Also note that Colpitts oscillator has a maximum of 16MHz while
Pierce supports up to 40MHz (Table A-15 i Device User Guide).
--- In , "Andrew Lohmann's New Email Server"
<andrew.lohmann@b...> wrote:
> Hi
> I found the colpits oscillator works just fine, you layouts could
be
adapted to get the same result. The extra precaution recomended is to
have no ground or power plane under the crystal pins compared with
the
pierce oscilator. My circuit includes the capacitor in the 0V lead of
the crystal, and the pcb layout is modified to put the crystal and
pll
components on the same side of the pcb as the processor. By
comparison
I moved tracks from pins 49..55 to inside the area enclosed by the
processor, I did not as you have break up the ground plane, and the
pll's parts rotated to make it a bit tidier.
>
>
> Andrew Lohmann AIIE
> Design Engineer
>
> PLEASE NOTE NEW EMAIL ADDRESS IS:
> andrew.lohmann@b...
>
> Bellingham + Stanley Ltd.
> Longfield Road, Tunbridge Wells, Kent, TN2 3EY, England.
> Tel: +44 (0) 1892 500400
> Fax: +44 (0) 1892 543115
> Website: www.bs-ltd.com
> ----- Original Message -----
> From: Robert Imhoff
> To:
> Sent: Sunday, February 22, 2004 10:53 AM
> Subject: Re: [68HC12] HCS12 Oscillator circuit
> *** This e-mail has been modified by Finjan's SurfinGate for
E-Mail.
> *** More details can be found in the attached
file, SurfinGate
Notification.txt
> Dear Doron
>
> Many thanks for your advice!
> (I noted your previous messages about the older mask sets, so
will
make
> sure of that).
>
> Best regards
> Robert
>
> On 22. Feb 2004, at 11:37, Doron Fael wrote:
>
> >
> > Robert,
> >
> > Please not that early MC9S12DG256 or MC9S12A256 mask-sets don't
> > support the
> > Pierce crystal configuration, but only Colpitts crystal or an
external
> > clock driver with 2.5V square wave.
> >
> > Only the newest mask-set 0L91N (or newer one if there are any)
of the
> > MC9S12DG256 or MC9S12A256, supports also the
Pierce crystal
> > configuration.
> > I suggest you make sure you can get this latest mask-set before
> > designing a
> > Pierce Oscillator for it.
> >
> > The Pierce crystal configuration IS supported by other HCS12
parts
> > like for
> > example DP512 / A512, DT128 / A128 and DJ64 / A64.
> >
> > The Pierce oscillator in general is less susceptible to noise
and is
> > more
> > robust than the Colpitts crystal, since it has a higher voltage
swing.
> > It
> > is therefore less sensitive to distance from the CPU and vias,
than the
> > Colpitts configuration.
> >
> > In your particular case, I believe both the shorter and longer
layouts
> > would be just fine. I would personally chose
the shortest,
since
it
> > has all
> > the crystal components on the opposite side from the CPU, and
also
> > most of
> > the the crystal system traces. It seems to me this may reduce
the
> > radiated
> > influence of the other CPU signals on the crystal signals and
> > vise-versa.
> > But again, in your case, I believe it doesn't matter much, and
both
> > layouts
> > would do just fine.
> >
> > Hope this helps,
> > Doron
> > Nohau Corporation
> > HC12 In-Circuit Emulators
> > www.nohau.com/emul12pc.html
> >
> > At 10:19 22/02/2004 +0100, you wrote:
> >> Hello,
> >>
> >> I am venturing into making a first circuit for a wind turbine
> >> controller based around the MC9S12DG256 or MC9S12A256 with a
Pierce
> >> oscillator.
> >> Would anyone be able to give me some feedback on the layout?
> >> In the device guide, Motorola says the tracks to the
oscillator
quartz
> >> should be made as short as possible, so I
came up with two
versions,
> >> one with shorter leads at the expense of
needing a few extra
vias to
> >> route the adjacent pins out from under
the MCU, and the other
avoiding
> >> these vias, but resulting in longer
tracks to the quartz.
> >>
> >> I would also be grateful for any recommendation of a
particular
16 MHz
> >> quartz and indicate suitable capacitor
and resistor values to
go use
> >> with it.
> >>
> >> Many thanks in advance!
> >> Robert Imhoff
> >>
> >> 1) layout with shortest tracks: http://www.issole.
info/shorter.gif
> >>
> >> 2) layout avoiding vias: http://www.issole.info/longer.gif
>
>
|