dab at froghouse.org
Fri Mar 31 07:10:57 CDT 2017
> 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.
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