Re: [avr-chat] Error in Timing.

[Erik Christiansen]

On Wed, Mar 01, 2006 at 11:31:14AM +0530, Royce Pereira wrote:
> I used 2 33pf caps in series at each pin to GND. This gives 16pF each.
> I set the low fuse bits to 0xFF, ckopt=1 (unprogrammed). This means 
> external xtal, 3-8 MHz, xtal osc-slowest rise time (right?).

Low fuse == 0xFF makes SUT == 0b11. That looks good for CKSEL == 0b1111.
It's hard to fault.

Stray capacitance may be increased unless the wiring to the capacitors,
crystal, and earth are kept very short. However, I wouldn't expect that
to pull the frequency very far.

Is the crystal just designed for a different oscillator circuit? e.g.
need 33 pf or more, or doesn't like the drive level? I'd try CKOPT = 0,
just for fun, before trying a more friendly crystal. It's unlikely that
you can pull the current one far enough by making one of the caps a
trimmer, as we did in the first digital clock for the Ford Falcon, a few
decades ago. (I remember asking the manufacturer to tweak the crystal,
to improve the adjustment range in our circuit, because of a smaller
frequency offset than you have.)

Erik