Spartan 3 IOSTANDARD vs VCCO

An output pin voltage swing is determined by VCCO, so how else does specifying LVCMOS25 vs LVCMOS18 as the IOSTANDARD affect output signals?

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Well,

Specifying the IO standard pretty much identifies to the bitgen part of the program which bits to turn on (and off) to get that drive strength for that standard.

Suppose you have 2.5 V Vcco, and you call for a 1.5 V 4 mA CMOS driver

-- running at 2.5 volts it will be much stronger than the 4 mA as the bits will have enabled most of the driver "legs" to get 4 mA min at 1.5V Vcco. It would require far fewer "legs" enabled to get 2.5 V 4 mA.

So, the IO will function at a different voltage, it just won't meet the data sheet specifications for that new voltage. Its behavior will be stronger if the Vcco is larger than it thought, or weaker if smaller than it was programmed for. Other specifications will also be affected: delay and input impedance if differentially terminated internally.

Austin

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