Prescaler for clock division in VHDL

Introduction

A prescaler divides a fast clock signal into a slower one. This helps when you need a slower clock for other parts of your circuit.

When you want to slow down a fast clock to control LEDs blinking speed.

When you need a slower clock for communication protocols like UART.

When testing circuits that require slower timing signals.

When saving power by running parts of a circuit at lower speeds.

Syntax

VHDL

process(clk)
  variable count : integer := 0;
begin
  if rising_edge(clk) then
    if count = N-1 then
      output_clk <= not output_clk;
      count := 0;
    else
      count := count + 1;
    end if;
  end if;
end process;

Replace N with the division factor you want.

This process toggles the output clock every N input clock cycles.

Examples

This example divides the input clock by 4.

VHDL

process(clk)
  variable count : integer := 0;
begin
  if rising_edge(clk) then
    if count = 3 then  -- divide clock by 4
      output_clk <= not output_clk;
      count := 0;
    else
      count := count + 1;
    end if;
  end if;
end process;

This example divides the input clock by 10.

VHDL

process(clk)
  variable count : integer := 0;
begin
  if rising_edge(clk) then
    if count = 9 then  -- divide clock by 10
      output_clk <= not output_clk;
      count := 0;
    else
      count := count + 1;
    end if;
  end if;
end process;

Sample Program

This VHDL code creates a prescaler that divides the input clock by 5. The output slow_clk toggles every 5 input clock cycles, making it slower.

VHDL

library ieee;
use ieee.std_logic_1164.all;

entity prescaler is
  port(
    clk       : in std_logic;
    reset     : in std_logic;
    slow_clk  : out std_logic
  );
end prescaler;

architecture Behavioral of prescaler is
  signal count : integer range 0 to 4 := 0;
  signal temp_clk : std_logic := '0';
begin
  process(clk, reset)
  begin
    if reset = '1' then
      count <= 0;
      temp_clk <= '0';
    elsif rising_edge(clk) then
      if count = 4 then  -- divide clock by 5
        temp_clk <= not temp_clk;
        count <= 0;
      else
        count <= count + 1;
      end if;
    end if;
  end process;

  slow_clk <= temp_clk;
end Behavioral;

Important Notes

Resetting the counter ensures the prescaler starts cleanly.

The output clock frequency is input frequency divided by (2 * N), because the output toggles every N cycles.

Use an integer range for the counter to save resources.

Summary

A prescaler divides a fast clock to a slower clock.

It uses a counter to toggle output after a set number of input clock cycles.

Resetting the counter helps start the prescaler correctly.

Practice

1. What is the main purpose of a prescaler in VHDL clock division?

easy

A. To generate random clock pulses

B. To multiply the input clock frequency

C. To reset the clock signal

D. To divide a fast input clock into a slower output clock

Solution

1. Step 1: Understand the function of a prescaler

   A prescaler reduces the frequency of a fast clock by dividing it to create a slower clock signal.

2. Step 2: Compare options with the prescaler function

   Only To divide a fast input clock into a slower output clock correctly describes this division function; others describe unrelated actions.

3. Final Answer:

   To divide a fast input clock into a slower output clock -> Option D

4. Quick Check:

   Prescaler divides clock = To divide clock [OK]

Hint: Prescaler always divides clock frequency, never multiplies [OK]

Common Mistakes:

• Thinking prescaler multiplies clock frequency
• Confusing prescaler with reset function
• Assuming prescaler generates random pulses

2. Which VHDL code snippet correctly declares a counter signal for a prescaler?

easy

A. signal counter : integer range 0 to 99 := 0;

B. signal counter : std_logic_vector(7 downto 0) := '0';

C. signal counter : boolean := false;

D. signal counter : integer := '0';

Solution

1. Step 1: Identify proper signal type for counting

   Counting requires an integer with a defined range and initial value, as in signal counter : integer range 0 to 99 := 0;.

