In the following video on SR latch (from University of Illinois) on youtube http://www.youtube.com/watch?v=CUfZladDqq8 ,
(The point illustrated in this composite screenshot!)
The "instructor" says the following 1mn 10 seconds into the main video --"Let's change S to 0(from 1),then with S and R as 0, both Q and Q' will hold their values"--Well that's what I don't get!!Now please try to see through the following clumsy description of my doubt and clear my confusion (I am a computer science student and this course is new for me,so please bear with me) :
How will Q and Q' hold their values?I mean, we are assuming that Q,which is fed as input to the bottom NOR gate,DOESN'T CHANGE IF S IS CHANGED FROM 1 TO 0!!!I completely don't get it.Isn't Q dependent on(is a function of) S and should change the moment S changes?I mean,S determines the value of Q'(For example S has to be 1 for Q' to be 0),and Q' in turn determines the value of Q.So, doesn't it mean it depends on S? Shouldn't Q change the moment S changes? You see, as per what I have studied so far, if there is a single logic gate, then we expect the output to be dependent on the inputs, and we expect it to change the moment any input changes, ISN'T IT?We don't expect the output to stay the same (or hold its value) if the inputs that determine it change.Then how come in the SR latch we expect the output Q of the upper NOR gate to hold its value when S changes from 1 to 0? Isn't it ironic that we automatically assume Q to remain the same(preserve its value) while the whole point of designing the SR latch is to preserve a value? Let me explain further.
Say in our circuit we use a voltage range of 0-2V to refer to 0(low) and a voltage range of 3-5V to refer to 1(high).So when we expect Q to be logical 1 or 3-5 Volts even after the input S that determines it changes, do we expect "SOMETHING" to hold that 3-5 volts of Q?What holds it?Is there somekind of ELECTRICAL REMNANCE,LATENCY or STORAGE involved?Or S changes much faster than Q could change!I am so utterly confused.I could just remember the whole thing by rote without understanding it, but I don't want to do that.So please answer what's up.I am sure you got what I mean to ask.The bottomline being---"S determines Q' which in turn determines Q, so when S changes, I expect Q to change or to be in an undefined state as the input indirectly determining it has changed.I just don't get why the instructor IMPLICITLY ASSUMES Q WILL STILL BE 1 when S changes to 0.