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My PIC datasheet (PIC24HJ128GP202) specifies three parameters to configure the PLL which are N1, N2 and M. Each of them have to satisfy some clock frequencie ranges in input/output of each block inside the PLL. I looked all around the datasheet and did not found the answer for this question:

what happens if I do not respect these ranges?

I mean, in the final output for example. The frequency should be between 12.5 MHz and 80MHz. What happens if I select PLLPOST (N2) which leads me to 80.5 MHz? Or yet select a PLLDIV (M) which leads me to 201MHz (when PLLPOST intput should be between 100-200 MHz)?

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  • \$\begingroup\$ Why would you wanna do that? Just for the sake of it? \$\endgroup\$ – Dzarda Jan 30 '14 at 22:35
  • \$\begingroup\$ First of all is curiosity. But mainly because I could use FRC frequencies and these are usually not a round number. So I can get only values like 198,... MHz or 201,..MHz. \$\endgroup\$ – Felipe_Ribas Jan 30 '14 at 22:38
  • \$\begingroup\$ Interesting... I am not a PIC-oriented guy though, so no specific tips from me unfortunately. \$\endgroup\$ – Dzarda Jan 30 '14 at 22:41
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Generally when you go outside what is recommended the behavior is undefined. It's not guaranteed to work, but neither is it guaranteed not to work. It might work when it's cold but not when it's hot. It might work at 3.3V Vdd but not at 3.29V Vdd. It might work on 19 units and fail on the 20th.

That said, if everything else was nominal (not too hot or cold, power supply near nominal) it's unlikely, in my (somewhat ill-considered) opinion, that they could make it tetchy enough that it would work perfectly at 200MHz and fail at 201, but if it does happen to behave that way you have nobody to blame but yourself.

From the datasheet:

enter image description here

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I don't know about all the parameters you've mentioned, but I know I ran into a problem when I accidentally set things up so I was above 70 MHz on one of the newer 70 MIPS PIC24E parts. I brought it back down to 69 MHz (leaving a little wiggle room since I was using the internal oscillator), and everything worked fine again.

BTW, to avoid you're having to ask, I don't remember anymore how far above 70 MHz I was.

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