Re: [EE]: PCB layout of crystal oscillator for MCU

Hi Martin,

This reminds me the old days when I used to wire a MCU by using a donut
board. The crystal has to sit 45 degree so that I could bend its legs
toward the MCU and solder them to MCU OSC1 and OSC2 legs. Then 2 SMD caps
were soldered to crystal legs and I would find a through resistor and cut
off one of its leg and use it to bridge the ground side of both caps to MCU
ground.

Thanks for the nostalgia.
WH Tan

On Sat, 22 Aug 2026 at 19:57, [email protected] <[email protected]>
wrote:

>
> Things were much easier twenty years ago with hand etched single layer
> boards. Admittedly only a 4MHz crystal and bigger parts:
>
>
>
> Cheers,
> Martin.
>
> On Saturday, 22 August 2026 at 05:59, David VanHorn <[email protected]>
> wrote:
>
> > Here's what I do, very similar to what you have here.
> >
> > I moat the ground on ly1 and ly2 only, and pour ground inside the moat
> > on ly1 and ly2.
> > Then I route a single track from the crystal area to the ground pin on
> the
> > CPU which is nearest to the crystal pins on the CPU.
> > I stitch ly1 and ly2 in the crystal area, but I do NOT stitch them to
> lower
> > layers.
> > The idea is that I want whatever currents are flowing to return only and
> > directly to the CPU.
> > The pin on the CPU where the crystal ground connects is then the only
> point
> > where the crystal ground touches the system ground.
> > This prevents any other currents that might be passing through from
> > affecting the crystal.
> >
> > Ly 3, 4 etc are then clear for use with tracks under the crystal as
> needed.
> >
> > Interesting side note: Check the skin depth in copper for the frequency
> you
> > are running. At lower crystal frequencies a single layer of copper isn't
> > thick enough to really isolate the crystal!
> > Engineering is the ART of compromise!
> >
> > Finally, check your crystal caps.  A crystal that calls out a 22pF load
> > capacitance needs significantly larger caps than 22pF!  C =
> (CL*2)-Cstray.
> > Cstray can be estimated at 5 or 6 pF usually. So for 22pF we get
> (44pf)-5pF
> > or 39pF caps.
> > On prototypes, I add a series resistor equivalent to 5x the xtal
> impedance
> > in the drive lead to check Barkhausen criteria at high and low
> temperature
> > and high and low operating voltage. If that works, then take the resistor
> > out on the final layout.
> > If not... :)  Troubleshoot.
> > --
> > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> > View/change your membership options at
> > https://mailman.mit.edu/mailman/listinfo/piclist
> > --
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>


-- 
WH Tan