Edge detector design in VHDL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a signal named 'prev_signal' of type std_logic.

VHDL

signal prev_signal : [1];

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

Astd_logic_vector(3 downto 0)

Bbit

Cinteger

Dstd_logic

Attempts:

3 left

💡 Hint

Common Mistakes

Using std_logic_vector instead of std_logic for a single bit signal.

Using integer type which is not suitable for single bit signals.

The signal 'prev_signal' should be of type std_logic to hold a single bit value.

2fill in blank

medium

Complete the process sensitivity list to detect changes on the input signal 'signal_in'.

VHDL

process([1])
begin
  -- process body
end process;

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

Asignal_in

Breset

Cclk

Dprev_signal

Attempts:

3 left

💡 Hint

Common Mistakes

Using clock signal instead of the input signal in sensitivity list.

Leaving sensitivity list empty or incorrect.

The process should be sensitive to changes on signal_in to detect edges.

3fill in blank

hard

Fix the error in the assignment to detect a rising edge on 'signal_in'.

VHDL

rising_edge_detected <= (signal_in = '1') and ([1] = '0');

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

Aclk

Bsignal_in

Cprev_signal

Dreset

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing signal_in with itself instead of previous signal.

Using clock or reset signals incorrectly.

To detect a rising edge, compare current signal_in with previous value stored in prev_signal.

4fill in blank

hard

Fill both blanks to update 'prev_signal' and detect a falling edge on 'signal_in'.

VHDL

process(clk)
begin
  if rising_edge(clk) then
    prev_signal <= [1];
    falling_edge_detected <= (prev_signal = '1') and ([2] = '0');
  end if;
end process;

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

Asignal_in

Bprev_signal

Dclk

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong signals for update or comparison.

Mixing clock signal with input signals.

Update prev_signal with current signal_in value. Detect falling edge when previous was '1' and current is '0'.

5fill in blank

hard

Fill all three blanks to create a complete edge detector process that detects rising edges on 'signal_in'.

VHDL

process([1])
begin
  if [2] then
    rising_edge_detected <= (signal_in = '1') and (prev_signal = '0');
    prev_signal <= signal_in;
  end if;
end process;

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

Aclk

Brising_edge(clk)

Dsignal_in

Attempts:

3 left

💡 Hint

Common Mistakes

Using input signal in sensitivity list instead of clock.

Not using rising_edge function for clock edge detection.

The process is sensitive to clock. The condition checks for rising edge of clock. Inside, it detects rising edge on signal_in by comparing with prev_signal and updates prev_signal.

Practice

1. What is the main purpose of an edge detector in VHDL?

easy

A. To generate a clock signal

B. To store a constant value permanently

C. To convert analog signals to digital

D. To detect changes in a signal from low to high or high to low

Solution

1. Step 1: Understand the role of edge detectors

   Edge detectors monitor signals to find when they change state, either rising or falling edges.

2. Step 2: Identify the correct function

   Detecting changes in signal levels is exactly what edge detectors do, unlike storing values or generating clocks.

3. Final Answer:

   To detect changes in a signal from low to high or high to low -> Option D

4. Quick Check:

   Edge detection = signal change detection [OK]

Hint: Edge detectors track signal changes, not constants or clocks [OK]

Common Mistakes:

• Confusing edge detection with clock generation
• Thinking edge detectors store constant values
• Assuming edge detectors convert signal types

2. Which VHDL code snippet correctly declares a signal to store the previous value for edge detection?

easy

A. signal prev_signal : integer range 0 to 1;

B. signal prev_signal : std_logic;

C. constant prev_signal : std_logic_vector(1 downto 0);

D. variable prev_signal : integer;

Solution

1. Step 1: Identify correct signal declaration for previous value

   For edge detection, we store the previous signal value as a signal of type std_logic.

2. Step 2: Compare options

   signal prev_signal : std_logic; declares a signal of type std_logic, suitable for storing previous signal state. Variables or constants are not appropriate here.

