how to write code to realize n times frequency?

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In order to multiply the frequency you need a fast oscillator (this can be done) a phase detector (this also can be done, perhaps)

and a method to modify the oscillator frequency (very difficult in FPGA).

Even if it works the performance should be very poor.

What about using an external PLL/DLL component?

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Thanks for your reply!

Yes，you're right. It's OK to use an external PLL/DLL, but It maybe a little complex if we don't have an external PLL/DLL and can't use the CPLD/FPGA's existing resources.

Now I only want to know how to use code to realize it. and I believe It's workable.