Check all citation links and add urldate=

Diplomschrift.bib

@@ -2,12 +2,14 @@
     author = {Olav Junker Kjær},
     title  = {The Nand Game},
     url    = {http://nandgame.com},
+    urldate = {2020-03-29},
 }
 
 @online{breadboard_computer,
     author = {Ben Eater},
     title  = {Building an 8-bit breadboard computer!},
     url    = {https://www.youtube.com/playlist?list=PLowKtXNTBypGqImE405J2565dvjafglHU},
+    urldate = {2020-03-29},
     year   = {2016},
 }
 
@@ -15,6 +17,7 @@
     author = {Clifford Wolf, Johann Glaser},
     title  = {Yosys - A Free Verilog Synthesis Suite},
     url    = {http://www.clifford.at/yosys/files/yosys-austrochip2013.pdf},
+    urldate = {2020-03-29},
     year   = {2013},
 }
 
@@ -22,70 +25,82 @@
     author = {Various Contributors},
     title  = {Yosys - Yosys Open SYnthesis Suite},
     url    = {https://github.com/YosysHQ/yosys},
+    urldate = {2020-03-29},
 }
 
 @software{nextpnr,
     author = {Various Contributors},
     title  = {nextpnr - a portable FPGA place and route tool},
     url    = {https://github.com/YosysHQ/nextpnr},
+    urldate = {2020-03-29},
 }
 
 @software{nextpnr-xilinx,
     author = {David Shah},
     title  = {nextpnr-xilinx},
     url    = {https://github.com/daveshah1/nextpnr-xilinx},
+    urldate = {2020-03-29},
 }
 
 @online{prjxray,
     author = {SymbiFlow},
     title  = {Project X-Ray},
     url    = {https://github.com/SymbiFlow/prjxray},
+    urldate = {2020-03-29},
 }
 
 @software{ghdlsynth-beta,
     author = {Tristan Gingold},
     title  = {ghdlsynth-beta},
     url    = {https://github.com/tgingold/ghdlsynth-beta},
+    urldate = {2020-03-29},
 }
 
 @software{ghdl,
     author = {Tristan Gingold},
     title  = {ghdl},
     url    = {https://github.com/ghdl/ghdl},
+    urldate = {2020-03-29},
 }
 
 @software{liteeth,
     author = {Florent Kermarrec},
     title  = {LiteEth},
-    url    = {https://github.com/enjoy-digital/liteeth}
+    url    = {https://github.com/enjoy-digital/liteeth},
+    urldate = {2020-03-29},
 }
 
 @software{litedram,
     author = {Florent Kermarrec},
     title  = {LiteDRAM},
-    url    = {https://github.com/enjoy-digital/litedram}
+    url    = {https://github.com/enjoy-digital/litedram},
+    urldate = {2020-03-29},
 }
 
 @software{open-fpga-loader,
     author = {Gwenhael Goavec-Merou},
     title  = {openFPGALoader},
     url    = {https://github.com/trabucayre/openFPGALoader},
+    urldate = {2020-03-29},
 }
 
 @software{gtkwave,
     author = {Tony Bybell},
     title  = {GTKWave},
     url    = {http://gtkwave.sourceforge.net},
+    urldate = {2020-03-29},
 }
 
 @online{riscv-compliance,
     author = {Jeremy Bennett, Lee Moore},
     title  = {RISC-V Compliance Task Group},
     url    = {https://github.com/riscv/riscv-compliance},
+    urldate = {2020-03-29},
 }
 
 @online{symbiyosys-slides,
     author = {Clifford Wolf},
     title  = {Formal Verification withSymbiYosys and Yosys-SMTBMC},
     url    = {http://www.clifford.at/papers/2017/smtbmc-sby/slides.pdf},
+    urldate = {2020-03-29},
 }