3. Final Answer:

   signal prev_signal : std_logic; -> Option B

4. Quick Check:

   Previous value storage = signal std_logic [OK]

Hint: Use 'signal' with std_logic for previous signal storage [OK]

Common Mistakes:

• Using variable instead of signal for previous value
• Declaring constant instead of signal
• Using incorrect data types like integer

3. Given the following VHDL process for a rising edge detector:

process(clk)
begin
  if rising_edge(clk) then
    prev_signal <= signal_in;
    if signal_in = '1' and prev_signal = '0' then
      edge_detected <= '1';
    else
      edge_detected <= '0';
    end if;
  end if;
end process;

What will be the value of edge_detected if signal_in changes from '0' to '1' at the clock edge?

medium

A. '0'

B. Undefined

C. '1'

D. Signal error

Solution

1. Step 1: Understand signal update timing in VHDL

   In VHDL, signal assignments inside a clocked process update after the process ends, so prev_signal still holds old value during evaluation.

2. Step 2: Evaluate condition at clock edge

   At the clock edge, prev_signal still equals '0' (old value), signal_in is '1'. prev_signal <= signal_in schedules update after process. Condition signal_in='1' and prev_signal='0' is true, so edge_detected <= '1'.

3. Final Answer:

   '1' -> Option C

4. Quick Check:

   Signal update delay enables edge detection with old prev_signal [OK]

Hint: prev_signal lags one cycle, allowing correct rising edge detection [OK]

Common Mistakes:

• Assuming prev_signal updates immediately within process
• Confusing signal and variable assignment timing
• Thinking edge_detected remains '0' due to update order

4. Identify the error in this VHDL code snippet for a falling edge detector:

process(clk)
begin
  if rising_edge(clk) then
    prev_signal <= signal_in;
    if signal_in = '0' and prev_signal = '1' then
      edge_detected <= '1';
    else
      edge_detected <= '0';
    end if;
  end if;
end process;

medium

A. No error; code is correct

B. prev_signal should be updated after edge_detected assignment

C. prev_signal must be a variable, not a signal

D. The condition should check for rising_edge(signal_in)

Solution

1. Step 1: Analyze signal update order

   Due to VHDL signal semantics, prev_signal retains its prior value ('1') during the condition check even though assigned first, allowing correct falling edge detection when signal_in='0'.

2. Step 2: Confirm code correctness

   This is the standard implementation for a synchronous falling edge detector; no error exists.

3. Final Answer:

   No error; code is correct -> Option A

4. Quick Check:

   Standard order works due to delta-delay updates [OK]

Hint: VHDL signal reads use pre-process values; code detects edge correctly [OK]

Common Mistakes:

• Misunderstanding that assignment order requires swapping
• Using rising_edge on signal_in instead of clk
• Declaring prev_signal as variable

5. You want to design an edge detector in VHDL that detects both rising and falling edges of a signal signal_in. Which approach correctly implements this?

hard

A. Compare current and previous values: edge_detected <= '1' when signal_in /= prev_signal else '0'; update prev_signal after

B. Use two separate processes for rising and falling edges with different clocks

C. Assign edge_detected <= '1' when signal_in = '1' else '0'; no previous value needed

D. Use a variable to store signal_in and assign edge_detected <= '1' when variable = '0'

Solution

1. Step 1: Understand dual edge detection logic

   Detecting both edges means detecting any change in signal_in compared to its previous value.

2. Step 2: Implement comparison and update order

   Comparing current and previous values with signal_in /= prev_signal detects any edge. Updating prev_signal after ensures correct timing.

3. Final Answer:

   Compare current and previous values: edge_detected <= '1' when signal_in /= prev_signal else '0'; update prev_signal after -> Option A

4. Quick Check:

   signal_in /= prev_signal detects any edge [OK]

Hint: Detect edges by comparing current and previous signal values [OK]

Common Mistakes:

• Using separate clocks for edge detection
• Ignoring previous signal value
• Assigning edge_detected based only on current signal