PLL and 18f252

jc71229 · Joined: 20 Jun 2006 Posts: 2

Hi folks,

I want to use the H4 fuse to enable PLL. My 18f252 gets an external clock of 6MHz on OSC1/CLKIN (same as http://www.ccsinfo.com/forum/viewtopic.php?t=27349&highlight=pll). However I can't get it into PLL mode (delays are 4 times slower as expected). Is this even possible with an external clock? I use

#fuses H4, NOWDT, NOPROTECT, PUT, NOBROWNOUT, NOLVP

for my fuse setting. I didn't notice that I have to do additional adjustments. My compiler is: CCS PCH C Compiler, Version 3.204, 24828 (Linux Version)

Anyone who can help?
Thanks
Jens

Ttelmah · Guest

It should work.
However there can be problems with the PLL, locking onto the fundamental, instead of running at *4, if the input signal is larger than expected (remember it expects to see the sinusoid from a crystal output, rather than a logic level 'square wave'). Double check that your programmer is setting the H4 fuse, and try with a series resistor 'in line' (perhaps 300R), or a series capacitor. I have used this with no trouble, at 4MHz, with a 220pF in series.

Best Wishes

asmallri · Joined: 12 Aug 2004 Posts: 1634 Location: Perth, Australia

Did you remember to power cycle the PIC after you programmed it the first time you set the H4 fuse?
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jc71229 · Joined: 20 Jun 2006 Posts: 2

Yep, that was it! Didn't know that I have to do a power cycle. Thanks so much asmallri + Ttelmah

Have fun
Jens