I'm not sure I exactly understood the question.
In most chips nowdays, core and IO power have different voltage anyway (3.3V and 1.2V).

If you are asking about separation of IO and core with the same voltage, especially digital IO and analog core,
you need the best separation. So, ideally, you put 2 regulators on the board and the board laid-out with 2 different ground planes and 2 different power planes. If it is impossible to put 2 regulators, use one, but keep different ground and power planes all the way to the regulator.

Was it the answer to you question?

Marc Murphy wrote on May 28^th, 2006, 11:49am:


You can do either, but unless there is a very clear reason (different voltage or power supply isolation/crosstalk issues etc.) then both supplys and both grounds can come from one supply.
If you use seperate supplies, still it is often they will share the PCB ground reference.

Usually although a chip may have several grounds supplies, often each domain has a substrtae connections somewhere in the die, which nominally means each ground pins is connected together electrically via the substrate (in some chip power/ESD strategies one does NOT do this). Anyway, even though they have nominally the same potential, the grounds can still be isolated from one another in terms of impedance (by deep trenches, substrate isolation structures, phyiscal die distance).

I would suggest, not to define you ground connections as GLOBALs. You can use different VSS's in the chip. Let's say VSS_CORE and VSS_IO. To separate them in the chip (for LVS) use the bulk separation layer (I think it is called PSUB or like that in TSMC).

Use also 2 different ground and power planes on the board and connect them together only very close to the voltage regulator.

Separation of the supplies also required special care about ESD protection. You need to think what will happen when you are zapping VDD_CORE <-> VSS_IO, VDD_CORE <-> VDD_IO... Do you have the discharge path or the current will find its own way and will burn out the chip.

Michael