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Basic Electronics » Understanding the clock rate and the crystal

May 27, 2010
by CyberGod
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What limits how fast the MCU can run. Why cant we just put 1GHz crystal instead of 14Mhz ? Is it how fast the switching in the internal transistors that limits the clock rate ?

May 27, 2010
by CyberGod
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One more thing. On some schematics people add two capacitors in series with the crystal to make sure it works fine. Is this really necessary ?

May 27, 2010
by n3ueaEMTP
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CyberGod, I cannot answer your first question, sorry. Your second question I can, yes (short answer).

Long Answer: When you use the crystal on the breadboard, the capacitors are not needed because there is a small about of capacitance in the rows of the breadboard in effect replacing them. When you make a project permanent, they are needed. The capacitors reduce noise in your circuit. For a more detailed explanation, check out section 8 of the MCUs datasheet (pg 26). Hope that helps.

May 27, 2010
by hevans
(NerdKits Staff)

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Hi CyberGod,

You have the right general idea the clock frequency is limited by the how fast you can get the signal across the logic components. At a basic level the chip is a bunch of logic gates (and,or,nand,etc) combined together to perform computations (add, subtract, and so on). In between these sets of combinational logic, you have memory elements that store state, and serve to hold the results of the logic gates. A digital circuit can only be clocked as fast as it takes for the signal to propagate across all the logic gates, and be stable again at the input to the storage elements. All of these elements that I am talking about are made up of transistors, so in a way it does all depend on how fast transistors can switch, but when settling on a clock frequency you have to consider that you are chaining lots of logic gates one after the other. This is a deep topic, if you want to read more about it, check out the MIT 6.004 (Computational Structures) Lecture Notes. Lecture 5 talks a lot about this, and you might want to take a look at Lecture 3 as well.

Hope that opens up a lot more questions =)

Humberto

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