dsPIC33 and 80Mhz Clock

[JimKueneman]

Can someone explain how to get an 80Mhz clock (40Mhz Tcy) with these parts? If you look at the datasheet it shows the output of the PLL to be a max of 80Mhz and the max output of the VCO to be 200Mhz. But if you look at how to calculate N2

N2 = 2*(PLLPOST + 1)

But PLLPOST only can be set down to 2 so N2 has a minimum value of 6.

So going from the PLL output back to the VCO it is 80Mhz*6 which is 480Mhz so it is outside the 200Mhz range of the VCO?

But on the next page it says that the default start up is

N1 = 2;
N2 = 4;
M = 50;

I don't understand this..... what I am I missing?

Thanks,
Jim

[JimKueneman]

Also my scope does not agree well with any math that I try.

Jim

[JimKueneman]

I get it. The binary value compensates for the equation.