In-Reply-To: <003301c04cbd$ae03fa00$9018ff3e@atlas800> Digital Signal Processing wrote: > Hi, > > I've been running a 16F876 with a 10MHz XTAL with 22pF load capacitors. > > I then wanted to move up to 16MHz (The PIC is a 20MHz part) > > I have used the formula... > > Load Capacitance Required =3D (C1*C2)/(C1+C2) > > Where C1 and C2 are the external parallel load capacitors. Now I've also = > assumed that the parasistic capacitance of the PIC pins to be around = > 5->8pF . So C1 =3D C1a + C1p (C_actual + C_parasitic) Of course the = > same goes for C2. > > Normally C1 and C2 are made the same so the formula becomes... > > Load Required =3D (C1*C1)(2*C1) =3D C1 / 2 > > So my crystal (16MHz) states in the data book that it needs 16pF load. > > Therefore > > 16pF =3D C1/2 =3D (C1a + C1p)/2 =3D (C1a + 5pF)/2 > 2*16 - 5 =3D C1a =3D 27pF > > So I put two 22pF capacitors as the load and nothing! This formula, using the specified load capacitance of the crystal is nothing to do with getting the oscillator to oscillate strongly and reliably. The specified load capacitance of the crystal is the load required for it to resonate at exactly the specified frequency, rather than at a slightly higher or lower frequency. If you do not need frequency accuracy down to a few tens of ppm (and one often doesn't) you'll do better to forget the specified load capacitance and refer to the suggested capacitors in the PIC data sheet. Having said that I would expect something to happen with a 16MHz crystal and two 22pF capacitors. Is it possible that these crystals are not of very good quality? Also have you considered using a ready made oscillator module. Brian Gregory. briang@cix.co.uk -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu