Lpc2000

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?

--Dave




--- In lpc2000@yahoogroups.com, Tom Walsh <tom@o...> wrote:
>
> While looking at the schematic for the Keil MCB2130, I noticed that
they 
> put a 560K resistor in series with the 32KHz crystal.  This seems odd, 
> the only reason I can see for a series resistance is to lower the 
> current through a crystal circuit.
> 
> Anyone see this?   From what I can tell from the LCP2138 manual, only 
> load capacitors are called for to pull the crystal on frequency.  
> AFAICT, my 32KHz crystal is running fine on my board without the series 
> resistor.
> 
> 
> Look at: http://www.keil.com/mcb2130/mcb2130-schematics.pdf  to see
what 
> I mean.
> 
> TomW
> 
> -- 
> Tom Walsh - WN3L - Embedded Systems Consultant
> http://openhardware.net, http://cyberiansoftware.com
> "Windows? No thanks, I have work to do..."
> ----------------------------------------------------
>