to Solution HW #10 and Studio 11 The objective of this studio is to start with the design of the Vending Machine Controller. Issues like interfaces with coin recognition, vending, actual dispensing of change etc. are beyond the scope of the lab. However, These might raise some interesting issues in the actual design of a real life vending machine controller. The "state" of the controller records how much credit is available. 1) There are 4 inputs : Q, N, D, CR There are 2 outputs: DG, RC. This can be easily seen from the block diagram provided. 2) The possible input sequences are: Q NNNNN NNNND NNNNQ NNND NNDN NDNN DNNN NNNQ NNDD NNQ NQ DDD DDN NDD DDQ DNQ NDQ DQ DND DDN NDNQ NNDQ DNNQ NNDQ There are 25 combinations... some of these are permutations of each other. 3) Assumptions: - When returning coins, there is no check to see if the required amount of change to be returned actually exists in the machine. It is assumed that this is taken care of elsewhere. The controller is only concerned with maintaining the right of amount credit that exisits in the machine. hence, if at any point, the user hits the Coin Return input, the state jumps back to the Zero Credit state, with the Return Coins ouput asserted high. - only one input active at a time.
4) State Diagram
5) Define the states according to the credit stored as: S0: S1: S2: S3: S4: 0 cents (also called "INIT" in a number of solutions) 5 cents 10 cents 15 cents 20 cents
State/Output table: Note that in each of the Next State/output columns, the outputs are in the order DG, CR Present (N,D,Q,CR) State 0001 0010 0100 1000 ----------------------------------------------S0 S0/0,1 S0/1,0 S2/0,0 S1 S0/0,1 S1/1,0 S3/0,0 S2 S0/0,1 S2/1,0 S4/0,0 S3 S0/0,1 S3/1,0 S0/1,0 S4 S0/0,1 S4/1,0 S1/1,0
S1/0,0 S2/0,0 S3/0,0 S4/0,0 S0/1,0
6) To build the state transition table, use the state/output table with grey code for the state encoding: S0= S1= S2= S3= S4= 000 001 011 010 110
7) As seen earlier, 5 states are needed. 8) Since 5 states are needed, 3 flip flops will be necessary