Using VHDL to Describe Counters


1. Introduction to Counters

A counter is a sequential circuit whose output progresses in a predictable repeating pattern, advancing by one state for each clock pulse. It has a number of binary outputs whose states progress through a fixed sequence. This count sequence can be ascending, descending, or nonlinear. The following are the definitions of some important terms used for counters:

Count Sequence -  The specific series of output states through 
		  which a counter progresses.

Modulus        -  The number of states through which a counter
		  sequences before repeating.

Recycle        -  It makes a transition from the last state of 
		  the count sequence to the first state.	

Up Counter     -  A counter with an ascending sequence.

Down Counter   -  A counter with a descending sequence.

Modulo-n (or mod-n) Counter is a counter with a modulus n.

Modulo Arithmetic is a closed system of counting and adding,
		  whereby a sum greater than the largest number 
		  in a sequence "rolls over" and starts from
		  the beginning of the sequence.

		  For Example, a regular clock is a typical counter.
		  On its face, there are 12 states. It is a Modulo-12
		  counter.  We know 4 hours after 10:00 a.m. is
		  2:00 p.m. In the mod-12 system, 10+4 = 2.   

In CS201, you have learned how to use sequential design procedure to build a counter. In the next section, you will see an example of using VHDL to describe a counter.

2. Using VHDL to Describe a Counter

Now let's look at an 8-bit counter. It counts up by one for each clock pulse. It also has a loading enable signal, which will determine if the counter keeps counting with the pulse or if the counter will start from a new number specified in loading mode. Here is the block design for it:
This counter can be described in VHDL as follows:

library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.numeric_std.all;

entity COUNT8 is 

		DIN	: in	std_logic_vector(7 downto 0);
		CLK	: in	std_logic;
		LOAD	: in	std_logic;
		DOUT	: out	std_logic_vector(7 downto 0)

end COUNT8;

architecture behavior of COUNT8 is

  	-- notice the process statement and the variable COUNT
  	variable COUNT:unsigned(7 downto 0) := "00000000";
  		if (CLK'EVENT AND CLK = '1') then
  			if LOAD = '1' then
  				COUNT := DIN;
  			else COUNT := COUNT + 1;
  			end if;
  		end if;
  		DOUT <= COUNT after 50 ns;
  	end process clk_proc;

end behavior;

When using VHDL to create a counter, we can take different approaches. We can encode the Boolean equations of the counter directly with concurrent signal assignment statements; we can use VHDL code to describe the behavior of the counter; we can use CASE statement to implement the state diagram of the counter; or we may be able to use a predefined counter in the software package available to you. Using concurrent signal assignment statement is an insufficient way to code many digital functions. Therefore, it is not preferable.

The above example is using behavior description. You will use the similar method to do your lab assignment number one.

Here are some useful terms for your reference:

  1. A binary counter is a counter that generates a binary sequence.
  2. The maximum modulus of a counter is the largest number of the counter states that can be represented by n bits, which is 2^n.
  3. A Full-Sequence Counter is a counter whose modulus is the same as it's maximum modulus (m = 2^n for an n-bit counter).
  4. A Truncated-Sequence Counter is a counter whose modulus is less than its maximum modulus (i.e. m < 2^n for an n-bit counter).

3. Formats of Process, IF-then-else, and CASE statements

A PROCESS is a construct containing statements that are executed if a signal in the sensitivity list of the PROCESS changes. The general format of a PROCESS is:

[label:] PROCESS (sensitivity list)




VHDL syntax requires an IF statement or a CASE statement to be obtained within a PROCESS.

An IF statement executes one or more VHDL statements, depending on the state of a Boolean test condition. It has the following syntax format:

        IF  ___expression THEN



        ELSIF   __expression  THEN






        END IF;

Here is the general format for the CASE statement:


PROCESS (sensitivity list)
  CASE __expression IS
     WHEN  __constant_value =>
     WHEN  __constant_value =>
     WHEN  OTHERS =>
END PROCESS __process_label;

The expression in the CASE statement is usually a port or signal whose value we wish test. For that case, one or more statements can be executed.

4. Lab Assignments

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