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maybe the reason of DDR SDRAM is different from the SDRAM
(...)
and the error" cannot place pin mem_dqs[1] to location T8" often generate while compiling quartusII project when I donnot use the demo backround.
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DDR is actually different from sdr SDRAM. In particular, with SDRAM you can't freely place signals on pins but you must follow some constraints on ddr signals pin location. This could explain the error with dqs signal on pin T8.
Anyway I'm missing some point: you refer to a demo project, so I suppose you are using a demo board and not a custom board. Is this your case?
If yes, then the pin connection should already be correct.
What C3 device are you using? Maybe you have wrongly set T8 for an alternate function: verify this in Quartus device settings.
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