If I have a PIC18 microcontroller with an 8MHz crystal and 4xPLL, is Fosc 8MHz or 32MHz?
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Fosc is the clock eventually with PLL, so with an 8MHz crystal and 4xPLL you would have an Fosc of 32MHz. However, note that this is divided by 4 internally, so that you will be able to handle 8 million instructions per second (MIPS).
I found this in the 18F2520/2620/4525/4620 datasheet, table 17-3 on page 187. You can see that Fcy, the cycle frequency, equals Fosc/4. Fosc therefore has to be with PLL.