Graduate Computer Architecture I VHDL Structure and Testing Michael Sorensen.

Graduate Computer Architecture I

VHDL Structure and Testing

Michael Sorensen

2 - CSE/ESE 560M – Graduate Computer Architecture I

Overview

• Everything You Always Wanted To Know About Synthesizable VHDL But Where Afraid To Ask by William D. Richard, PhD.

• Components• VHDL 2 Process Structure

– ALU using 2 Processes• Using Modelsim (Tips)• Testing

– VHDL Test bench / Test Vectors– Modelsim Do files

• VHDL Debugging Notes and Tips


Everything You Always Wanted To Know

• Everything You Always Wanted To Know About Synthesizable VHDL But Where Afraid To Ask by William D. Richard, PhD. – It is available on the website, just remember William D. Richard.– It is a great resource for those that don’t have a VHDL book.


Components

Component AND2entity and2 is Port ( a : in std_logic; b : in std_logic; s : out std_logic);end and2;architecture Behavioral of and2 isBegin

s <= a and b;

end Behavioral;

Component AND4entity and4 is Port ( a : in std_logic; b : in std_logic; c : in std_logic; b : in std_logic; s : out std_logic);end and4;

architecture Behavioral of and4 is

component and2 Port ( a : in std_logic; -- Ports must match b : in std_logic; s : out std_logic);end component;

signal temp1, temp2 : std_logic;

Begin

and2one:and2 port map ( a => a, b => b, s => temp1);

and2two:and2 port map ( a => c, b => d, s => temp2);

and2three:and2 port map ( a => temp1, b => temp2, s => s);

end Behavioral;


VHDL 2 Process Structure

• Benefits– Design Pattern, easily recognized– “Easier” to program– “Easier” to debug– Works best for finite state machines

• Basics– First process defines what occurs on a rising clock– Second process defines the combinational logic

• Basics (in terms of finite state machine)– First process sets the state from the next_state on a rising clock– Second process calculates the next_state


VHDL 2 Process Structureentity example is Port ( clk : in std_logic; reset : in std_logic; a : in std_logic_vector(31 downto 0); z : out std_logic_vector(31 downto 0));end example;architecture Behavioral of example issignal a_int, z_int : std_logic_vector(31 downto 0);Begin

-- D-Flip Flopsprocess (clk) -- <-- process of only clockbegin

if clk'event and clk = '1' then -- <-- proper way to trigger on a rising clock edgeif reset = '1' then -- <-- proper way to create a reset

z <= “00000000000000000000000000000000”;a_int <= “00000000000000000000000000000000”;

elsea_int <= a;z <= z_int;

end if;end if;

end process;

-- Combinational Logicprocess (a_int) -- <-- process of signals used in c/l but no clockbegin

-- z_int <= F(a_int);end process;end Behavioral;


ALU using the 2 Processesentity alu32bit is Port ( clk : in std_logic; a : in std_logic_vector(31 downto 0); b : in std_logic_vector(31 downto 0); ctrl : in std_logic_vector(2 downto 0); z : out std_logic_vector(31 downto 0); ovr : out std_logic; zero : out std_logic);end alu32bit;

architecture Behavioral of alu32bit is

signal a_int, b_int, z_int : std_logic_vector(31 downto 0);signal ctrl_int : std_logic_vector(2 downto 0);signal ovr_int, zero_int : std_logic;signal tmp1, tmp2 : std_logic_vector(31 downto 0);

begin

process (clk)begin

if clk'event and clk = '1' thena_int <= a;b_int <= b;ctrl_int <= ctrl;z <= z_int;ovr <= ovr_int;zero <= zero_int;

end if;end process;


ALU using the 2 Processes (cont.)process (a_int, b_int, ctrl_int, tmp1, tmp2)begin

ovr_int <= '0';zero_int <= '1';z_int <= tmp1;case ctrl is

when "000" => -- ADD-- REMOVED FOR SPACE REASONS

when "001" => --SUBTRACTtmp1 <= a_int - b_int;if tmp1 = "00000000000000000000000000000000" then zero_int <= '0';end if;

when "010" => --bitwise ANDtmp1 <= a_int and b_int;

when "011" => --bitwise ORtmp1 <= a_int or b_int;

when "100" => --bitwise XORtmp1 <= a_int xor b_int;

when "101" => --shifttmp1 <= to_stdlogicvector(to_bitvector(a_int) sll conv_integer(b_int));

when "110" => --lessif a_int < b_int then tmp1 <= "00000000000000000000000000000001";else tmp1 <= "00000000000000000000000000000000";end if;

when others => null;end case;

end process;

end Behavioral;


