Lpc2000

Tom,

In production environments, you would like to "tune" the oscillator
circuit so that the rtc runs on time. Having a 32768 hz output makes
this really easy. National Semi had some rtc chips that could output
the clock for "tuning." This is the only time I ever used it. 

To do tuning, you need a calibrated crystal from the crystal
manufacturer. You can then change the load caps until the frequency
come out right. You have to do this as the stray capacitance on the
board can only be approximated.

Adding 3-5 pf with a jfet to tap off the signal can makes measuring
the clock tricky. Bob Pease in his troubleshooting books claims ~1pf
and the best I could ever do was ~ 3-5 pf. 

If you need a 32kc clock and low power, this is the best solution I've
found. Every other oscillator I built with gates or transistors
consumed more power.

As I recall, even taking the signal from the output of the chip and in
front of the series resistor still pulled the crystals frequency.

Ed

--- In lpc2000@yahoogroups.com, Tom Walsh <tom@...> wrote:
>
> alan_in_nz wrote:
> 
> >I am using an LPC2148 with a 32.768KHz crystal on the RTC oscillator 
> >pins. I need to internally route the resulting 32.768KHz clock and 
> >output it to a GPIO pin. Is this possible? If so how and what pin is 
> >best to use as the output GPIO pin? The datasheet I have been looking 
> >at does not seem to have (as far as I can see) a detailed block 
> >diagram which shows the clock schemes for this device and how I could 
> >achieve this. If anyone has done this or knows how to do this I would 
> >appreciate the help. Thanks. Alan
> >
> >
> >  
> >
> That sounds odd.  I had to put a resistor in series with the crystal to 
> drop the drive level of the LPC2xxx oscilator circuit.  That circuit 
> should have plenty of output level to drive other logic?  If not, why 
> not just stick a FET into the osc circuit and buffer your 32KHz into 
> your other logic?
> 
> TomW
> 
> 
> >
> >
> > 
> >Yahoo! Groups Links
> >
> >
> >
> > 
> >
> >
> >  
> >
> 
> 
> -- 
> Tom Walsh - WN3L - Embedded Systems Consultant
> http://openhardware.net, http://cyberiansoftware.com
> "Windows? No thanks, I have work to do..."
> ----------------------------------------------------
>