SSOs and Vcco on Spartan3

Marco,

All of them.

We do all testing with all Vcco/gnds connected.

There is something called SSI, simultaneously switching inputs. If the inputs have overshoot or undershoot, or some small ringing (as they all do unless perfectly matched), then there will be currents flowing which will create ground/Vcco bounce.

Just because it is an input is no reason to think it doesn't need all its Vcco pins.

Aust> Hi, on xapp623 on PDS there are all the step to define the number and

Thanks Austin, I'll consider both inputs and outputs the same way to compute how many Vcco I need to connect, ok? Marco

Hi Austin, Where do these currents flow? Through the protection diodes? Thanks, Syms.

Marco,

I'm a bit confused: are you considering not hooking up all the Vcco pins? You want to supply power through a limited subset of the available power pins?

Symon,

Through the substrate diodes that are intrinsic to the IO output devices.

The output stack acts as the "protection" for inputs.

This is the foundry standard "self-protecting" IO design that we changed to starting with Virtex II. Prior to that we had various proprietary ESD protection structures (that caused us immense headaches).

SSI is not as much as SSO, but it should not be ignored!

Aust>

I'm a bit confused too, so do I have to connect all the Vcco (as reported on the datasheet) and only a number of decoupling capacitor according to the xapp623 or do I have to put a decoupling capacitor for each Vcco? Marco