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Electronics: Clock Signals
Clock signals control the pulse of synchronous digital system circuits - action within them is synchronised to the clock pulse. The action can be set to happen on the rising edge or falling edge of the pulse within the hardware design.
A clock pulse signal is a square wave. In a square wave, voltage varies by periodically switching from high to low state (no 'in between' values), it is therefore a digital signal as there are only two states. The trace above shows equal times of high and low - but that does not have to be the case. Some clock signals spend longer high than low or vice versa. The period (T) of the clock pulse is the time in seconds taken for one complete cycle. The frequency (f) measured in hertz (Hz) of the clock cycle can be found by calculating the reciprocal of the period. f = 1/T
The triangle < next to the CLOCK input on a combinational logic device shows that it is edge-triggered, that is, it responds to sudden changes in voltage, but not to slow changes or to steady logic levels.
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A clock input to a register or flip-flop is represented as shown in the diagram on the right. Sometimes the letters CLK are not present - the < is enough for an electronics engineer to know that the input is a clock signal. 
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