http://www.doctronics.co.uk/pdf_files/CLK_02.pdf

http://www.electronics-tutorials.ws/sequential/seq_3.html

http://www.edutek.ltd.uk/CBricks_Pages/Monostable_LOGIC.html

http://pfnicholls.com/electronics/astable.html

http://www.ti.com/lit/an/scea046/scea046.pdf

http://www.electronics-tutorials.ws/waveforms/generators.html