Vending machine FSM example in VHDL - Step-by-Step Execution

Concept Flow - Vending machine FSM example

Start: Idle State

↓

Wait for Coin

↓

Coin Inserted?

No→Stay Idle

Yes↓

Update Credit

↓

Credit >= Price?

No→Wait for More Coins

Yes↓

Dispense Item

↓

Return Change if any

↓

Reset Credit

↓

Back to Idle

The vending machine waits for coins, updates credit, checks if enough money is inserted, dispenses item, returns change, then resets to idle.

Execution Sample

VHDL

library ieee;
use ieee.std_logic_1164.all;

entity vending_machine is
  port(clk: in std_logic; coin: in std_logic; dispense: out std_logic);
end vending_machine;

This VHDL code snippet defines a vending machine entity with clock, coin input, and dispense output signals.

Execution Table

Step	State	Input Coin	Credit	Condition (Credit >= Price)	Action	Output Dispense
1	Idle	0	0	False	Wait for coin	0
2	Idle	1	1	False	Add coin to credit	0
3	Idle	1	2	True (Price=2)	Dispense item	1
4	Dispense	0	0	False	Reset credit	1
5	Idle	0	0	False	Wait for coin	0

💡 After dispensing, credit resets and machine returns to idle waiting for coins.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
Credit	0	1	2	0	0
Dispense	0	0	1	1	0
State	Idle	Idle	Idle	Dispense	Idle

Key Moments - 2 Insights

Why does the machine not dispense when the first coin is inserted?

Because the credit is less than the price (see Step 2 in execution_table), so it waits for more coins.

What happens to credit after dispensing the item?

Credit resets to zero after dispensing (Step 4), preparing the machine for the next customer.

Visual Quiz - 3 Questions

Look at the execution_table, what is the credit value after Step 3?

A 2

B 1

C 0

D 3

💡 Hint

Check the 'Credit' column in row for Step 3 in execution_table.

At which step does the machine dispense the item?

A Step 4

B Step 2

C Step 3

D Step 5

💡 Hint

Look at the 'Action' and 'Output Dispense' columns in execution_table.

If the price was 3 instead of 2, what would happen at Step 3?

A Dispense item

B Wait for more coins

C Add coin to credit

D Reset credit

💡 Hint

Refer to the condition 'Credit >= Price' in execution_table and imagine price=3.

Concept Snapshot

Vending Machine FSM in VHDL:
- States: Idle, Dispense
- Inputs: coin signal increments credit
- Check if credit >= price
- If yes, dispense item and reset credit
- Else, wait for more coins
- Output: dispense signal triggers item release

Full Transcript

This example shows a vending machine finite state machine in VHDL. It starts in Idle state waiting for coins. Each coin input adds to credit. When credit reaches or exceeds the price (2), the machine dispenses the item by setting the dispense output high. After dispensing, credit resets to zero and the machine returns to Idle. The execution table traces each step showing state, input, credit, condition checks, actions, and outputs. Key moments clarify why dispensing waits until enough credit and how credit resets after dispensing. The visual quiz tests understanding of credit values, dispensing step, and behavior if price changes.

Practice

1. What is the main purpose of a vending machine FSM in VHDL?

easy

A. To perform complex mathematical calculations

B. To control the sequence of accepting coins and dispensing items

C. To store large amounts of data permanently

D. To display graphics on a screen

Solution

1. Step 1: Understand FSM role in vending machine

   The FSM (Finite State Machine) controls the order of operations like accepting coins and dispensing items.

2. Step 2: Compare options to FSM purpose

   Only To control the sequence of accepting coins and dispensing items describes controlling the sequence of actions, which matches FSM's role.

3. Final Answer:

   To control the sequence of accepting coins and dispensing items -> Option B

4. Quick Check:

   FSM controls sequence = To control the sequence of accepting coins and dispensing items [OK]

Hint: FSM controls steps in process flow, not data storage or math [OK]

Common Mistakes:

• Confusing FSM with memory storage
• Thinking FSM does math or graphics
• Ignoring the sequence control aspect

2. Which VHDL statement correctly declares a state type for a vending machine FSM?

easy

A. signal state : integer := 0;

B. variable state : std_logic_vector(1 downto 0);

C. type state_type is (IDLE, COIN_INSERTED, DISPENSE);

D. constant state : boolean := true;

Solution

1. Step 1: Identify correct state type declaration

   States in FSM are usually declared as an enumerated type listing all states.

