SR latch

so in previous videos we've looked at the or gate which if you remember in this case we have two inputs if one input or the other or both are on then the output is on otherwise the output is off in this video I want to start thinking about scenarios where we have a gate where the output is connected back to the input or or some series of gates where the output is connected back to the input and so you might think well maybe we can just come up with a truth table like this or the input it can be there a 0 or a 1 if it's a 0 then the output ought to be something and if the 1 and the output might be something different but what we'll see is is that it's not quite as simple as that so to build a little bit of intuition for what's going on here I think it's easiest to kind of build a circuit and and kind of see what happens so this is that circuit it's got a this chip is this as a 74 LS 32 chip which has four or gates on it and this is the pin out for it and you can see with the four or gates you've got a B and why are the input a input B and Y is the output so that the first three pins and this chip is is upside down so it's these first three pins here and what we've got just like the diagram here is input a is connected to the switch input B is connected via this little blue wire to the output and then we also have an LED connected to the output and you might be wondering if we need a resistor for this LED but if if we look at again at the data sheet for the 74 LS 32 there's already on the output there's already a resistor here connected through this transistor and so when this is switched on into the gates on we already have 120 ohm resistor built into the end of the chip here in this in this case that's going to limit the current so we should be okay with that led the input is connected to the switch which it's actually connected across this resistor to ground and then when we push the button the button will will connect us to this 5 volt source here so if we're not pushing the button it's inputting a 0 if we push the button it's inputting a 1 and this is a momentary switch so let's hook the power up and see what happens so cut the power up and you can see the output is off and so that kind of makes sense maybe the inputs start as zeros and so zero and zero should give us give us a zero output and of course if that's a zero then that's going to feed it another zero back into this B so it just stays everything's a zero does that make sense so now let's see what happens if we change the input to a 1 by pushing the button so if we push the button the inputs a 1 so 2 1 and a 0 and we get a 1 as the output we see the output comes on but actually this output is now coming back into this input so B is a 1 as well now so actually we've got we're pushing the button so we got a 1 coming in a we've got a 1 in being in so the output is still 1 but now if I let go of the button 2 a is now 0 B is still 1 and so the output is still 1 so this this stays on even though I've now let go of the button and in fact if I keep pushing the button there's really nothing I can do to get this to turn off once this input turns on even if it turns off later the output stays on and it latches on essentially it's a really the only way to reset this is to disconnect power from the entire circuit and then you know we can reconnect it and of course it starts out at 0 again but if at any point after this thing is powered up that input ever goes to a 1 even if it's just momentary if I just hit it really quickly that output latches on and remembers that that input was at one point a 1 so this is pretty interesting you can imagine why we can't just come up with a simple truth table like this like we might like to but what would be even more interesting is if we had some way to turn it off again some other way to reset this other than having to disconnect power from the entire circuit because that might not be you know too practical so what I want to do is look at another circuit that actually does work that way and this is this is that other circuit and this might look a little confusing at first you know but we'll walk through it one thing you'll notice is different we're using nor gates instead of or gates so it's the same as an or gate but the output is inverted we'll talk a little bit more in a minute about that and again I think you know a good way to build some intuition for for this is to actually build the circuit and so I've done that here this this chip now is that quad nor gate it's a 74 LS 0 2 chip which has 4 nor gates of course we're just using two of them and to kind of walk through what's going on in here the bottom pin here is this bottom input and then the next pin up is the is input here and you can see the yellow wire is connecting it to the output of the topic or gate the third pin is the output of the bottom nor gate here and you can see that's connected to an LEDs we can see what the output is and you also see this little yellow wire is connecting it to the input of the top nor gate the other input at the top of nor gate is connected to this blue wire to the switch here so we can set the input there and then finally the output of the top nor gate as we saw we've got a hook to an LED here and it's also going back down to an input on the bottom or gate and again these switches are hooked up so that they're normally a zero and when we're pushing them they're a 1 so let's say you know before we think too too much about this let's let's just sort of power this up and see what happens I was like to kind of build the thing and see what happens good plug it in and the top LED comes on so this nor gate on the top is is switched on so this is basically what is happening here so I've kind of drawn exactly what states different things are in to kind of make this a little bit easier to understand and also as a reminder it got the truth table for an or gate so again if either input 1 input or the other or both are on then it's off otherwise it's on so sort of the opposite output of an or gate so in this case the top or gate is on and so we see the LED there and that implies that this input down here must also be on and so if we just kind of Reason through this so hopefully it'll make sense so the bottom nor gate we've got a 1 and a 0 and of course this one for some reason this top LED came on we'll talk about why a little bit later but for now we'll just sort of take it on faith that it's on because we can see that it's on and the bottom is is 0 because we're not we're not pushing this button so that's a zero so one in a zero means that the output is a zero and that makes sense do we see the bottom LED is off and so then that's being fed back into the top nor gate and of course we know we're not pushing the top switch either so it's a zero and a zero