Flip Flop Operating Characteristics


>>Good day, this is Jim Paytel from Columbia Gorge Community
College Renewable Energy Technology Program. This is ET-122, Digital 2. Today, we are going
to discuss flip-flop operating characteristics. And in the grand scheme
of things compared to all the gee-whiz stuff
that we’ve already learned, this is a very boring lecture. Okay, these are just talking about the very real
characteristics of a flip-flop. Yes, ideally they should
be responding instantly, but we all know that
is not true. Okay, there’s a couple
propagation delays that we have to take
into account. So let’s first talk
about propagation delays. And again, a propagation
delay is what you put in, there’s a delay and
then there is an out. And let’s specifically talk
about, let’s say, we’re dealing with a positive-edge device
where this is our positive edge and we’re talking about
from the 50 percent point of the positive edge. Well, let’s talk about the TPLH. So time propagation
from low to high. So there’s TPHL, time
propagation delay high to low. So we’re talking about
TPLH, low to high. So again, this is a
positive-edge device, assuming that the
device is already in a – let’s make that prettier. Let’s assume the
device is already in a reset mode,
this right here. And then we want
it to go to a set. So we put it in the set mode
and we get the positive edge, and ideally it should set right
at that moment, but it doesn’t. There’s a slight delay
between here and here. Between the positive edge and
from low to high, TPLH, okay? So that is the propagation
delay from low to high, and that’s the synchronous
delays. There’s also the
asynchronous delays, remember our asynchronous
which overrides the clock, our active low presets
and our active low clears. So imagine right here,
here’s our active low preset and it goes down like this. Yes, in a perfect world
it looks like that, but in reality there’s
a bit of a delay. Excuse me, a bit
of a slope to that, and that’s a 50 percent point. So how long does it take for
the output to really preset? So for example, it was
in the zero condition, and then it receives
the command right here. Ideally, it would
preset right here. So again, we’re talking about
presets, which is an active low. It would preset here, but
there’s a bit of a delay between here and here, and
that is going to be our time for propagation delay from
low to high for our preset. Okay? Just think of
this as, you know, way back when when you’re a kid. If your mom told you
to clean your room. How long did you
actually just sit in your room and just be mad? This is the reason – that’s
the time there that it took you to actually just be mad, and then you actually start
cleaning your room right here. Okay? So those are
propagation delays. Okay? There’s also another thing in flip-flop operating
characteristics that you’ll get in a data sheet, and
that’s our setup time. Okay? It’s basically the
minimum time needed for a mode. Remember, our modes are
set mode, our reset mode, our toggle mode, and finally our
latch or just stay there mode. Just keep what you’ve got mode. You know, the minimum time
needed for those modes to be established prior to
the arrival of a clock edge. So let’s say, for
example, here’s our J input for a flip-flop and here’s
our K input for a flip-flop. And here is our clock. Okay, so we are,
let’s say, okay, it’s previously in latch mode. Zero, zero, just stay
with what you’ve got. And now we want it to
go to toggle mode, okay? So they both rise up at this
time, but now our clock, if it just suddenly occurred
right here, yeah, it might jump on those signals at the wrong
time, so what you’ve got to do is you’ve got to give it
some setup time right there. So it really reads
that right there. And this is what’s referred
to as the setup time. Give the JK flip-flop right
there a chance to stabilize in the toggle mode in
this particular case prior to the arrival of
a positive edge. If it was a negative edge, yeah. It’d be over here. Okay? So last one to talk
about is our hold time. Okay? Again, for edge-triggered
devices, the time required for stable inputs, basically
it’s got to stay at – stay still at least this long. Think of it as like a camera. Just hold on for a second
while I take this picture. For example, let’s say here’s
our clock and that’s when it’s at the 50 percent point. And what it’s saying,
let’s say – let’s just use D flip-flop here. Okay, positive edge
detection D flip-flop, and here’s our data
or our D line-in. Let’s say it’s going like this
and it just suddenly changes, you know, right here,
from zero to one. It might catch it
at the wrong time, so basically what it’s saying is “I know I’m a positive
edge detection device, but at least give me some time
to capture that initial zero.” Whoops. Sorry about that. At least give me
some time to capture that initial time, initial zero. Okay? You just can’t have the
D suddenly be changing at the – for a positive-edge
detection device right at the positive edge. It just wouldn’t work. Okay, clock speed. Clock speed is kind of the
grand operating characteristic that you need to be concerned. This is how fast you
can drive your car. Clock frequencies,
again, as we know, the higher frequency
numbers, the faster we can go. Basically it’s the highest
frequency it can be – a flip-flop can be driven;
a thirty megahertz chip versus a 50-megahertz chip. Which one’s faster? Well, it’s the 50-megahertz, because your clock can be
changing that much faster. Okay, last one is – oh no,
actually there’s a couple more. A pulse with – this is
kind of the minimum time for active low preset and clear. So we talked about clock speed and then we are talking
about pulse width. This is typically referred
to for the active low preset and clear and here’s
our timing diagram. And here’s our active
low preset coming along and it goes super-low real
fast and it comes back up. Do you think everything can
catch it in a nanosecond? No. No, it’s not
going to happen. So what you need is
a minimum pulse width for our active low preset of
whatever the datasheet says, TW. Okay? And there’s also pulse
widths for the clock, too. You would expect – and
that’s for our pre – and you would expect,
for our clock – again, it should be a 50
percent duty cycle. Well, not always. Most of the time. There is a minimum specification
for the width of the clock, which, if you’re
thinking correctly, you’ve already got it right
there with the clock space. Remember, width is directly
related to frequency. And you should have gone through
ET-112 to figure that out. If you don’t know that
by now, I can’t help you. Okay. Power dissipation. Power dissipation is the last – power dissipation
is exactly that. How much power each
flip-flop is dissipating, so a 10 milliwatt flip-flop
versus a 25 milliwatt flip-flop, and so many flip-flops for,
you know, like four flip-flops, four tens versus 425 milliwatts. It’d be 40 milliwatts
versus 100 milliwatts. So you make the call, you
know, with the understanding that sometimes you need devices that are lower power are
sometimes a little bit more expensive. Okay, so super rough discussion about flip-flop operating
characteristics and now it’s going to
more exciting things. How we can apply
flip-flops and a bunch of different applications,
including some counters, which will be coming up.

2 Comments

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  • Thanks a lot sir ! really made me clear of this concept !!

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