Reset strategy selection in VHDL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an asynchronous reset signal named 'rst'.

VHDL

signal rst : std_logic := [1];

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

A'0'

B'1'

C"0"

D"1"

Attempts:

3 left

💡 Hint

Common Mistakes

Using double quotes instead of single quotes for std_logic.

Initializing reset to '1' by mistake.

The reset signal is usually initialized to '0' in VHDL using single quotes for std_logic.

2fill in blank

medium

Complete the process sensitivity list to include the clock and asynchronous reset signals.

VHDL

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

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

Aclk

Bclk, rst

Crst

Dclk or rst

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'or' instead of commas in sensitivity list.

Including only clock or only reset.

In VHDL, the sensitivity list uses commas to separate signals, so 'clk, rst' is correct.

3fill in blank

hard

Fix the error in the reset condition check inside the process.

VHDL

if [1] = '1' then
  q <= '0';
elsif rising_edge(clk) then
  q <= d;
end if;

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

Ad

Bclk

Cq

Drst

Attempts:

3 left

💡 Hint

Common Mistakes

Checking clock signal in reset condition.

Checking output signal instead of reset.

The reset condition should check the reset signal 'rst' to asynchronously reset the output.

4fill in blank

hard

Fill both blanks to complete the synchronous reset process.

VHDL

process(clk)
begin
  if rising_edge([1]) then
    if [2] = '1' then
      q <= '0';
    else
      q <= d;
    end if;
  end if;
end process;

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

Aclk

Brst

Cq

Dd

Attempts:

3 left

💡 Hint

Common Mistakes

Using reset in rising_edge.

Checking output signal instead of reset.

The clock signal is used in rising_edge, and the reset signal is checked synchronously inside.

5fill in blank

hard

Fill all three blanks to complete the process with asynchronous reset and data assignment.

VHDL

process([1])
begin
  if [2] = '1' then
    q <= '0';
  elsif rising_edge([3]) then
    q <= d;
  end if;
end process;

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

Aclk, rst

Brst

Cclk

Dd

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'or' in sensitivity list.

Checking clock in reset condition.

Wrong order of conditions.

The sensitivity list includes clock and reset separated by ','. Reset is checked first, then clock edge for data assignment.

Practice

1. Which statement best describes an asynchronous reset in VHDL?

easy

A. It requires a special reset clock signal to activate.

B. It resets the circuit immediately, independent of the clock.

C. It resets the circuit only on the rising edge of the clock.

D. It resets the circuit only after a delay of several clock cycles.

Solution

1. Step 1: Understand asynchronous reset behavior

   An asynchronous reset acts immediately when activated, without waiting for the clock signal.

2. Step 2: Compare with synchronous reset

   Synchronous reset waits for the clock edge, unlike asynchronous reset which is independent of the clock.

3. Final Answer:

   It resets the circuit immediately, independent of the clock. -> Option B

4. Quick Check:

   Asynchronous reset = immediate reset [OK]

Hint: Asynchronous reset ignores clock, acts immediately [OK]

Common Mistakes:

• Confusing asynchronous with synchronous reset
• Thinking reset waits for clock edge
• Assuming reset needs a special clock

2. Which VHDL code snippet correctly implements a synchronous reset for a flip-flop?

easy

A. process(reset) begin if reset = '1' then q <= '0'; end if; end process;

B. process(clk, reset) begin if reset = '1' then q <= '0'; elsif rising_edge(clk) then q <= d; end if; end process;

C. process(clk) begin if rising_edge(clk) then if reset = '1' then q <= '0'; else q <= d; end if; end if; end process;

D. process(clk) begin if reset = '1' then q <= '0'; elsif rising_edge(clk) then q <= d; end if; end process;

Solution

1. Step 1: Identify synchronous reset structure

   Synchronous reset checks reset inside the clock edge condition, so reset is applied only on clock rising edge.

