Hi,

I've been running a 16F876 with a 10MHz XTAL with 22pF load capacitors.=20


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!

I just cannot get the oscillators/crystals to start up. Any ideas what I =
should check next? I'm going to buy some 32pF caps and try them soon.

Is the calculation correct? I don't understand as my 10MHz crystal works =
fine with 22pF caps and that's spec'ed  at 30pF load which would mean =
the 22pF caps are wrong, but yet it works fine? (I don't know the exact =
frequency though)

Thanks

Peter

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu