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There's no way than trying it yourself!
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Generally, it's simply good engineering praxis to avoid floating inputs. Floating input will at least cause additional current consumption, because the respective CMOS input buffers are possibly floating through their linear range. At worst case, very high frequent self oscillations can be brought up, causing excessive supply currents and interferences.
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Of course I can try it myself, but that doesn't mean that I get a statistically relevant result, which is why an answer with more insight might give me a bit more confidence about the risks I'm running.
As to floating, strictly speaking I don't keep them floating, I fix them to a narrow range (specifically the range of the LVDS signals), which should keep them from floating and in theory, this should be within spec of a Schmitt Trigger input.
The reason I want to implement it like this is that I have a remote unit which I want to program (re)using some of the wires to it which are normally used for LVDS dataexchange.
This means that I have an FPGAmain on my mainboard from which three wires (plus power) are running to the FPGAremote unit.
The FPGAmain will have one LVDS inputpair which I connect to two separate TTL outputs also on FPGAmain. Both lines are connected to DCLK and DATA0 on FPGAremote in addition to some LVDS outputpair on FPGAremote. There is one line from FPGAmain (TTL output) which is connected to nCONFIG on FPGAremote.
During startup FPGAmain is programmed using active mode via a flashrom.
Then FPGAmain will manipulate DCLK/DATA0/nCONFIG to FPGAremote using TTL levels to program it, after which FPGAmain will tri-state the drivers on the TTL-outputs for DCLK and DATA0 to FPGAremote, and subsequently will FPGAremote (after startup) use LVDS on that same signalpair to communicate to the corresponding LVDS inputpair on FPGAmain.
My assumption was that it might work because the inputs strictly aren't floating anymore and therefore should not pickup stray signals, and the fact that they're Schmitt Triggered should keep them from oscillating wildly because the LVDS levels do not hover around the Schmitt Trigger levels.
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