2. Step 2: Check options for enumerated type

   type state_type is (IDLE, COIN_INSERTED, DISPENSE); declares a type with named states, which is correct for FSM states.

3. Final Answer:

   type state_type is (IDLE, COIN_INSERTED, DISPENSE); -> Option C

4. Quick Check:

   State type enum = type state_type is (IDLE, COIN_INSERTED, DISPENSE); [OK]

Hint: FSM states use enumerated types, not integers or booleans [OK]

Common Mistakes:

• Using integer or boolean instead of enumerated type
• Confusing signal and type declarations
• Using variable instead of type for states

3. Given this VHDL snippet inside a vending machine FSM process:

case state is
  when IDLE =>
    if coin = '1' then
      next_state <= COIN_INSERTED;
    else
      next_state <= IDLE;
    end if;
  when COIN_INSERTED =>
    next_state <= DISPENSE;
  when DISPENSE =>
    next_state <= IDLE;
end case;

What will be the next state if current state is IDLE and coin = '0'?

medium

A. COIN_INSERTED

B. Undefined

C. DISPENSE

D. IDLE

Solution

1. Step 1: Analyze the IDLE state condition

   If coin = '1', next_state is COIN_INSERTED; else next_state remains IDLE.

2. Step 2: Apply coin = '0' condition

   Since coin = '0', the else branch applies, so next_state stays IDLE.

3. Final Answer:

   IDLE -> Option D

4. Quick Check:

   coin '0' in IDLE = next_state IDLE [OK]

Hint: Check coin input condition in IDLE state carefully [OK]

Common Mistakes:

• Assuming coin '0' moves to COIN_INSERTED
• Confusing next_state with current state
• Ignoring else branch in if statement

4. Identify the error in this VHDL FSM process snippet for a vending machine:

process(clk)
begin
  if rising_edge(clk) then
    case state is
      when IDLE =>
        if coin = '1' then
          state <= COIN_INSERTED;
        end if;
      when COIN_INSERTED =>
        state <= DISPENSE;
      when DISPENSE =>
        state <= IDLE;
    end case;
  end if;
end process;

medium

A. No reset condition for state initialization

B. State variable should be a signal, not variable

C. No default case in case statement

D. Missing else branch in IDLE state causes latch

Solution

1. Step 1: Check for reset condition in FSM process

   FSM processes usually include a reset to initialize the state at start.

2. Step 2: Identify missing reset

   This code lacks any reset signal or condition to set initial state, which is an error.

3. Final Answer:

   No reset condition for state initialization -> Option A

4. Quick Check:

   Missing reset = No reset condition for state initialization [OK]

Hint: Always include reset to set initial FSM state [OK]

Common Mistakes:

• Thinking else branch missing causes latch in synchronous process
• Confusing signal and variable usage
• Ignoring importance of reset in FSM

5. You want to extend a vending machine FSM to accept two coin types: 5 cents and 10 cents. The FSM should dispense an item when total inserted coins reach 15 cents or more. Which approach correctly updates the FSM to handle this?

hard

A. Add a coin_value input and a coin_total signal; update coin_total on each coin and transition to DISPENSE when coin_total >= 15

B. Use two separate FSMs for 5 and 10 cent coins and combine outputs

C. Ignore coin values and dispense after 3 coins inserted

D. Add a timer to dispense item after fixed time regardless of coins

Solution

1. Step 1: Understand requirement to track coin values

   We must track total coin value inserted, not just count coins.

2. Step 2: Choose approach to accumulate coin values

   Add a coin_value input and a coin_total signal; update coin_total on each coin and transition to DISPENSE when coin_total >= 15 uses a coin_value input and coin_total signal to sum coins and trigger dispense when total >= 15, which fits requirement.

3. Final Answer:

   Add a coin_value input and a coin_total signal; update coin_total on each coin and transition to DISPENSE when coin_total >= 15 -> Option A

4. Quick Check:

   Track coin values and total = Add coin_value and coin_total, dispense at 15+ [OK]

Hint: Sum coin values, dispense when total reaches target [OK]

Common Mistakes:

• Using separate FSMs instead of one with coin total
• Counting coins instead of summing values
• Dispensing by time, not coin total