[SI-LIST] Re: Single crystal driving two different inputs of a chip
- From: "Andrew Ingraham" <a.ingraham@xxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Wed, 29 Jun 2005 08:23:38 -0400
> Drivig two CMOS inputs should be no problem - they present much less load
> than AS and F chips used to present in the past (somewhat more
> capacitance,
> but should be no issue).
I'd check on that if I were you. I've seen clock inputs even on recent
chips that presented substantial loads.
Plus, it used to be practice on some ASICs to omit input buffers on clock
inputs, instead relying on the external clock source to directly drive the
internal clock tree. That could raise the input capacitance, as well as
cause other input specs (input thresholds) to differ from normal.
> Sorry for the confusion what i meant is connecting a single crystal
> resonator to two
> oscillators within a chip
Oooh, now you have a very different situation! I think most people would
not recommend doing this.
Understand that a crystal is a passive device. Typically the crystal pins
on an IC connect it to the output and feedback pins of a gain device (a
buffer). In any event, some signal needs to come out of the IC to the
external crystal, for the crystal to do anything. At least one of the pins
must be an output from the IC (or bidirectional).
If you connected a crystal to two of these pairs of pins, you could be
connecting the outputs of two buffers together. I really don't think you
want to attempt this, without consulting the IC vendor for their advice.
Often, one of the two pins for each oscillator gain block can be driven by
an external source. It might be possible to connect the output pin of one
gain device, to the input pin of the other (and leave its output pin not
connected). Again, consult the IC vendor.
I am a little puzzled about the problem you are trying to solve. How does
process variation cause jitter? How does it degrade the audio quality?
Maybe jitter isn't the problem.
Regards,
Andy
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