Hi all,

I reviewed a bunch of designs, spun my own board, and had a real
board house put parts down. Here is the issue:

I have a couple boards all acting the same. Namely, the oscillator
isn't going. I'm using an Abracon 14.7456Mhz oscillator (535-9070-1-
ND digikey) with the required 18pF caps. I've used this before on an
LPC 2106 board, and it buzzes along just fine on that unit.

Scoping the unit, there is 1 volt present on both sides of the
crystal (pins 61 and 62 on the LPC2138 chip). I removed the crystal,
and there is still 1V present on both pads, as well as both pins on
the chip. The resistance measured between these pads with the
crystal or out is in the multi-megaohms (little tough to measure due
to extra capacitance, but there is not a short between the pins).

I replaced the crystal with a new one...same results. I'm trying to
avoid my next step...replacing the LPC on the hope that it is just a
bad chip...

I have about a 5 msec R/C on the reset line. That looks clean. But
when I reset the unit, I see no activity on the oscillator lines. As
a matter of fact, when the unit is in reset (the reset pin has been
scoped as low, then transitioning to high) the pins still have 1V on
them. This doesn't seem right to me...but I have two boards now
doing the identical thing.

So, long story short (too late)...does anyone know if 1V on these
lines seems normal? Do I have too much capacitance somewhere and is
the oscillator not getting kicked enough? I'm not very up on
crystal / oscillator testing in or out of circuit. I'm only doing
a "my LPC2106 board shows 1V peak to peak oscillations, but my
LPC2138 isn't" comparison...kind of simple minded.

In advance:
1) Yes, I scoped at the pins, every pin on the 2138. Grounds are
grounded, 3.3V is 3.3V, indeterminate pins are a mix between ground
and about 2.0 v (no program in it yet...fresh chip).
2) Yes, both on the bare board and the assembled, I've buzzed out /
ohmed out the traces to make sure there is no goofy-ness. I have
almost no other pins connected at this point anyway, but none other
than the oscillator pins show 1 volt. This makes me think it's
normal, and that the oscillator circuit has some extra capacitance or
something...but the tolerances in the datasheet say I should be more
than fine...

Any ideas on why the oscialltor won't start? Did I miss something in
the manual?

Help?

Thanks in advance... Start a New Thread

matterama3444 wrote:
>
> Hi all,
>
> I reviewed a bunch of designs, spun my own board, and had a real
> board house put parts down. Here is the issue:
>
> I have a couple boards all acting the same. Namely, the oscillator
> isn't going. I'm using an Abracon 14.7456Mhz oscillator (535-9070-1-
> ND digikey) with the required 18pF caps. I've used this before on an
> LPC 2106 board, and it buzzes along just fine on that unit.
>

Of this new design, do you have *any* boards that *are* working??? As I
read this, it sounds like you designed a new board, then sent it out to
have the PCB fabricated and stuffed? Now, none of this new design is
working?

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