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clock skew
The operation of most digital circuit systems, such as computer systems, is synchronized by a periodic signal known as a "clock" that dictates the sequence and pacing of the devices on the circuit. This clock is distributed from a single source to all the memory elements of the circuit, which are also called registers or flip-flops. In a circuit using edge-triggered registers, when the clock edge or tick arrives at a register, the register transfers the register input to the register output, and these new output values flow through combinational logic to provide the values at register inputs for the next clock tick. Ideally, the input to each memory element reaches its final value in time for the next clock tick so that the behavior of the whole circuit can be predicted exactly. The maximum speed at which a system can run must account for the variance that occurs between the various elements of a circuit due to differences in physical composition, temperature, and path length.
In a synchronous circuit, two registers, or flip-flops, are said to be "sequentially adjacent" if a logic path connects them. Given two sequentially-adjacent registers R''i'' and R''j'' with clock arrival times at destination and source register clock pins equal to TC''i'' and TC''j'' respectively, clock skew can be defined as: .
==In circuit design==
In circuit designs, clock skew (sometimes called timing skew) is a phenomenon in synchronous circuits in which the clock signal (sent from the clock circuit) arrives at different components at different times. This can be caused by many different things, such as wire-interconnect length, temperature variations, variation in intermediate devices, capacitive coupling, material imperfections, and differences in input capacitance on the clock inputs of devices using the clock. As the clock rate of a circuit increases, timing becomes more critical and less variation can be tolerated if the circuit is to function properly.
There are two types of clock skew: negative skew and positive skew. Positive skew occurs when the transmitting register receives the clock tick earlier than the receiving register. Negative skew is the opposite: the receiving register gets the clock tick earlier than the sending register. Zero clock skew refers to the arrival of the clock tick simultaneously at transmitting and receiving register.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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