elson at pico-systems.com
Fri Mar 31 11:26:16 CDT 2017
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
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