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Enabling PLL on 16F1847 - SOLVED
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Ttelmah



Joined: 11 Mar 2010
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PostPosted: Wed Apr 27, 2016 12:30 am     Reply with quote
What there should be is perhaps a warning saying 'software PLL mode automatically selected'.

You'll find if you fuse enable the PLL, with a crystal selected (or an external oscillator), the fuse will function exactly as you expect.
The chip can't actually boot in the PLL mode with the internal oscillator.
If you look at the data sheet, you have:

Quote:

"Note:"
"Following any Reset, the IRCF<3:0> bits"
"of the OSCCON register are set to ‘0111’"
"and the frequency selection is set to"
"500 kHz. The user can modify the IRCF"
"bits to select a different frequency."


Now the PLL can _only_ be used with 8Mhz selected. So the chip cannot boot up at 8MHz * 4.

The actual boot is done at 500Khz, then the compiler switches the oscillator to 8Mhz, and then enables the PLL bit.

This is the sequence you would have to do if working in assembler. The compiler does it for you.

Behind the scenes, the compiler prevents the PLL fuse from being selected if in RC mode (since this would result in a non-running chip)....
gjs_rsdi



Joined: 06 Feb 2006
Posts: 476
Location: Bali

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PostPosted: Wed Apr 27, 2016 3:43 pm     Reply with quote
Thank you for the explanation Ttelmah.

Building a new board and will test with an external oscillator at 8MHz and 32MHz (using PLL)
Will post the results

Best wishes
Joe
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