I'm working on a legacy product, so I can't adjust the crystal or board in any way. It was originally written in PicBasicPro and I'm moving things over to Hi-Tech C.

It's a Pic18F4525 and has a 20Mhz crystal. According to the datasheet, the PLL just multiplies the frequency by 4.

So, this would give me a frequency of 20Mhz


This would give me a frequency of 80Mhz


Surprisingly, by enabling HSPLL, the serial port outputs correctly, lcd still outputs, other than delay times I didn't see anything break.

So, my question is, what frequency is the clock running? Is it really running 80Mhz with PLL? or does the Hi-Tech C compiler know the max clock speed is 40Mhz and limit it there?

I will probably just stay safe and leave it at 20Mhz, but I would have expected serious problems clocking a 40Mhz chip to 80Mhz and didn't see any.


No, you can't enable the PLL with a 20 MHz crystal since 80 MHz clock rate is way out of spec. The PIC may appear to work, but there is no guarantee what it might screw up at what time without warning. Overclocking is a no-win game some clueless people engage in that think there is a free lunch to be had. You said "product", so you really really don't want to go there.

  • \$\begingroup\$ Thanks. I figured it was not a good idea, but was surprised to see it working. \$\endgroup\$ Mar 6 '12 at 18:25
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Please see my answer at here re "“Overclocking” an AVR" which applies well enough to be well worth looking at, even if I do say so myself :-).


I don't think


enables the PLL, which is why things are working.

I think you need:


because in \dat\cfgdata\18f4525.cfgdata, I see:

CVALUE:6:HSPLL:HS oscillator, PLL enabled (Clock Frequency = 4 x FOSC1)
CVALUE:2:HS:HS oscillator
  • \$\begingroup\$ That's what I have, I had a small typo, it got changed by editing. I'll clarify \$\endgroup\$ Mar 6 '12 at 18:15

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