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