Post by thread:LP XTAL

Hi all, I have a problem regarding the LP (low frequency Xtal) setting. I have tried to run the PIC (F84) with a 35Khz Xtal ratted from an old cheap digital watch. For some reason it isn't working. I have 33pF cap's on it and it seems that when I load it up with a CRO lead, or even just touch it, the oscillator winds up and kicks into life. Is 33pF too small for the lower frequency?? Anyone know how this animal works? Sincerely John

John Orhan wrote: > I have a problem regarding the LP (low frequency Xtal) setting. I have tried > to run the PIC (F84) with a 35Khz Xtal ratted from an old cheap digital > watch. For some reason it isn't working. I have 33pF cap's on it and it > seems that when I load it up with a CRO lead, or even just touch it, the > oscillator winds up and kicks into life. Is 33pF too small for the lower > frequency?? Anyone know how this animal works? See the section on 'Crystal/Ceramic resonator operation' in the databook. You need to add a small series resistance to OCS2 to create noise. "2: A series resistor (RS) may be required for AT strip cut crystals". AFAIK 32khz crystals are strip cut. AT refers to the crystal axis used for that cut (just as XT refers to that axis). The various axis of a crystal have different properties (tempco, stability, etc.) and so a crystal is not -just- a crystal, you have to get one that is cut to match your application. -- Robert.Rolf-AT-UAlberta.ca

Hi Robert, Thanks for your tip. John -----Original Message----- From: Robert Rolf [spam_OUTRobert.RolfTakeThisOuTUALBERTA.CA] Sent: Thursday, 30 March 2000 2:23 To: .....PICLISTKILLspam@spam@MITVMA.MIT.EDU Subject: Re: LP XTAL John Orhan wrote: > I have a problem regarding the LP (low frequency Xtal) setting. I have tried > to run the PIC (F84) with a 35Khz Xtal ratted from an old cheap digital > watch. For some reason it isn't working. I have 33pF cap's on it and it > seems that when I load it up with a CRO lead, or even just touch it, the > oscillator winds up and kicks into life. Is 33pF too small for the lower > frequency?? Anyone know how this animal works? See the section on 'Crystal/Ceramic resonator operation' in the databook. You need to add a small series resistance to OCS2 to create noise. "2: A series resistor (RS) may be required for AT strip cut crystals". AFAIK 32khz crystals are strip cut. AT refers to the crystal axis used for that cut (just as XT refers to that axis). The various axis of a crystal have different properties (tempco, stability, etc.) and so a crystal is not -just- a crystal, you have to get one that is cut to match your application. -- Robert.Rolf-AT-UAlberta.ca

Also on the PIC data sheets you'll note the oscillator selection, LP with a 32KHz crystal needs about 68-100pf caps. I'm running mine now with 100pf, no series resistance, and everything is fine. -Adam John Orhan wrote: {Quote hidden}> > Hi all, > > I have a problem regarding the LP (low frequency Xtal) setting. I have tried > to run the PIC (F84) with a 35Khz Xtal ratted from an old cheap digital > watch. For some reason it isn't working. I have 33pF cap's on it and it > seems that when I load it up with a CRO lead, or even just touch it, the > oscillator winds up and kicks into life. Is 33pF too small for the lower > frequency?? Anyone know how this animal works? > > Sincerely John