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

Larry Smith (ldsmith@lisboa)
Thu, 28 Aug 1997 08:46:04 -0700

> 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?
> Thanks in advance
> Jose Luis Gonzalez

It is true. Just take a look at the power dissipated by internal logic
circuits and IO drivers and ask yourself which one has the capability of
making the most noise. IO circuitry may dissipate a few watts but
internal circuitry on some modern processors and ASICs can dissipate 10's
of watts. It is now common for the internal circuitry to run at a lower
power supply voltage (2.5 or 2.0 volts) compared to the IO circuitry
(3.3 volts). Not only is there more energy to make noise in the
internal circuitry, but it is likely to be more sensitive because 100 mV
of noise is a bigger proportion of a 2.0 volt power supply voltage.

Noise transients in the internal circuitry tend to be code dependent.
For example, a processor can be actively doing something for a few clock
cycles and then stop doing it for a few clock cycles. This creates a
clock sub-harmonic. Various forms of energy conservation also create
noisy power surges in internal circuitry when circuits are activated or
deactivated. My personal opinion is that power distribution and noise
management will be the next big challenge for the package/semiconductor
industry. It is likely to surpass simultaneous switch as the major
problem to be solved.

There is a special session in the upcoming Electrical Performance of
Electrical Packages conference (San Jose, October 27-29, 1997) to deal
with this topic.

Larry D Smith
Sun Microsystems