USB1.1 needs a 48MHz clock to be divided, so a 12MHz crystal oscillator is required, and the frequency is 4 times that of 48MHz. Why don’t choose 48MHz Crystal Oscillator directly? Because a 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 a low-frequency crystal oscillator with second harmonic generation. But it’s not absolute, many circuits are often used a crystal oscillator of 50M or more. That’s a key point that we need special attention to when we do PCB layout.
USB1.1 needs a 48MHz clock to be divided, so a 12MHz crystal oscillator is required, and the frequency is 4 times that of 48MHz. Why don’t choose 48MHz Crystal Oscillator directly?
Because a 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 a low-frequency crystal oscillator with second harmonic generation. But it’s not absolute, many circuits are often used a crystal oscillator of 50M or more. That’s a key point that we need special attention to when we do PCB layout.
Why we use a 32.768KHz crystal oscillator when the RTC clock needs a more accurate 1S (1 second)?
Because all digital circuits are expressed by the binary system,32768 is 1000000000000000 in a 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 a clock, for example, an RC circuit, but it’s not very accurate. Many single-chip microcomputers do not have a crystal oscillator because there is an RC circuit in its internal.
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