Thanks Aaron and Peter for your length replies and info.
I think the best course it to use the inductor to start
with. Ill probably
get a summer student to do all the experimentation
mentioned. Basically
probably an issue that can take as much time up as desired!
What freq range are we actually trying to block here? Im
guessing just as
low as 1M and up?
Regards,
chris
----- Original Message -----
From: "Aaron Shepard" <aarons glw.com>
To: <chris_beckershaw.ca>
Sent: Tuesday, April 11, 2006 9:07 AM
Subject: Re: [ColdFire] coldfire layout
> Chris,
>
> I can't answer specifically to the 5213, but I have
dealt with plenty of
> other parts that utilize internal PLLs.
>
> You should try to use a filter of some sort to minimize
switching noise
> when you to connect PLL power to a Vcc power plane
shared by the processor
> or other components. Be aware that noise on the planes
can vary
> significantly between different board layouts and
component selections, so
> it's always safer to have the pads there for the
ferrites in case you need
> them. You could put zero-ohm jumpers on the pads if
you decide you don't
> need the ferrites.
>
> One ferrite-less solution is to use a fat trace (50 mil
or wider) and
> connect it back to the power source. This actually
works very well. I
> usually put a few extra bypass capacitors along the
trace for insurance.
>
> Another ferrite-less solution is to partition the plane
to create a clean
> PLL power region, but don't do this unless you know
exactly how it should
> be done. Plane partitioning is often times the
cleanest solution, but if
> it's done incorrectly it can cause headaches.
>
> Try not to have an aversion to using the ferrites. The
type of ferrites
> your application requires are cheap and come in tiny
packages. I don't
> recommend getting into the habit of connecting the PLL
power pins directly
> to the power plane... it might work now, but eventually
it will bite you.
>
> A flaky PLL can cause you a ton of grief... often times
it's not worth it
> to skimp on these things.
>
>
> Aaron Shepard
>
>
> Chris Becker wrote:
>
>> Hi,
>>
>> We are laying out a 5213 coldfire part. As usual
we like to copy the
>> evaluation board schematic as much as possible.
This part has seperate
>> vdd and vss pins for the pll, and the eval board
puts ferite beads on
>> both this lines. I am wondering if this is really
necessary....
>> Has anyone bothered using ferrite beads on the pll
power pins? Or has
>> just tying them to the power and ground planes been
trouble free?
>>
>> Thanks for comments,
>> Chris
>>
>>
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>
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