Quick question regarding setting the IO standard level in a projects UCF file.
I have a port which interfaces with some 5V logic, on bank 3 of my spartan 3A. So I have connected the VCCO3 to the 5V supply. but now when I'm looking to set IOSTANDARD attribute in my UCF file (and checking the documentation on page 340) it seems that there is no 5V standard available for this FPGA :-?
Is there any standard which will use the 5V attached to VCCO3, allowing proper communication without damaging the FPGA?... or will I have to start cutting some tracks :-x
An at least casual reading of a Spartan 3 datasheet would be strongly suggested. Simply start with the tables Absolute Maximum Ratings and General Recommended Operating Conditions. It completely answers your question.
Connecting 5V logic outputs through series resistors, utilizing the FPGA's built-in clamp diodes is in fact a simple methode to do what you want.
uhm... Well, lets hope it's still alive. that certainly is not the intended use. The idea is to use 3.3 Volt for that.
As pointed out, no 5V IO standard. You can define your 5V tolerant inputs as an LVTTL input with a VCC of 3.3 volt, and then add a series resistor as suggested. Note that I said 5v tolerant input, not output. If you want 5V outputs then you might need level shifters, or get creative.
The google string du jour is "large swing single ended", and the app note is xapp459.