SR flip flop question please help ty

Thread Starter

jake077

Joined Mar 14, 2012
8
if the current state of SR flip flop is 0 and the next state is also 0 the following inputs were used.

a) s = 1 r = 0
b) s = 0 r = x
c) s = x r = 0
d) none of the above.

Can someone please help me understand, i dont understand why is the asnwer is c?? ty
 

Thread Starter

jake077

Joined Mar 14, 2012
8
:((( i thought it was B but i don't understand why the answer key is C instead.
 

mlog

Joined Feb 11, 2012
276
The answer key must be wrong. The answer simply cannot be C.

Recall that answer C is "s = x r = 0." The "x" as applied to the set input (S) is a "don't care." A "don't care" means the input can be either a 0 or a 1. If the set input (S) is a logic 1, then it will set the SR latch, and the resulting output (Q) will be 1. Clearly the next state cannot be 0.

http://www.allaboutcircuits.com/vol_4/chpt_10/2.html
 

Thread Starter

jake077

Joined Mar 14, 2012
8
that's what i thought so too but i put my answered as B but i got marked it wrong and the answer key my prof post online the answer for this question is C. So i'm like how the hell did he come up with C as the answer. Doesn't make any sense to me at all.
 

mlog

Joined Feb 11, 2012
276
I would challenge the professor -- of course it's easy for me to say -- by proving that "c" is wrong. Actually you want to challenge the key and not the professor.

I basically did it for you. It is not uncommon for answer keys to be wrong for years.

Look at the link that I posted earlier. It shows the input-output state diagrams.

 
Last edited:

Georacer

Joined Nov 25, 2009
5,154
Could it be that your book examines a NAND SR latch? The inputs are reversed in that one.

In any case, a schematic post would be useful.
 
Last edited:

Brownout

Joined Jan 10, 2012
2,364
an S-R latch required a 'low' on S for a reset, or a low on R to set it. To get a zero ouput, the inputs are R=0, S=X. At least that was the way it was last time I looked at discreete logic. It would, of course, depend on the specific technology used to create the latch.
 
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