A circuit includes a voltage generator and a sensing device. The voltage generator includes a first output and a second output. The first output is configured to output a word line voltage, and the second output is configured to output a flag signal indicates a relationship between the word line voltage and a reference signal. The sensing device includes a first input and a third output. The first input is coupled to the second output of the voltage generator, and the third output of the sensing device is configured to output a value corresponding to capacitance change of word line capacitance loading based on the flag signal.