[SI-LIST] : Split vs. common chip busses

Mike Degerstrom ([email protected])
Fri, 10 Jul 1998 15:32:26 -0500

Fellow si-listers:

Presently we have a CMOS ASIC design with VDD and VSS supplies
for the core logic and separate VDD and VSS supplies for
the I/O. I believe that historically the use of separate
supplies for the core and I/O was to keep the noisy I/O
supplies isolated from the core supplies.

Has anyone analyzed the problem to determine the benefit
or harm that would occur from using common VDD and VSS
supplies for both core and I/O - specifically as it
applies to today's submicron/million+ gate ASICs? Any
designs out there that worked with common supplies
to core and I/O?

We are presently evaluating a package that will have low
inductance VSS path but a high inductance VDD path. With
common supplies on the chip we could use the core's VDD to VSS
capacitance to lower the AC impedance of the VDD supply
and thus reduce switching noise when switching from


Mike Degerstrom                       Email:    [email protected]	
Mayo Clinic - Gugg. Bldg. RM 1011-B    Phone:    (507) 284-3292
Rochester, MN 55905                    FAX:    (507) 284-9171
WWW:  http://www.mayo.edu/sppdg/sppdg_home_page.html
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