7-seg display not turning off digits

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I was playing around with my new Papilio One 500K and LogicStart MegaWing. I've mostly been following the projects in Mike Field's book.


First few projects with LEDs and switches seem to work fine, but when I try the 7-seg display, all 4 digits are always lit-up at the same time and have the selected segments displayed.

Setting anodes to "0000" or "1111" or any other combination makes no difference.

The only way to turn off digits (all of them) is to set "1111111" to the segments.


I tried measuring voltage on the P18 pin (which should be the rightmost digit's anode, I think?) without the LogicStart MW and it correctly goes from low to high depending on the value in VHDL. Measuring the pin's voltage with the MW attached always show 2.6 V, regardless of the pin's output value. I don't know whether that's supposed to be like that due to the transistor or if that's wrong.


Am I right to expect that all segments should go dark with anodes set to "1111", regardless of the segments' value? If so, does anyone have any idea what might be wrong and how to test it?

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Are you sure you're using the right .ucf file? IIRC the anode control is active low, so yes, setting them to "1111" should result in all displays off. Check the constraints file and make sure the anode signals are routed to the correct pins for the Papilio board you have.

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This is the .ucf part related to the display:

NET "anodes<0>" LOC="P18";NET "anodes<1>" LOC="P26";NET "anodes<2>" LOC="P60";NET "anodes<3>" LOC="P67";NET "segments<6>" LOC="P62";NET "segments<5>" LOC="P35";NET "segments<4>" LOC="P33";NET "segments<3>" LOC="P53";NET "segments<2>" LOC="P40";NET "segments<1>" LOC="P65";NET "segments<0>" LOC="P57";

It's copy-pasted from Mike Field's book and I believe it's consistent with the LogicWing's page, too (

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