Impedance I/O SPARTAN3 board

The IO impedance of Spartan3 pins depends on the IO standard selected. As already noted, you can get the IBIS files which specify (in excruciating detail) the properties of the IO drivers.

Cheers

PeteS

PeteS a =E9crit :

I have read IBIS files, my I/O is :LVCMOS25-S-12mA. What's the impedance of this I/O standard ?

Thanks.

PeteS a écrit :

I have read IBIS files, my I/O is :LVCMOS25-S-12mA. What's the impedance of this I/O standard ?

Thanks.

I just got the latest IBIS models, and I'm not going to plug them into anything right now, but the information you need to use is the package data (which will give you the packaging equivalent circuit) which is driven by the model you show.

If you take the IBIS drive model, you will see it also has loading components listed (these are the components on the testbed where the data were gathered).

So draw a little circuit;

Driver -> package model -> load circuit

Now plug in the numbers (a delta is more useful than any one number) and you'll get a V/I curve vs. the entire circuit. That's your impedance, as close as you should need. If you look at the impedance of the two external circuits, you'll see they don't give you that same impedance; what's left is the buffer impedance.

Xilinx don't (nor do Altera as far as I know) specify impedances in the datasheet for at least 2 very good reasons:

1. They would have to list them (vs. frequency) for every single IO buffer model, and the datasheet would be swamped (although I will admit it would be nice as a separate design note - hint to Xilinx as you have the data and it can't be too hard to automate the process and get at least *predicted* impedances into a specific load :)

1. It depends on the package, so the same issues as in (1) apply.

Use Spice or equivalent or even Matlab if you have it if you want to automate the process a little. I remember doing this for a Virtex part a few years ago and I used a Perl script to generate the numbers to plug into a spreadsheet ;)

Cheers

PeteS