going in here which means that the top nor gate should be on and that's what we see so this hopefully makes some sense so now what happens if we start changing these inputs so the bottom input is fairly straight forward if we turn the bottom input on then this gate will be the inputs will be one and one and we could see the output then is still zero so this doesn't change which means this top nothing else changes so if if we change this bottom we if we input a 1 if we push this button nothing should happen and in fact nothing happens we can push this button as much as we want nothing's changing because we're just going from 1 0 to 1 1 you can see 1 0 1 1 the output stays the same here doesn't change this nothing changes now the top one is a little bit more interesting so if we push the top button we're basically changing this here to a 1 so this I've just redrawn the circuit here and I'm changing just this top input to a 1 if we do that the top gate now has inputs of 1 and 0 and so 1 and 0 it ought to change it ought to turn off in output a 0 here if it does that that puts us here so 1 0 and 0 so that makes sense but now that changes the input of the bottom gain so now the bottom gate is getting a 0 and a 0 coming in well if a 0 and a 0 are coming in the bottom gate ought to be outputting a 1 so then it's going to switch on and when it switches on that changes the input on the top gate again so now the top gate is getting a 1 and a 1 well in that case a 1 and a 1 is still a 0 so the top gate doesn't switch it's already off and so now the circuit is stabilized in this configuration but what you see happens is before we had the top LED on or the top gate on and at the end of this whole sequence of events the bottom gate is on the top gate is off so let's give it a try I push the button here on top indeed we see the top goes off the bottom comes on so we're now in this state and really it did actually flip through all three of these states it just happened very quickly I mean on the order of nanoseconds really to get to this state here well now if we release the top button and this goes to a zero that puts us in this state here and a 0 and a 1 input on the top gate is still going to output a 0 0 and a 1 is still 0 and so nothing changes and so now with both inputs off we've swapped which of the gates is on so this is essentially a mirror image of where we started so we started with the top gate on the bottom gate off now we have the bottom gate or excuse me the top gate off and the bottom gate on and since this is the mirror image of this you know now pushing the top button doesn't have any effect whereas before remember when pushing the bottom button it didn't have any effect but pushing the bottom button is basically going to run through this same sequence of events but sort of in mirror image and flip from the bottom to the top and now the bottom is now in effect and so you can see by pushing these buttons we can kind of flip it between one state and the other and then once it's in that state it stays in that state until we until we push the button again or don't we push the other button I should say and so this is a fairly common circuit and it has a name it's called it's called an SR latch an SR stands for set reset because the the latch can essentially be set or reset and so these inputs are labeled you know typically labeled R s and then the outputs are labeled Q and inverse Q or not q and the way to sort of think about this is that if you want to set the Q output then you input a 1 on the on the s input or the set input so this sets it in the sense that the output Q is on and then this resets it in the sense that the output Q is off and then inverse Q is just always the opposite of whatever Q is and so sometimes you might see this this SR latch in circuit diagrams just as a box will draw it like this and it will have these inputs and they'll be labeled R and s and that's how you'll know it's an SR latch and it'll have the outputs Q and not Q and then usually they'll have a little bubble here to show that that's an inverted output and so if you see this in a circuit diagram you can basically think of it as this as this circuit or thing that works equivalently in the sense that if you press the if you or if you input a one on that on that set input then the queue will latch high and if you input a one on the reset input then the queue will latch low and then the not queue will sort of do the opposite and you might be wondering you know what happens if you input a one on both the set and reset at the same time and you know in that case the outputs are both you know both Q and not Q go go low and you know sometimes you might see this referred to as an invalid state because you you you know if you're sort of using this as a set reset latch you should only be setting it or resetting it at any point in time shouldn't be doing both so sometimes you might see that as an invalid State the other question you might have is you know when we first when we first powered this up you know it started with this top gate on and the bottom gate off and you might be wondering well why you know why does it prefer the top gate this thing is the circuit is fairly symmetrical and really it's it's just kind of almost random or arbitrary which one which one turns on first because when it first starts up all of these inputs are zeros and so if both inputs are zeros both gates want to turn on and in fact both gates might turn on but what will happen is one of them will turn on maybe a little bit faster and this signal will get to the other gate and this other gate will now have this one input and a 1 and a 0 will turn one of the gates off and so it's really just a question of which one turns on first and it's you know usually probably on the order of a few nanoseconds or one it's just a little bit faster than the other and so it just sort of arbitrarily one of them is going to turn on first and so if you're using the circuit in and using this you know this SR latch in a circuit you're going to want to make sure you you have some way of resetting it or setting it or getting it into some known state before you rely on its output I suppose and so because the SR latch has this property that you can set and reset it and it remembers the state that it's in you can imagine it might be useful in all sorts of computer memory and other applications and so in future videos we'll we'll build on this and look at some other types of latches and and other components that are used in computers

Channel: Ben Eater

Views: 778,289

Rating: 4.9622664 out of 5

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Id: KM0DdEaY5sY

Length: 12min 58sec (778 seconds)

Published: Wed Feb 17 2016

Reddit Comments

[u/BakersfieldChimp]

The first thing I thought was, "A latch diagram!"