct because this
is a master-slave JK flip-flop and you
remember that the out the final output
the Q output doesn't actually change
until the clock you know both goes high
and then goes low
and so unlike the first JK flip-flop
that we we tried to build the triggers
on the rising edge of the clock the
master-slave JK flip-flop because it
requires the clock to go high and then low
again the output triggers on the falling
edge of the clock and so that's
indicated in a in a symbol for it with
this little bubble here showing that
this is a you know this clock input is
looking at the falling edge with that
bubble means. So the next video we'll
will hook a couple of the 74 and 76's
together and build a binary counter