Discussion in 'Homework Help' started by jake077, Mar 26, 2012.

1. ### jake077 Thread Starter New Member

Mar 14, 2012
8
0
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.

2. ### mlog Member

Feb 11, 2012
276
36
That makes two of us.

3. ### jake077 Thread Starter New Member

Mar 14, 2012
8
0
(( i thought it was B but i don't understand why the answer key is C instead.

4. ### mlog Member

Feb 11, 2012
276
36
I think it's B too.

Mar 14, 2012
8
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6. ### mlog Member

Feb 11, 2012
276
36
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.

7. ### jake077 Thread Starter New Member

Mar 14, 2012
8
0
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.

8. ### mlog Member

Feb 11, 2012
276
36
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: Mar 27, 2012
9. ### Georacer Moderator

Nov 25, 2009
5,142
1,266
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: Mar 27, 2012
10. ### Brownout Well-Known Member

Jan 10, 2012
2,375
998
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.