Power monitoring circuitry is provided, comprising a capacitor configured to receive a current, so as to charge the capacitor and a switching device, connected to the capacitor. The switching device is configured to periodically discharge the capacitor in response to receipt of a clock signal from a circuit being monitored. The power monitoring circuitry also comprises a comparator, configured to perform a comparison of a voltage developed by the capacitor with a threshold voltage, and to output an indication of a change in power supplied to the circuit in response to the comparison. Other embodiments are also described.