Which of the following input sequences for a cross-coupled R-S flip-flop…

Answer: 00, 11

Explanation: For a cross-coupled R–S latch implemented with NAND gates, a 0 on an input is the active (asserting) level. The input pair 0,0 is the forbidden state where both outputs are driven high. If the inputs then change simultaneously to 1,1, both outputs attempt to change based on the other's value.

• Step 1: At inputs 0,0 both outputs go to 1 (forbidden state).

• Step 2: When inputs change simultaneously to 1,1, each output must switch according to the feedback from the other output.

• Step 3: Because gates have propagation delays and the loop is symmetric, the outputs can alternately drive each other and produce an oscillation or prolonged metastability until some asymmetry settles the loop.

Why other sequences do not oscillate: Sequences that assert only one input at a time (for example 0,1 or 1,0 in either order) implement a definite set or reset; the feedback loop drives the latch to a stable complementary output pair, so no oscillation occurs.

Mitigation: Avoid applying both inputs low simultaneously, use edge-triggered flip-flops, or add controlled timing/hysteresis so inputs do not transition to 1,1 simultaneously from 0,0.