Re: [SI-LIST] : Output driver versus internal logic switching noise

Arthur Collard (artc@lobsang.sps.mot.com)
Thu, 28 Aug 1997 08:59:54 -0500

On Thu Aug 28 04:29 CDT 1997 Jose Luis Gonzalez writes:
> Hello,
>=20
> I'have been reading in several switching noise related papers that
> traditionally the only important source of noise in ICs was due to the
> simultaneous switching of the output drivers but that in today =
circuits the
> switching noise generated by the internal logic would be also very
> important. Does anybody have quantitative confirmation of this? It is =
any
> study of the ratio between internal logic/output driver switching =
noise in
> any commercial IC or ASIC?
>=20
> Thanks in advance
>=20
> Jose Luis Gonzalez
>=20
> =
_________________________________________________________________________=
___
>=20

Interesting enough I have done some interesting experiments measuring =
radiated=20
emissions (EMI) using the SAE J1752/3 test method (1GhZ TEM cell) on =
various=20
Motorola micrcontroller product. One of these experiments was presented =
at the=20
RF ERxpo West 1995 (EMC/ESD International) which I co-authored. The =
title of the=20
paper is "An Investigation of the SAE J1753/3 Test Method for Integrated =
Circuit=20
Radiated Emissions" by Kevin Anderson and Art Collard (page 328).=20

It is true as you point out that the only important noise in IC's is due =
to=20
output drivers switching (assuming that they are on) however some IC's, =
like=20
Motorolas, can operate in "single chip mode" (SCM). In SCM, memory is =
accessed=20
internally and the I/O is limited to serial ports and/or general purpose =
I/O or=20
neither. These types of pins switch at much lower frequencies than a =
system=20
clock. When this configuration is operating the noise source is mostly =
internal=20
but is conducted out through the power/ground pins.=20

The dominant noise source in either mode of operation is the internal =
clock and=20
the peak emissions of the clock harmonics can be equal in either =
configuration.=20
I have measured up to a 20dB decrease in non-clock harmonic noise in SCM =
=20
compared to expanded mode.

Other contributers to internal noise are cpu engine choice, internal=20
power/ground distribution, internal clock distribution and software =
code. =08

Also important to IC noise is the external decoupling filtering and =
placement,=20
memory choice, IC placement in relationship to external components and=20
connectors, pcb layout.=20

Regards

Art Collard

< > Motorola Confidential Proprietary < > Motorola Internal Use Only
=20
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