The veteran’s answer about Crystal Oscillator (Part two)

USB1.1 needs 48MHz clock to be divided, so 12MHz crystal oscillator is required, and the frequency is 4 times that of 48MHz. Why don’t choose 48MHz Crystal Oscillator directly?

Because high frequency crystal oscillator is easy to be affected by peripheral circuits or affect peripheral circuits. Most of the time,we get high frequency by using low frequency crystal oscillator with second harmonic generation. But it’s not absolute,many circuits is often used a crystal oscillator of 50M or more. That’s a key point that we need a special attention when we do PCB layout.

 

Why we use 32.768KHz crystal oscillator when the RTC clock needs a more accurate 1S (1 second)?

Because all digital circuits are expressed by binary system,32768 is 1000000000000000 in binary system. So as long as we detect the highest bit changes, we will know that 1 second, don’t need to check every bit, the circuit is more simple.

 

Does the clock have to be generated by the crystal oscillator?

No,there are several ways to generate clock,for example RC circuit,but it’s not very accurate. Many single-chip microcomputers do not have the crystal oscillator because of there is a RC circuit in it’s internal.

 

(the end)

 

 

 

 

 

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Mike



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