You are to find the state- machine diagram for the following electronic vending- machine specification. The vending machine sells soda for $1.50 per bottle. The machine accepts only D ($1 bills) and Q (quarters = 25¢). When the sum of money is greater than $1.50, i.e., two $1 bills, the machine returns change in the coin return (two quarters). When $1.50 has been paid, the machine lights an LED to indicate that a soda flavor may be selected. The choices by pushbutton are C (Cola), L (Lemon soda), O (Orange soda), and R (Root Beer). When one pushbutton is pushed, the selected soda is dispensed and the machine returns to its initial state. One other feature is that an LED comes on to warn the user that two quarters are not available for change, so if a second $1 bill is inserted, no change will be given.

(a) Find the state- machine diagram for the soda vending machine as specified.

(b) The specification as given is not very user friendly. Rewrite it to provide a remedy for every possible situation that the user might encounter in using the machine.

All iles referred to in the remaining problems are available in ASCII form for simulation and editing on the Companion Website for the text. A VHDL or Verilog compiler/simulator is necessary for the problems or portions of problems requesting simulation. Descriptions can still be written, however, for many of the problems without using compilation or simulation.