Timer with terminal count output in VHDL

Introduction

A timer counts clock pulses to measure time intervals. The terminal count output tells us when the timer reaches a set value.

To create a delay in a digital circuit.

To measure how long an event lasts.

To trigger an action after a specific time.

To generate periodic signals like blinking lights.

Syntax

VHDL

process(clk, reset)
  variable count : integer := 0;
begin
  if reset = '1' then
    count := 0;
    terminal_count <= '0';
  elsif rising_edge(clk) then
    if count = MAX_COUNT then
      terminal_count <= '1';
      count := 0;
    else
      count := count + 1;
      terminal_count <= '0';
    end if;
  end if;
end process;

The process runs on clock and reset signals.

Terminal count output goes high when count reaches MAX_COUNT.

Examples

This example counts from 0 to 10 and sets terminal_count high at 10.

VHDL

signal terminal_count : std_logic;
constant MAX_COUNT : integer := 10;

process(clk, reset)
  variable count : integer := 0;
begin
  if reset = '1' then
    count := 0;
    terminal_count <= '0';
  elsif rising_edge(clk) then
    if count = MAX_COUNT then
      terminal_count <= '1';
      count := 0;
    else
      count := count + 1;
      terminal_count <= '0';
    end if;
  end if;
end process;

This variation sets terminal_count high after counting up to MAX_COUNT, then resets.

VHDL

process(clk, reset)
  variable count : integer := 0;
begin
  if reset = '1' then
    count := 0;
    terminal_count <= '0';
  elsif rising_edge(clk) then
    if count < MAX_COUNT then
      count := count + 1;
      terminal_count <= '0';
    else
      terminal_count <= '1';
      count := 0;
    end if;
  end if;
end process;

Sample Program

This timer counts clock pulses up to 5. When it reaches 5, terminal_count goes high for one clock cycle, then the count resets.

VHDL

library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;

entity Timer is
  port(
    clk : in std_logic;
    reset : in std_logic;
    terminal_count : out std_logic
  );
end Timer;

architecture Behavioral of Timer is
  constant MAX_COUNT : integer := 5;
  signal count : integer := 0;
begin
  process(clk, reset)
  begin
    if reset = '1' then
      count <= 0;
      terminal_count <= '0';
    elsif rising_edge(clk) then
      if count = MAX_COUNT then
        terminal_count <= '1';
        count <= 0;
      else
        count <= count + 1;
        terminal_count <= '0';
      end if;
    end if;
  end process;
end Behavioral;

Important Notes

Terminal count output is usually a single clock pulse indicating the timer reached the limit.

Resetting the count after terminal count helps the timer repeat cycles.

Summary

A timer counts clock pulses to measure time.

Terminal count output signals when the timer reaches a set value.

Resetting the count allows the timer to repeat its cycle.

Practice

1. What does the terminal count output signal in a VHDL timer?

easy

A. The timer has reached its preset count value

B. The timer is reset to zero

C. The clock signal is inactive

D. The timer is paused

Solution

1. Step 1: Understand the timer function

   A timer counts clock pulses until it reaches a preset value.

2. Step 2: Define terminal count output meaning

   The terminal count output signals when the timer hits that preset value.

3. Final Answer:

   The timer has reached its preset count value -> Option A

4. Quick Check:

   Terminal count = timer reached preset [OK]

Hint: Terminal count means timer reached set value [OK]

Common Mistakes:

• Confusing terminal count with reset signal
• Thinking terminal count means timer stopped
• Mixing terminal count with clock inactivity

2. Which of the following is the correct VHDL syntax to declare a terminal count output signal named tc as a std_logic?

easy

A. signal tc std_logic;

B. signal tc : std_logic

C. signal tc : std_logic;

D. signal tc = std_logic;

Solution

1. Step 1: Recall VHDL signal declaration syntax

   Signals are declared with keyword 'signal', name, colon, type, and semicolon.

2. Step 2: Check each option

   signal tc : std_logic; uses correct syntax: 'signal tc : std_logic;'. Others miss colon or semicolon or use wrong assignment.

