Lpc2000

Some opinions needed:
If I'm running CPU clock at 18MHz with an external 18MHz crystal,
which is more reliable (over full temperature/voltage range):
-  Switch off the PLL (clock source from external crystal/OSC)
-  Turn on 1x PLL  (Clock source from PLL VCO/CCO)
Regards

At 01:12 AM 4/4/2006 +0000, unity0724 wrote:
>Some opinions needed:
>If I'm running CPU clock at 18MHz with an external 18MHz crystal,
>which is more reliable (over full temperature/voltage range):
>-  Switch off the PLL (clock source from external crystal/OSC)
>-  Turn on 1x PLL  (Clock source from PLL VCO/CCO)

The PLL almost certainly can't be more reliable than the oscillator since 
it uses the oscillator as an input. I'd just use the oscillator and leave 
the PLL off.  You do gain in having a simpler circuit and marginally 
simpler software but I doubt it makes a great deal of difference in 
reliability.  More noticeable would be a reduction in power requirements 
and perhaps EMI.

On the other hand unless there is a real difference in reliability why 
bother with the PLL

Robert

" 'Freedom' has no meaning of itself.  There are always restrictions,   be 
they legal, genetic, or physical.  If you don't believe me, try to chew a 
radio signal. "  -- Kelvin Throop, III
http://www.aeolusdevelopment.com/

--- In lpc2000@yahoogroups.com, Robert Adsett <subscriptions@...> 
wrote:
>
> At 01:12 AM 4/4/2006 +0000, unity0724 wrote:
> >Some opinions needed:
> >If I'm running CPU clock at 18MHz with an external 18MHz crystal,
> >which is more reliable (over full temperature/voltage range):
> >-  Switch off the PLL (clock source from external crystal/OSC)
> >-  Turn on 1x PLL  (Clock source from PLL VCO/CCO)
> 
> The PLL almost certainly can't be more reliable than the 
> oscillator since it uses the oscillator as an input. I'd just 
> use the oscillator and leave the PLL off.  You do gain in 
> having a simpler circuit and marginally simpler software
> but I doubt it makes a great deal of difference in 
> reliability.  More noticeable would be a reduction in power 
> requirements and perhaps EMI.
> 
> On the other hand unless there is a real difference in 
> reliability why bother with the PLL
> 
> Robert
> 

Umm... Somehow I thought of PLL/VCO with low pass filter might:
- Reject any noise, glitch, short pulses on the oscillator
  (e.g.,  there might be a surge/lightning nearby affecting the 
   OSC, but not the PLL)
- PLL/VCO gives 50% duty cycle clock to CPU
Any idea which clock source is more stable??
Regards