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Here's a copy of the FLEX10K data sheet, I couldn't find a link on Altera's site.
The information you want is in Table 19 on p44, and in Figure 20 on p46. The output high voltage depends on how much current your external device is drawing. If you really were driving a high-impedance input and were drawing no current, then Figure 20 indicate you should get a high voltage of 4V. Since you are getting 3.5V, either your VCCIO voltage is low, or your external device is drawing a few milliamps.
As you comment, your interface logic levels are compatible, so you're fine.
Cheers,
Dave
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It seems pretty much in line with what I was getting. I don't remember what kind of load I had but it can't have been much, so yeah... All good :-) Figure 20 is pretty informative, thanks for pointing that out!
-Mux
Honored Contributor
