2.2 INCREASING PROCESSOR CLOCK FREQUENCY

Increasing the system clock frequency allows the computer to execute more instructions per unit time. However, logic gates need time to switch states and system buses need time to be charged or discharged through bus drivers. These delays are closely tied to the underlying silicon technology such as NMOS, CMOS, and bipolar. The type of gate circuits also dictate the clock speed, such as using CMOS or domino logic or current-mode logic. There is also a fundamental limit on how fast a chip could run based on dynamic power dissipation. Dynamic power dissipation is given approximately by

(2.1)

where C is the total parasitic capacitance, f is the clock frequency, and V is the power supply voltage. Engineers developed many techniques to reduce power consumption of the chip while raising the clock frequency. One obvious solution was to reduce the value of C through finer lithographic process resolution. A bigger impact resulted when the chip power supply voltage was reduced from 5.0 to 2.2 V and then 1.2 V, and the question is how much the supply voltage can keep scaling down without affecting the gate switching noise margin.