A computing device’s power consumption depends on its current activity. The consumption depends on changes of state of its components, rather than on the states themselves, because of the nature of CMOS transistors. When an input voltage is applied to a CMOS inverter, a transient short-circuit is induced. The rise of the current during this transient is much higher than the static dissipation caused by parasitic current leakage. Using a 10-20 resistor in the power supply line, these current fluctuations can be measured. To achieve good results, measurements should be made with at least 12-bit resolution and 50 MHz sampling frequency. Such acquisition parameters allow us to distinguish between different CPU instructions and estimate the number of bus bits changing at a time.

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