2. Step 2: Check other options for correctness

   signal counter : std_logic_vector(7 downto 0) := '0'; uses std_logic_vector but initializes incorrectly with a single quote; signal counter : boolean := false; uses boolean which can't count; signal counter : integer := '0'; assigns a character to integer, which is invalid.

3. Final Answer:

   signal counter : integer range 0 to 99 := 0; -> Option A

4. Quick Check:

   Integer counter with range and init = Correct declaration [OK]

Hint: Use integer with range and initial value for counters [OK]

Common Mistakes:

• Using std_logic_vector without proper initialization
• Using boolean type for counting
• Assigning character literals to integer signals

3. Given this VHDL process for a prescaler with a max count of 3:

process(clk, reset)
begin
  if reset = '1' then
    counter <= 0;
    clk_out <= '0';
  elsif rising_edge(clk) then
    if counter = 3 then
      counter <= 0;
      clk_out <= not clk_out;
    else
      counter <= counter + 1;
    end if;
  end if;
end process;

What is the frequency division factor of clk_out relative to clk?

medium

A. 4

B. 8

C. 6

D. 3

Solution

1. Step 1: Understand counter behavior

   The counter counts from 0 to 3, so it counts 4 clock cycles before toggling clk_out.

2. Step 2: Calculate total division factor

   Since clk_out toggles every 4 input clocks, one full cycle of clk_out is 8 input clocks (4 up, 4 down).

3. Final Answer:

   8 -> Option B

4. Quick Check:

   Toggle every 4 clocks means full period 8 clocks [OK]

Hint: Multiply count by 2 for full output clock period [OK]

Common Mistakes:

• Using counter max value as division factor directly
• Forgetting clk_out toggles, doubling period
• Counting only half the clock cycles

4. Identify the error in this prescaler process code:

process(clk, reset)
begin
  if reset = '1' then
    counter <= 0;
    clk_out <= '0';
  elsif rising_edge(clk) then
    if counter = 4 then
      counter <= 0;
      clk_out <= not clk_out;
    else
      counter <= counter + 1;
    end if;
  end if;
end process;

Assuming counter is declared as integer range 0 to 3, what will happen?

medium

A. Counter never reaches 4, so clk_out never toggles

B. Counter resets correctly at 4

C. Runtime error due to counter overflow

D. clk_out toggles every clock cycle

Solution

1. Step 1: Check counter range and comparison

   The counter is declared with range 0 to 3, but the code checks if counter = 4, which is outside the range.

2. Step 2: Understand effect of out-of-range value

   Since counter can only count up to 3, it will never equal 4, so the condition is never true and clk_out never toggles.

3. Final Answer:

   Counter never reaches 4, so clk_out never toggles -> Option A

4. Quick Check:

   Counter max 3 but checks 4 = Condition never met [OK]

Hint: Check counter max matches comparison value [OK]

Common Mistakes:

• Ignoring counter range limits
• Assuming counter can reach out-of-range values
• Thinking clk_out toggles without counter reset

5. You want to design a prescaler in VHDL that divides a 50 MHz clock down to 1 kHz. Which counter range is closest to the correct choice?
Use the formula: division factor = 50,000,000 / 1,000 = 50,000.
Remember the output clock toggles every time the counter reaches max and resets.

hard

A. signal counter : integer range 0 to 99999 := 0;

B. signal counter : integer range 0 to 49999 := 0;

C. signal counter : integer range 0 to 24999 := 0;

D. signal counter : integer range 0 to 12499 := 0;

Solution

1. Step 1: Calculate division factor for toggling

   The full division factor is 50,000, but clk_out toggles every time counter reaches max, so counter max = 50,000 / 2 = 25,000.

2. Step 2: Choose closest counter range

   Since counting starts at 0, range should be 0 to 24,999 to count 25,000 cycles.

3. Final Answer:

   signal counter : integer range 0 to 24999 := 0; -> Option C

4. Quick Check:

   Divide by 2 for toggle count = 24999 max [OK]

Hint: Divide total count by 2 for toggle max count [OK]

Common Mistakes:

• Using full division factor as counter max
• Not dividing by 2 for toggle behavior
• Choosing incorrect range limits