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All CPU circuitry operates according to the system clock. Some CPU circuits work according to the rising edge of the clock and some on the falling edge of the clock. The ideal clock is a square wave with specified duty cycle. The required duty cycles (100% * time of high in a period/ period) for the clock input differ from different microprocessors. Each CPU specifies a maximum clock speed and minimum clock speed. The source of clock can be from a quartz crystal, or an RC circuits, or an external circuit. Most CPUs are driven by quartz crystal. Each CPU has its own suggested ways to connect a clock source to it. The frequency of the crystal should be chosen as an integer multiple of the desired system clock frequency (depends on the given CPU).