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External Clock Source

Started by James Dabbs March 8, 2004
Are there any non-obvious considerations in using an external oscillator
to drive the clock pin of the LPC210X (instead of a crystal using the
internal oscillator)?



An Engineer's Guide to the LPC2100 Series

It capacitively couples at a low level. It is not a direct connect
from the oscillator to the pin on the LPC.

Regards
-Bill Knight
R O SoftWare
On Mon, 8 Mar 2004 13:00:18 -0500, James Dabbs wrote:

Are there any non-obvious considerations in using an external oscillator
to drive the clock pin of the LPC210X (instead of a crystal using the
internal oscillator)?

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> It capacitively couples at a low level. It is not a direct
> connect from the oscillator to the pin on the LPC.

So, if I need to clock the LPC from an external oscillator, what should
I do? In this case, it's actually an sine waye from a UHSO, but I was
planning to square it up for this chip.



--- In , "James Dabbs" <jdabbs@t...> wrote:
> > It capacitively couples at a low level. It is not a direct
> > connect from the oscillator to the pin on the LPC.
>
> So, if I need to clock the LPC from an external oscillator, what
should
> I do? In this case, it's actually an sine waye from a UHSO, but I
was
> planning to square it up for this chip.
I use an 3.3V oscillator IC and use a resistor divider (330 Ohms from
the oscillator to a common point, 75 Ohms from the common point to
ground). I then run a 100pf cap from that sommon point to the X1
input of the processor.

Robert


--- In , "James Dabbs" <jdabbs@t...> wrote:
> > It capacitively couples at a low level. It is not a direct
> > connect from the oscillator to the pin on the LPC.
>
> So, if I need to clock the LPC from an external oscillator, what
> should I do? In this case, it's actually an sine waye from a UHSO,
> but I was planning to square it up for this chip.

Check out page 39 in the LPC210x User Manual (download from
Philips website). It shows you exactly how to do it. Basically,
you need to put a 100pF capacitor between your oscillator and
X1. The oscillator should have an amplitude of at least 200mV
and no more than 1.8v.

-bob