2. Literature review
An early vending machine is invented by Hero of Alexandria, a first-century engineer and mathematician . In 1876, Simeon Denham built a vending machine dispensing stamps called “the first fully automatic vending machine”. A modern coin-operated vending machine was first introduced in London in 1883 by Percival Everitt. After that, vending machines have become well-known and been widely used in England.
The following is details about background knowledge related to the
middle of document…
That signal can be translated as the button has been pressed and released several time before it become steady. Also, if wiring the push button straight to the vending machine, when the buyer push and hold a button, the machine will read the signal as item has been purchased several times. So, the signal from a push button needs to be passed through a single pulser to make it steady and short before passing through the vending machine. The concepts of single pulser are reading input signal every clock instead of reading it all the time to make a steady output and generating only a single clock of high signal after the button has been pressed. The signal can become high again when the button is pressed after it has been released. So, the machine won’t misunderstand when user hold a button.
A multiplexer, or MUX, is a combinational logic network with 2n data inputs, n control inputs, and one data output. 
2.6 Karnaugh Map
Karnaugh Map or K-Map is a useful method for Boolean expression simplification. The goal of using this method is to implement the expression with fewest literals and fewer wires as possible. The concept of Karnaugh map is that when the output does not depend on some inputs, those inputs can be cut off from the expression since its do not affect a result of the output. The Kernaugh map is another way to represent a truth table so the designer can easily grouping