Basic Design
Contents
Introduction
As you know, FSMs can be implemented in a Mealy or in a Moore fashion. In a state of sugar-fueled euphoria, you forget about the culinary implications and opt for a Mealy implementation. Your extensive experience by now convinces you that a price of 4 B$ would undercut the competition while ensuring a steady stream of profits, particularly if you can ensure high volumes to give you negotiation leverage. You decide that the Mealy FSM should receive as input the three denominations of 1 B$, 3 B$, and 5 B$ bills1.
VendingMachine Component
This part is the top level of the design. Its job is to take in the money deposited and provide a chocolate bar once at least 4 B$ is deposited and to return the appropriate amount of change to the customer. To handle the case of vacillating customers midstream changing their minds about the purchase, in case they reassess the nutritional benefits of chocolate, you also assign an input code for a refund. The two-bit input signal indicates the amount of money deposited into the machine or if a refund is requested. The encoding is in the table below:
Table 1
Encoding for what bill is deposited
| Code | Description |
|---|---|
| 00 | Refund2 |
| 01 | 1 B$ |
| 10 | 3 B$ |
| 11 | 5 B$ |
For this vending machine, there are two sets of outputs to consider. The first output indicates whether a chocolate is dispensed by the machine where ‘0’ means no chocolate and ‘1’ means a chocolate is dispensed. The next output to consider is concerned with how much change/refund we need to return to the user. Suppose that someone inserts a 3 B$ coin and then a 5 B$ coin. To maximize your profits, you decide to sell two chocolate bars and offer no refunds. However, now suppose that someone inserts a 3 B$ coin and another 3 B$ coin, resulting in a refund of 2 B$ and a successful purchase of a chocolate bar. Now suppose we get an indecisive gym bro who inserts a 3 B$ coin, but decides that the macros aren’t there in our chocolate bar. When he refunds the amount, we have to give the gym bro 3 B$. We can therefore surmise that the amount of change/refund returned by the machine may run as high as 3 B$. For now, the FSM will output the entire change/refund amount in one swoop by using a 2 bit-output signal whose encoding is shown below.
Table 2
Encoding for the amount to refund
| Code | Refund Amount |
|---|---|
| 00 | 0 B$ |
| 01 | 1 B$ |
| 10 | 2 B$ |
| 11 | 3 B$ |
Table 3
Encoding for the amount of chocolate to refund
| Code | Chocolate Amount |
|---|---|
| 00 | 0 chocolates |
| 01 | 1 chocolate |
| 1X | 2 chocolates |
The input ports VendingMachine that we are asking you to implement takes in the money and the output ports provides a chocolate bar and the change shown in the figure below. You should need no more than four states to implement this FSM (including the initial state denoting no money having been received yet). It should take you no more than two flipflops to capture the state information, and with the two coin inputs, you will be working with 4-input K-maps to generate your next-state logic and the 3 output bits.
You need a reset signal because the initial state for the flipflops in real life is unknown. It is much easier to start in a 0 state than it is to start in an unknown one!
Footnotes
-
After a deep 45 minute search we found that the Bahamas is one of very few countries to follow this denomination scheme. ↩
-
People who take too long depositing 4 B$ into our machine can keep it moving! We have chocolate to sell! But seriously, when testing your circuit, a real-time clock might operate too quickly for you to test your circuit correctly. Either manually advance the clock, or use a testbench to automate this process for you. Our testbenches won’t ask for a refund unless we mean it to, meaning that if we deposit bills back to back, we will do so in subsequent clock cycles, with no gaps left behind. ↩