A processing circuit including a first oscillation circuit, a second oscillation circuit, a counting circuit, and a control circuit is provided. The first oscillation circuit receives an input voltage and generates a first clock signal according to the input voltage. The second oscillation circuit receives an output voltage and generates a second clock signal according to the output voltage. The counting circuit receives the output voltage. The counting circuit adjusts a first counter value according to the first clock signal and adjusts a second counter value according to the second clock signal. The control circuit receives the output voltage and determines whether the input voltage is experiencing an attack according to the first counter value and the second counter value. The first oscillation circuit operates in an un-protected power domain. The second oscillation circuit, the counting circuit, and the control circuit operate in a protected power domain.