Avian’s Blog

Recently I've heard some quite interesting (and wrong) theories from people that are overclocking their CPUs. So I compiled this short list of overclocker's facts together with the underlying reasons on the level of transistor circuits. As far as I know, these simplified explanations are correct for static and dynamic CMOS circuits (which covers all consumer VLSI (CPUs, GPUs, etc.) circuits on the market today).

Heat output is proportional to clock frequency.

Reason: CMOS circuits consume power (and generate heat) only when they switch from one state to the other. Energy consumed by one gate for one state transition is approximately constant, because this is the energy stored in the input capacitance of the next gate in the circuit (static CMOS circuits also consume power for a very short time during state transition when a direct connection of supply and ground lines exists, but the principle is the same). So the power consumption of the whole circuit is proportional to the number of state transitions per second. In a complicated microprocessor a portion of all gates will switch each clock cycle. So more clock cycles per second means more gate switches means higher heat output.

A circuit can operate at higher frequencies when cooled to a lower temperature.

Reason: MOS transistors in a CMOS circuit act as switches. Unlike mechanical switches that have a constant resistance in "on" state, MOSFETs have a more complicated relationship between current and voltage. If a higher current can flow through the transistor at a given voltage, it will charge or discharge the input capacitance of the next gate faster, thus allowing the circuit to operate reliable at more state transitions per second (higher clock frequency). It turns out that the current/voltage relationship for a MOS transistor is proportional to mobility of electrons and holes (charge carriers) in silicon. Charge mobility (see this article) is higher at lower temperatures, so circuits will work faster when cooled.

Heat output is proportional to the square of supply voltage.

Reason: As I mentioned earlier, the power consumption of CMOS circuits comes from charge and discharge cycles of gate capacitances. The energy stored in a capacitor is proportional to the square of voltage on its terminals (W=(C U²)/2). Since capacitors in CMOS circuits are charged to supply voltage and discharged to 0V, each state transition consumes energy that is proportional to the square of supply voltage.

A circuit can operate at higher frequencies when connected to a higher supply voltage.

Reason: Normally when a capacitor is discharged through a resistor, time needed for the capacitor voltage to reach, let's say 25%, of its initial value, is independent of the initial voltage. MOS transistor switch however will let through more current when it has a higher control (gate) voltage. This is similar to the case where the resistor through which the capacitor is discharging has lower resistance. In CMOS circuits, the transistor gate voltage is equal to the supply voltage, so the gate capacitances will be charged or discharged faster with higher supply voltages and the circuit will work with higher clock frequencies.