2. Step 2: Analyze each option

   process(clk) begin if rising_edge(clk) then if reset = '1' then q <= '0'; else q <= d; end if; end if; end process; checks reset inside rising_edge(clk), which is correct for synchronous reset. process(clk, reset) begin if reset = '1' then q <= '0'; elsif rising_edge(clk) then q <= d; end if; end process; uses asynchronous reset sensitivity. process(reset) begin if reset = '1' then q <= '0'; end if; end process; is asynchronous and incomplete. process(clk) begin if reset = '1' then q <= '0'; elsif rising_edge(clk) then q <= d; end if; end process; checks reset outside clock edge, incorrect for synchronous.

3. Final Answer:

   process(clk) begin if rising_edge(clk) then if reset = '1' then q <= '0'; else q <= d; end if; end if; end process; -> Option C

4. Quick Check:

   Synchronous reset inside clock edge = process(clk) begin if rising_edge(clk) then if reset = '1' then q <= '0'; else q <= d; end if; end if; end process; [OK]

Hint: Synchronous reset inside rising_edge(clk) block [OK]

Common Mistakes:

• Putting reset check outside clock edge
• Using reset in sensitivity list for synchronous reset
• Confusing asynchronous and synchronous reset syntax

3. Given this VHDL process with asynchronous reset:

process(clk, rst) begin
  if rst = '1' then
    q <= '0';
  elsif rising_edge(clk) then
    q <= d;
  end if;
end process;

What is the value of q after reset rst goes high, regardless of clock?

medium

A. Previous value of q retained

B. Value of d at next clock edge

C. Undefined until next clock edge

D. '0' immediately

Solution

1. Step 1: Understand asynchronous reset effect

   When rst is '1', the process sets q to '0' immediately, ignoring clock.

2. Step 2: Confirm behavior regardless of clock

   Because rst is in sensitivity list and checked first, q resets immediately when rst = '1'.

3. Final Answer:

   '0' immediately -> Option D

4. Quick Check:

   Asynchronous reset sets q to '0' immediately [OK]

Hint: Asynchronous reset sets output immediately, ignoring clock [OK]

Common Mistakes:

• Thinking reset waits for clock edge
• Assuming q keeps old value during reset
• Confusing asynchronous with synchronous reset

4. Identify the error in this VHDL code implementing synchronous reset:

process(clk, reset) begin
  if rising_edge(clk) then
    if reset = '1' then
      q <= '0';
    else
      q <= d;
    end if;
  end if;
end process;

medium

A. Reset should be in sensitivity list only if asynchronous

B. Reset check must be outside rising_edge(clk)

C. Missing else clause for reset condition

D. Process should not have clk in sensitivity list

Solution

1. Step 1: Check sensitivity list for synchronous reset

   Synchronous reset requires only clock in sensitivity list; including reset causes unnecessary triggers.

2. Step 2: Confirm reset check inside clock edge is correct

   Reset is checked inside rising_edge(clk), which is correct for synchronous reset.

3. Final Answer:

   Reset should be in sensitivity list only if asynchronous -> Option A

4. Quick Check:

   Synchronous reset excludes reset from sensitivity list [OK]

Hint: Synchronous reset excludes reset from sensitivity list [OK]

Common Mistakes:

• Including reset in sensitivity list for synchronous reset
• Moving reset check outside clock edge for synchronous reset
• Confusing sensitivity list rules

5. You want a reset that ensures your circuit starts immediately after power-on but avoids glitches during clock transitions. Which reset strategy should you choose?

hard

A. Asynchronous reset with glitch filter circuit

B. Synchronous reset only

C. Asynchronous reset without any filtering

D. No reset, rely on power-on default states

Solution

1. Step 1: Understand immediate reset need

   Immediate reset after power-on requires asynchronous reset to act without waiting for clock.

2. Step 2: Avoid glitches during clock transitions

   Asynchronous resets can cause glitches; adding a glitch filter or synchronizer avoids this problem.

3. Final Answer:

   Asynchronous reset with glitch filter circuit -> Option A

4. Quick Check:

   Immediate reset + glitch avoidance = asynchronous with filter [OK]

Hint: Use async reset plus filter to avoid glitches [OK]

Common Mistakes:

• Choosing synchronous reset only (delays reset)
• Using async reset without glitch protection
• Assuming power-on defaults are reliable