The system clock is needed to synchronize all components on the motherboard, which means they all do their work only if the clock is high; never when it's low. And because the clock speed is set above the longest time any signal needs to propagate through any circuit on the board, this system is preventing signals from arriving before other signals are ready and thus keeps everything safe and synchronized. The CPU clock has the same purpose, but is only used on the chip itself. Because the CPU needs to perform more operations per time than the motherboard, the CPU clock is much higher. And because we don't want to have another oscillator (e.g. because they also would need to be synchronized), the CPU just takes the system clock and multiplies it by a number, which is either fixed or unlocked (black edition models).

title={What are system clock and CPU clock; and what are their functions?},
author={Benjoyo (https://cs.stackexchange.com/users/27547/benjoyo)},
note={URL: https://cs.stackexchange.com/q/38243 (version: 2015-02-11)},
organization={Computer Science Stack Exchange}

This answer states that the clock components change state only when the signal is high. Why is it so?

  • $\begingroup$ Continue to read. $\endgroup$
    – quicksort
    Commented Oct 22, 2017 at 19:11
  • $\begingroup$ Please make your question self-contained, so we don't have to click through a link to understand what you are asking. As it stands, if the link stops working, the question becomes incomprehensible. Also provide a full citation for all references you refer to, if possible (e.g., title, authors, etc.). $\endgroup$
    – D.W.
    Commented Oct 23, 2017 at 6:03

1 Answer 1


It is not necessarily true, this is only one possible way of achieving synchronization. One could arbitrarily decide that it happens when the signal is low, or when the signal goes from low to high (rising edge), or high to low (falling edge), or even both. This wikipedia page contains more information.

  • $\begingroup$ I got one more question. How does one know when the state will change? $\endgroup$
    – user79161
    Commented Oct 23, 2017 at 12:34
  • $\begingroup$ The quick answer is you don't really know/it's irrelevant. Each subcircuit can use a different convention. If you have the schematic of the circuit you can find out. You can also look at the power consumption and see how it correlates with the clock. But modern circuits have several clock and clock domain anyway so different parts of the circuit can change at different times. $\endgroup$ Commented Oct 23, 2017 at 15:44

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