3. Final Answer:

   signal tc : std_logic; -> Option C

4. Quick Check:

   Signal declaration = signal name : type; [OK]

Hint: Signal declaration needs colon and semicolon [OK]

Common Mistakes:

• Omitting colon between name and type
• Forgetting semicolon at end
• Using equals sign instead of colon

3. Given this VHDL process for a timer:

process(clk, reset)
begin
  if reset = '1' then
    count <= 0;
    tc <= '0';
  elsif rising_edge(clk) then
    if count = 9 then
      count <= 0;
      tc <= '1';
    else
      count <= count + 1;
      tc <= '0';
    end if;
  end if;
end process;

What is the value of tc after 10 clock cycles without reset?

medium

A. '1' on the 10th cycle, '0' otherwise

B. '0' on all cycles

C. '1' on every cycle

D. '1' on the 9th cycle only

Solution

1. Step 1: Analyze counting and terminal count logic

   The counter counts from 0 to 9. When count = 9, tc is set to '1' and count resets to 0.

2. Step 2: Trace tc output over 10 clock cycles

   For cycles 1 to 9, tc = '0'. On the 10th cycle (count=9), tc = '1'. Then count resets.

3. Final Answer:

   '1' on the 10th cycle, '0' otherwise -> Option A

4. Quick Check:

   tc = '1' at terminal count (10th cycle) [OK]

Hint: tc is '1' only when count reaches max [OK]

Common Mistakes:

• Assuming tc is '1' every cycle
• Confusing count value for tc output
• Forgetting count resets after terminal count

4. Identify the error in this VHDL timer process:

process(clk, reset)
begin
  if reset = '1' then
    count <= 0;
    tc <= '0';
  elsif clk'event and clk = '1' then
    if count = 10 then
      count <= 0;
      tc <= '1';
    else
      count <= count + 1;
      tc <= '0';
    end if;
  end if;
end process;

medium

A. tc signal should be declared as integer

B. Using clk'event and clk = '1' instead of rising_edge(clk)

C. Reset condition should be checked after clock event

D. Count comparison should be count = 9, not 10

Solution

1. Step 1: Check count comparison logic

   The timer counts from 0, so max count for 10 cycles is 9, not 10.

2. Step 2: Understand effect of wrong count value

   If count = 10 is checked, terminal count never triggers because count resets at 9+1=10, causing off-by-one error.

3. Final Answer:

   Count comparison should be count = 9, not 10 -> Option D

4. Quick Check:

   Max count for 10 cycles = 9 [OK]

Hint: Count max is one less than total cycles [OK]

Common Mistakes:

• Using clk'event instead of rising_edge(clk) (still valid but less modern)
• Checking reset after clock event
• Declaring tc as wrong type

5. You want to design a VHDL timer that counts from 0 to 15 and sets terminal count output tc high for exactly one clock cycle when count reaches 15. Which code snippet correctly implements this behavior?

hard

A. if count = 15 then
count <= 15;
tc <= '1';
else
count <= count + 1;
tc <= '0';
end if;

B. if count = 15 then
count <= 0;
tc <= '1';
else
count <= count + 1;
tc <= '0';
end if;

C. if count >= 15 then
tc <= '1';
else
count <= count + 1;
tc <= '0';
end if;

D. if count = 14 then
count <= 0;
tc <= '1';
else
count <= count + 1;
tc <= '0';
end if;

Solution

1. Step 1: Understand terminal count condition

   The timer must reset count to 0 and set tc='1' exactly when count = 15.

2. Step 2: Evaluate each option

   if count = 15 then count <= 0; tc <= '1'; else count <= count + 1; tc <= '0'; end if; resets count at 15 and sets tc='1' for one cycle. if count = 15 then count <= 15; tc <= '1'; else count <= count + 1; tc <= '0'; end if; holds count at 15, causing no reset. if count >= 15 then tc <= '1'; else count <= count + 1; tc <= '0'; end if; does not reset count, causing tc to remain high for multiple cycles. if count = 14 then count <= 0; tc <= '1'; else count <= count + 1; tc <= '0'; end if; triggers at 14, which is incorrect.

3. Final Answer:

   if count = 15 then count <= 0; tc <= '1'; else count <= count + 1; tc <= '0'; end if; -> Option B

4. Quick Check:

   tc high only at count = 15, then reset [OK]

Hint: Reset count and set tc only at exact max count [OK]

Common Mistakes:

• Not resetting count after terminal count
• Setting tc high for multiple cycles
• Triggering terminal count too early