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I am not a very experienced firmware developer, so I came across this doubt when I was reading the datasheet of a microcontroller:

In my application, I will use a crystal to generate a clock signal of 26 MHz, the same frequency I want in my system's clock bus. Reading the section that explain about the PLL module, I found the table below:

Table from the manufacturer datasheet

In the first line, it is shown a use case where the output frequency is the same as the input frequency.

So, my question is: Is there any advantage to make use of a PLL module with frequency gain equals to one, as my source clock is already in the desired frequency?

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The use case for that would be for the PLL cleaning up the clock signal, i.e. lowering jitter. If you already have a good clock source, then there is no real benefit.

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    \$\begingroup\$ If your XTAL oscillator environment is so noisy the jitter needs to be run thru a PLL (on the same chip) for zero-crossing cleanup, then I suggest you not bother. Clean up the environment instead. \$\endgroup\$ – analogsystemsrf Jan 26 '18 at 5:21
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Is there any advantage to make use of a PLL module with frequency gain equals to one

Maybe not in your circuit but if you want a clock signal that is in quadrature to an existing clock signal then you can use a PLL. Even if the existing clock frequency moved +/- 20% (or more), the PLL clock would remain at 90 degrees to the original. I am describing a type I phase detecting PLL: -

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An EXOR is a type I phase detector and if used within a PLL loop it would naturally align the output to be mid-range on the detector i.e. \$\pi/2\$ or 90 degrees. Quite useful in some radio receivers.

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