Vending machine FSM example in VHDL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare the state type for the vending machine FSM.

VHDL

type state_type is ([1]);

Drag options to blanks, or click blank then click option'

AIDLE

BSTART, STOP

CIDLE, COIN_INSERTED, DISPENSE

DWAIT

Attempts:

3 left

💡 Hint

Common Mistakes

Listing multiple states without commas.

Using invalid identifiers for states.

The state_type should be declared as an enumeration listing all states. Here, multiple states are declared to represent the vending machine states.

2fill in blank

medium

Complete the code to define the signal for the current state.

VHDL

signal current_state : [1] := IDLE;

Drag options to blanks, or click blank then click option'

Astate_type

Bbit

Cstd_logic

Dinteger

Attempts:

3 left

💡 Hint

Common Mistakes

Using std_logic or bit instead of the state_type.

Using integer which is not suitable for states.

The current_state signal must be of the state_type declared for the FSM states.

3fill in blank

hard

Fix the error in the process sensitivity list to include the clock and reset signals.

VHDL

process([1])
begin
  if reset = '1' then
    current_state <= IDLE;
  elsif rising_edge(clock) then
    -- state transitions
  end if;
end process;

Drag options to blanks, or click blank then click option'

Areset

Bclock, reset

Cclock or reset

Dclock

Attempts:

3 left

💡 Hint

Common Mistakes

Including only clock or only reset.

Using 'or' instead of a comma in sensitivity list.

The sensitivity list must include both clock and reset signals separated by a comma (,). This makes the process sensitive to changes in either signal.

4fill in blank

hard

Fill both blanks to complete the state transition for coin insertion and dispensing.

VHDL

case current_state is
  when IDLE =>
    if coin = '1' then
      current_state <= [1];
    else
      current_state <= IDLE;
    end if;
  when [2] =>
    current_state <= IDLE;
  when others =>
    current_state <= IDLE;
end case;

Drag options to blanks, or click blank then click option'

ACOIN_INSERTED

BDISPENSE

DWAIT

Attempts:

3 left

💡 Hint

Common Mistakes

Using WAIT instead of COIN_INSERTED or DISPENSE.

Mixing up the order of states.

When a coin is inserted, the state moves to COIN_INSERTED. After dispensing, the state returns to IDLE.

5fill in blank

hard

Fill all three blanks to complete the output logic for dispensing and coin return.

VHDL

with current_state select
  dispense <= [1] when DISPENSE,
             [2] when COIN_INSERTED,
             [3] when others;

Drag options to blanks, or click blank then click option'

A'1'

B'0'

Attempts:

3 left

💡 Hint

Common Mistakes

Setting dispense to '1' in COIN_INSERTED state.

Using inconsistent values for others.

Dispense signal is '1' only in DISPENSE state; otherwise, it is '0'.

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