Re: [SI-LIST] : Micro Noise

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From: jjcahill@us.ibm.com
Date: Wed Nov 03 1999 - 08:19:26 PST


A few thoughts, not necessarily connected.

Many times self-test operations generate more power supply transients than
actual operation. Is there decoupling explicitly on the chip or the
package? If you allow the part to start operating on a very low activity
code loop, does it generate a stronger or weaker near field?

Are all of the off-chip drivers tristated during reset or are they toggling
or static?

If the crystal is pulled, what is generating the clock to the device?
Something internal to the chip? What frequency does it operate at without
external guidance?

Joe

CEC Analysis and I/O Design
Phone: 507.253.0762 Fax: 507.253.4966
jjcahill@us.ibm.com

Lfresearch@aol.com on 11/03/99 09:31:26 AM

Please respond to si-list@silab.eng.sun.com

To: si-list@silab.eng.sun.com
cc:
Subject: [SI-LIST] : Micro Noise

Hi all,

I have a problem with a microprocessor which appears to be radiating very
strongly from the package. I've deactivated all other circuits ( pulled
crystals, pins etc ) on the board. Additionally, the micro is held in
reset,
PROM's are pulled, but it's a noisy as heck. Close field probes ( both E
and
H field ) show the package to be the dominent source of radiation.

The device has a maximum clock rate of 50 MHz, this is where we're using
it.
Clock it at 32 MHz and the emissions fall about 10 dB up to about 300 MHz,
more above 300 MHz. Yes it's a CMOS device.......

Am I stuck with putting a can over the chip? The product has a plastic
package, so there is no shielding available at all.....

Thanks,

Derek

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