Spartan 3A port IO standards

[allsey87]

Hi all,

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

Cheers,

Mike

 

[permute]

does that FPGA support 5V IO?

I know that more modern FPGA's have issues with 3.3V io. perhaps you need a level translator for the 5V io.

(some designs might be tolarant of 5V inputs, but less tolarent of 5V outputs)

 

[hamed_sotoudi]

Hi,
No you cant use it and it may be damage reverse diodes in your FGPA input buffer so put a 270 ohm resistor in series with your inputs.

 

[FvM]

it seems that there is no 5V standard available for this FPGA

Correctly guessed.

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.

 

[mrflibble]

So I have connected the VCCO3 to the 5V supply.

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.

https://www.xilinx.com/support/documentation/application_notes/xapp459.pdf

Hope that helps.