Lpc2000

derbaier wrote:

>The 32KHz "tuning fork" crystals used in watches and microcontroller
>sleep circuits have a maximum drive rating.  IIRC it is on the order
>of 5 microwatts or less.  The resistor is not intended to have any
>direct effect on the frequency of oscillation, although it does tend
>to suppress any overtone oscillation tendancy. The purpose of the
>resistor is to prevent overdriving the crystal.  If the crystal is
>overdriven it's frequency accuracy will degrade over time at a rate
>much faster than it is specified for. In extreme cases it could
>actually crack, but that is not likely with a low voltage CMOS
>oscillator.  In cell phone ASICs we always designed the crystal power
>limiting resistance internal to the ASIC itself.  I wonder what Keil
>found out concerning the crystal drive circuit in the 2130?
>
>  
>
Either that, or their designer has had problems with excessive heating 
of the crystals in past designs and simply carried over a tried-n-true 
solution.

I did not consider that the colpitts circuit would do that, overheat the 
crystal, but I guess it is possible.  You always learn something new.  ;-)

I'll have to look at the specs to see if over-driving the crystal may 
indeed be a potential problem...


TomW


-- 
Tom Walsh - WN3L - Embedded Systems Consultant
http://openhardware.net, http://cyberiansoftware.com
"Windows? No thanks, I have work to do..."
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