https://www.seas.upenn.edu/~cit595/cit595s10/lectures/seqcircuitsPart1.pdf

http://www.ece.utah.edu/~kalla/ECE3700/verlogic3_chapter6.pdf

http://www.ee.ncu.edu.tw/~jimmy/courses/DSD06/05_seq.pdf

http://shodhganga.inflibnet.ac.in/bitstream/10603/174523/5/05_chapter

http://www.pitt.edu/~kmram/0132/lectures/registers+counters.pdf

http://users.eecs.northwestern.edu/~haizhou/303/Lec09.ppt

http://lms.uop.edu.jo/lms/pluginfile.php/2465/mod_resource/content/0/Lecture8-Logic-Registers  Counters-abu-arqoub-2013.pdf

http://userweb.eng.gla.ac.uk/scott.roy/DCD3/07_Memory.pdf

https://en.wikipedia.org/wiki/Ring_counter

https://www.electronics-tutorials.ws/sequential/seq_6.html

https://www.researchgate.net/profile/Mary_Sheeran/publication/220884447/inline/jsViewer/02bfe5110f2e5a50f8000000?inViewer=1&pdfJsDownload=1&origin=publication_detail&previewAsPdf=false