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Chapter: Digital Logic Circuits - VHDL

VHDL for Serial Comparator

Flip-flop implementation: reset priority, event, rising edge sensitive.

VHDL for Serial Comparator

 

Things to observe:

 

1.        Flip-flop implementation: reset priority, event, rising edge sensitive.

2.        If and case -- sequential statements -- are valid only within a process.

3.        Concurrent assignment is a ``process.''

4.        Semantics of a process: sensitivity list, assignments:

5.        b <= a;

6.        c <= b;

does not behave as it would in C.

7.        VHDL architecture broken into three processes:

a)        State storage.

b)       Next state generation.

c)        Output generation.


 

--  VHDL for serial comparator. The inputs a and b are input lsb first.

--  The Mealy machine uses rising edge sensitive flip-flops and an

--  asynchronous active low reset.

--

-- The output is 1 if b > a, otherwise 0.

library ieee;

use ieee.std_logic_1164.all;

entity comparator is port

(a, b, clk, reset : in std_logic; o : out std_logic );

end comparator;

 

architecture process_defn of comparator is

 

-- Two states needed.

 

type state_type is (S0, S1); -- State assignment.

 

attribute enum_encoding : string; attribute enum_encoding of state_type :

type is "0 1";

 

signal state, next_state : state_type;

 

-- For convenience, concatenate a and b. signal inputs : std_logic_vector (1 downto 0);

 

begin

 

--        Concurrent assignment executes the rhs changes.

--        Concatenate a and b into inputs.

inputs <= a & b;

 

 

--        Processes execute whenever something on their sensitivity list

--        changes. All assignments take place when the process exits.

 

--

-- This process implements the D flip-flop.

 

state_register : process (clk, reset) begin

 

-- If/else construct only valid within a process. if (reset = '0') then

state <= S0;

 

elsif (clk'event AND clk = '1') then state <= next_state;

 

end if; end process;

 

 

-- This process computes the next state.

 

next_state_process : process (inputs, state) begin

 

case state is

 

when S0 =>

 

if (inputs = "01") then next_state <= S1;

else

 

next_state <= S0; end if;

 

 

when S1 =>

 

if (inputs = "10") then next_state <= S0;

else

 

next_state <= S1; end if;

 

end case; end process;

 

 

-- This process computes the output.

 

output_process : process (inputs, state) begin

case state is

 

when S0 =>

 

if (inputs = "01") then o <= '1';

 

else

 

o <= '0'; end if;

 

when S1 =>

 

if (inputs = "10") then o <= '0';

 

else

 

o <= '1'; end if;

 

end case; end process;

 

 

end process_defn;

 

 

 

A test bench is a virtual environment used to verify the correctness or soundness of a design or model (e.g., a software product).

 

The term has its roots in the testing of electronic devices, where an engineer would sit at a lab bench with tools of measurement and manipulation, such as oscilloscopes, multimeters, soldering irons, wire cutters, and so on, and manually verify the correctness of the device under test.

 

In the context of software or firmware or hardware engineering, a test bench refers to an environment in which the product under development is tested with the aid of a collection of testing tools. Often, though not always, the suite of testing tools is designed specifically for the product under test.

 

A test bench or testing workbench has four components.

 

1.INPUT: The entrance criteria or deliverables needed to perform work

 

2.PROCEDURES TO DO: The tasks or processes that will transform the input into the output

 

3.PROCEDURES TO CHECK: The processes that determine that the output meets the standards.

 

4.OUTPUT: The exit criteria or deliverables produced from the workbench

 

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·        Reactive Test Bench Option example

 

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The highest level of testbench generation is provided by TestBencher Pro, which allows a user to design bus functional models using multiple timing diagrams to define transactors and a sequencer process to apply the diagram transactions. TestBencher can be added to BugHunter or purchased as a standalone product.

 

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