Lpc2000

Laurens Swaans wrote:

>Tom,
>   
>  Could you please tell me what kind of oscillator circuit you use? I'm quite confused about what the LPC2129 datasheet specifies.
>  What I get from it is that I need an oscillator that has a minimum rise/fall-time of 5ns, a minimum RMS-voltage of 200mV, but the input pin (XTAL1) can not withstand voltages exceeding V18 (which is 1.8V???). All oscillators I see are 5V or rarely 3.3V... What do you use??? Do you use a 5V oscillator and attenuate the signal?
>   
>  
>
I use a 3.3v osc and attenuate that.  It is digikey P/N: 535-9255-1-ND

Along with that osc, I also have 1)3.9K, 2)2.2K, and 2)100p caps.  The 
two 2.2K resistors are each locate near one of the CPUs.  Those 2.2K 
resistors go to GND on one side and the osc feed for the other.  The osc 
feed is the 3.9K resistor in series with the output of the osc output.  
This forms an attenuator for the osc signal (which is HUGE!).  Then, the 
100p caps are used to connect each CPUs' X1 input to the attenuated 
signal, see diagram:


OSC--------3.9K---.-------------------.
                  |                   |
  X1----100p------|    X1----100p-----|
                  |                   |
                 2.2K                2.2k
                  |                   |
                  |                   |
                 GND                 GND

Hope this comes out, heh.

Anyhow, I have a 400mv signal @ each X1 input.  The DC voltage of the X1 
input is around 1.2volts, well within spec.

Regards,

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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