problem whit pll h4 in pic18f6620

Roberto da Rivoli · Joined: 09 Feb 2004 Posts: 37 Location: TURIN -Italy-

I use pic18f6620 with xtal 10 MHz and fuses HS, H4 set. but micro it works only to 10MHz

in fact the seriale works correctly only with

# use delay(clock=10000000)

Ttelmah · Guest

You should only have H4 set, not 'HS'. However the patterns should 'OR' together without causing a problem.
Try with just H4.
If this fails, try adding a resistor _in series_ with your crystal of about 50R. There was a problem in the past on many PIC's, that the PLL, is sensitive to being overdriven, and will not run properly if the gain of the crystal is higher than expected. This is why the spec requires a parallel cut crystal. Adding the resistor can help if the crystal gain is just a little too high.

Best Wishes

sseidman · Joined: 14 Mar 2005 Posts: 159

Also, I seem to remember the need to actually power down the chip when changing between HS and H4

Scott

Guest · Posted: Fri Jul 29, 2005 1:36 pm

sseidman is absolutely right. It caused me hours of troubleshooting in the past. You need to program the config, completely power down the PIC, and then it should be running with the PLL enabled.

I have used that exact circuit configuration in the past, PIC + 10MHz ceramic resonator without any problems.

Darren Rook · Joined: 06 Sep 2003 Posts: 287 Location: Milwaukee, WI

Also, there's a flaw in the 18F6620 in that it doesn't work properly over 25MHz.