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Offline faceu  
#1 Posted : Thursday, November 17, 2016 8:20:28 AM(UTC)
faceu


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Joined: 11/16/2016(UTC)
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Location: hongkong

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

Quote:
__CONFIG(1, HS & IESOEN & FCMEN);

This would give me a frequency of 80Mhz

Quote:
__CONFIG(1, HSPLL & IESOEN & FCMEN);

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.

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