Cross-talk square-wave?

[Jon Elson]

On 03/31/2017 07:10 AM, David Bridgham via cctalk wrote:
>> Don't trust ANYTHING!  Recent Xilinx FPGAs have permanent "weak
>> keepers" on all pins, they can not be turned off.
>> What this is is a non-inverting receiver on the pad, that is driving
>> back to the pad with about a 50K Ohm resistor.
>> Plays hob with analog stuff like crystal oscillators.  The weak keeper
>> would PERFECTLY explain your square wave!
>> When it gets a narrow pulse to high, it holds the line high.  When it
>> gets a narrow pulse to low, it will switch to holding the line low.
>> So, if you are using a Xilinx FPGA of recent vintage, or some of their
>> CPLDs, they will do exactly this.
> Looks like we have an explanation then.  We're using an XC7A75T-CSG324,
> a Xilinx Artix 7 series FPGA.  Thank you very much.
>
>
Yup, that definitely has such a feature.  I think there are 
some Xilinx knowledgebase articles on the specifics of 
certain family's pad options and how to deal with them in 
pseudo-analog functions like crystal oscillators, switch 
debounce, etc.

Jon