SR flip flop question please help ty

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.

    Can someone please help me understand, i dont understand why is the asnwer is c?? ty
     
  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.
     
  5. jake077

    Thread Starter New Member

    Mar 14, 2012
    8
    0
    anyone?? please help
     
  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.

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

    [​IMG]
     
    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.
     
Loading...