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Pll multiplication factor Pre and post

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I have seen in several instances in micro controllers that for example in pll there is always a pre multiplication factor and post multiplication factors. Why such pre and post ? why can't there be single multiplication factor. What is the advantage of having two factors.

Ian Rogers

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Take the pic32.... You have a peripheral bus clock .. A USB clock... A system clock..

To run the PLL for the USB you need a 4Mhz crystal as the PLL is 24:1... Therefore if you hookup an 8Mhz crystal you need to divide the FOSC by 2... We use an 8Mhz crystal as the system can run at 80Mhz with a simple 10:1 and the peripheral bus can then run at 40Mhz...

All these frequencies???? We need to run the USB at full speed.... WE want to run the system as fast as possible... The peripherals cannot run at the system speed....

That's why we are given the option to pre and post scale....
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