Re: Problem with TI TMS320DM6446 and crystal not starting

On Sat, 15 Aug 2009 16:49:15 -0700, Joerg <invalid@xxxxxxxxxxxxxxx>
wrote:

KJ wrote:
On Aug 15, 4:07 pm, n...@xxxxxxxxxxx (Nico Coesel) wrote:

Some crystal oscillators require a 1M resistor across the crytal. Did
you try that? What does TI say about this?


The resistor across the crystal is to force some form of 'digital'
invertor to operate in a linear region. This can also be accomplished
on chip with a transistor. Since TI says to only connect the caps and
no resistor, the implication is that the 6446 takes care of the
biasing on chip.


Well, maybe it does. But maybe not. I second Nico's suggestion. Take one
of the worst boards and just try it. Also, does this processor offer the
ability to select different burden caps inside? If yes then those code
settings might be off.

If all that fails I suggest you post the layout, only the area with the
crystal and the bypassing in the vicinity.

The data *** shows the "burden" capacitors as external. One mistake
often made by amateurs is not following the directions for the right
values of those capacitors, dependent on _frequency_.

Also, depending upon who designed it, the oscillator may or may not
include the drive-limiting series R. Particularly problematic if the
designers are located in that up and coming "English-speaking"
democracy... you know the one I mean... Microchip is fond of doing
that ;-)

My rule of thumb is to use an RC from the output of the oscillator,
providing 45° of phase shift at the crystal frequency, the other
capacitor is not nearly as critical (particularly with high-Q
resonators) unless you overdo it and lose loop gain.

...Jim Thompson
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