18F4520 and internal osc & PLL

[Bruce]

Change OSCCON = $72 to OSCCON = $70 (%01110000).

[pwhitt]

Hi Bruce,

Thanks very much. I actually got the same advice from PIC Basic support - although the $72 value was given to me by support earlier, and matched teh value I was using. This does indeed fix it - I see now that the PIC is configured to use the "main oscillator" rather than the internal - but could you please explain why? I was assuming the internal oscillator was everything internal and the main oscillator was an external crystal or RC.

To avoid any confusion to those that may notice - the first OSCCON value I posted had OSCCON=$7F. This was copied/pasted from a file in which I was thoughtlessly monkeying with the values attempting to fix it.

[Bruce]

The primary oscillators include the External Crystal and Resonator modes, the
External RC modes, the External Clock modes and the internal oscillator block.

You set the "primary" oscillator with FOSC3:FOSC0 configuration bits - so with
the internal oscillator block set in CONFIG1H, and 8MHz or 4MHz selected in
OSCCON, you need System Clock Select bits SCS1:SCS0 set for "primary" for
PLL to work.

The PLL is available only when it's configured to use the internal oscillator
block as its "primary" clock source (FOSC3:FOSC0 = 1001 or 1000).

It's not exactly hit-you-in-the-face clear from looking at the datasheet, but
that's the only way to turn on PLL when using the internal osc.

[pwhitt]

Thanks so much for clearing that up Bruce - excellect.

[juliogbox]

I had exactly the same problem with PIC18F1330. But I could fix it using the Bruce advice:
movlw B'0111000'
movwf OSCCON, A

But there is a strange behavior of the MPLAB-SIM. If I do this:
movlw B'0111011'
movwf OSCCON, A

The simulator do not write the two lower bits, as you can see in this image (click to enlarge):



But in the hardware the two bits are written, that's the reason because the PLL hadn't worked in my prototype.