" a cross-coupled R-S flip-flop realized with two NAND gates may lead to an oscillation "
Oscillation means output is going to change every time.
As, 2 NAND gates are there , other 2 might be AND gate.
Now, circuit diagram will be like this
A) Now, at first R is getting 1 , that corresponding NAND gate producing 0.
That 0 is input of another NAND gate,
So, NAND gate corresponding to S getting input as 1 NAND 0=1 (As shown in picture below)
Now, it's 1st output is 0 1
Next, 0 1 ouput is giving input to R and S
So, AND gate of R is producing 0 AND 1=0
NAND gate of R is giving 0 NAND 1=1(See picture again)
Similarly , AND gate of S getting input 1AND 1=1
R value previously was 0 (We donot know, R will produce first or S will produce first, So, taking previous value of R)
for NAND gate it is getting 0 NAND 1=1
So, 2nd output here is 1 1
Similarly keep check 1 1 as input as next AND gates
it will produce 0 0
So, if we put 11 as first input, it will keep toggleing , producing 01--11--00--11--00-------------------
D) if we give 00 first in same circuit it will produce as output 00--11--11--11--11------------------
So, output will not toggle ,after reaching 11 state.
B) if we give 01 as input it will produce 10--11--10--11----------------
means only S (or we can say any one output) will toggle , but not both.
C)Similar reason as B)