Pre scaling and post scaling is some kind of multiplier or divider to the actual clock signal, which will give the same signal as of we have connected a large value crystal.
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Here pre means at the input side while the post means operation done at the output.
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I know a predivider or postdivider with a PLL.
Because a PLL has a limited input range (e.g. form 1MHz to 2MHz) we need the pre dividing to connect a clock with a high frequency
A PLL often has also a limited output range (e.g. 250MHz to 500MHz) so you need a post divider to get the correct output frequency
What is the circuit which uses your pre/post scaled clock?