From: Seth <[email protected]>
Reply-To: "[email protected]" <[email protected]>
Date: Friday, December 5, 2014 at 5:07 AM
To: "[email protected]" <[email protected]>
Subject: Re: [beagleboard] Purpose of the GND_OSC zero ohm resistors?
> Thanks for the info, John. I thought it might have something to do with that.
> If I get a chance I'll have to pick up that book - thanks for the suggestion!
>
> I see in the earlier BBB designs the GND_OSC wasn't connected to the main
> DGND. Only later were they connected (via those zero ohm resistors), and that
> apparently helped the BBB function better. To me it seems that would introduce
> more switching noise (as John put it) than leaving them not connected to DGND.
> I guess I'm a little confused about John's use of the word "isolated" ("If you
> don¹t isolate the OSC gnd...") - were you suggesting the zero ohm resistors
> acted as isolation from digital ground in this case?
>
> Gerald - do you mean that I could connect the GND_OSC net to the DGND net
> directly, or were you saying that I didn't need to connect those two at all
> (as in the earlier versions of the BBB)? What's the reason for having the
> resistors on the BBB if they're not needed? I was speculating that maybe you
> weren't initially sure if connecting the ground nets together would solve the
> "ground bounce" problem or maybe introduce worse problems, so you used those
> resistors to allow yourself the ability to leave them unpopulated in the event
> that it caused more issues than it solved (rather than re-spin new boards
> without the connection at all).
GND represents the return path for each signal, so DGND represents the
return path for digital circuits and GND_OSC represents the return path for
the OSC circuit. Ideally, you create a reference point (normally GND pin of
the power plug and then using a star topology, you connect to each of the
GND circuits, such as AGND, DGND, etc. This way, no GND circuit will see the
return current of the other circuits. Normally, the PCB layout software
won¹t allow you to connect nets of different names, so you use zero ohm
resistors to connect these nets on the PCB.
If GND_OSC isn¹t connected to DGND, then it will be connected inside the
chip and this isn¹t ideal. Measure the voltage signal between GND_OSC and
DGND with and without the zero ohm resistor. You will see the noise between
the two nets increase without the zero ohm resistor. BTW, the zero ohm
resistor isn¹t idea either and we normally have special schematic symbols
that allow us to connect two nets with different names. The PCB
representation is just a copper trace which connects the two nets together,
but most important is that you can move this copper trace so that it is
close to the reference point (normally the GND pin of the power connector).
Hope this helps.
Regards,
John
>
>
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