The 2550/4550 etc type chips have a really nice PLL in the oscillator stage.

The PLL always ends up running at 96 Mhz, and to do so, it needs a reference of 4mhz. So if you're using a 20Mhz crystal, it has to be divided by 5 with the prescaler to get back down to 4Mhz. That's the _PLLDIV_?_1L

Code:
Crystal
4  - _PLLDIV_1_1L
8  - _PLLDIV_2_1L
12 - _PLLDIV_3_1L
16 - _PLLDIV_4_1L
20 - _PLLDIV_5_1L
24 - _PLLDIV_6_1L

External Oscillator
40 - _PLLDIV_10_1L
48 - _PLLDIV_12_1L
Then once you have the 4 Mhz reference, the PLL will run at 96 Mhz.
A postscaler can divide that by 2 to give 48 Mhz to the USB. _USBDIV_2_1L

And a seperate postscaler can give the CPU 48, 32, 24 or 16 Mhz depending on the _CPUDIV_OSC1_PLL2_1L config setting.

In effect, you can get any of these frequencies, no matter which one of the (multiple of 4Mhz) crystal you use.

You can have a 4Mhz crystal supplying 48Mhz to the USB and 16 Mhz to the CPU, or both USB and CPU at 48Mhz.

Clear as Mud, right.
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