Using Modelsim (Tips)

• Commands: – vcom

• Modelsim’s vhdl compiler. Run Vcom and a GUI pops up to allow you to select files to compile. Make sure you do them in order of dependency.

– vsim• Start a simulation. Example: vsim alu32bit (notice no .vhd)

– quit -sim • Exits out of a simulation. For you to recompile if you make changes.

– restart -f• Restarts a simulation. For you to change your do file and rerun it.

• Zoom Full, the blue magnify glass, Auto Scales the wave form• Change Radix, right click on signals, select Radix

– View signals or vectors in Binary, Unsigned, Decimal, etc.


Testing (VHDL Test Vectors)

• To create one select, New Source, VHDL Test Bench• OR – Create a new VHDL Module and rewrite• Benefit – can be set to check desired results

Example: Synchronized And Gateentity syncand2 is Port ( clk : in std_logic; a : in std_logic; b : in std_logic; s : out std_logic);end syncand2;

architecture Behavioral of syncand2 is

begin

process (clk)begin

if clk'event and clk = '1' thens <= a and b;

end if;end process;

end Behavioral;


Testing (VHDL Test Vectors)library ieee;use ieee.std_logic_1164.all; entity syncand2tb is -- No Ports for VHDL Test Benchend syncand2tb; architecture stimulus of syncand2tb is component syncand2 Port ( clk : in std_logic; a : in std_logic; b : in std_logic; s : out std_logic); end component;

signal clk : std_logic; signal a: std_logic; signal b: std_logic; signal s: std_logic; signal vector_cnt: integer := 1; signal error_flag: std_logic := '0'; type test_record is record -- Declare a record type a: std_logic; -- Signals of component to test b: std_logic; s: std_logic; -- Expected result end record;



type test_array is array(positive range <>) of test_record; -- Collect them -- in an array -- The following constant declaration describes the test vectors to be -- applied to the design during simulation, and the expected result after a -- rising clock edge. constant test_vectors : test_array := ( -- a, b, s ('0', '0', '-'), -- RESET (IGNORE THE RESULT) ('0', '0', '0'), ('1', '0', '0'), ('0', '1', '0'), ('1', '1', '1'), ('0', '0', '0') ); begin -- instantiate the component UUT: syncand2 -- instance declared as UUT (Unit Under Testing)

port map ( clk => clk, a => a, b => b,

s => s);

The Test Vectors


Testing (VHDL Test Vectors) -- provide stimulus and check the result

testrun: process variable vector : test_record; begin for index in test_vectors'range loop vector_cnt <= index; vector := test_vectors(index); -- Get the current test vector -- Apply the input stimulus... a <= vector.a; b <= vector.b; -- Clock (low-high-low) with a 100 ns cycle... clk <= '0'; wait for 25 ns; -- Quarter of the desired clock cycle clk <= '1'; wait for 50 ns; -- Half of the desired clock cycle clk <= '0'; wait for 25 ns; -- Quarter of the desired clock cycle

-- Check the results... if (vector.s /= '-' and s /= vector.s) then error_flag <= '1'; assert false report "Output did not match!" severity WARNING; else error_flag <= '0'; end if; end loop; wait; end process;end stimulus;



Results


Testing (Modelsim Do files)

• To create a Do file use any standard text editor

Example: Synchronized And Gateentity syncand2 is Port ( clk : in std_logic; a : in std_logic; b : in std_logic; s : out std_logic);end syncand2;

architecture Behavioral of syncand2 is

begin

process (clk)begin

if clk'event and clk = '1' thens <= a and b;

end if;end process;

end Behavioral;



• Copy generated commands to do file• To read bidirectional lines after Freezing them, select Force,

set value to Z and set Kind to Deposit

force -freeze sim:/syncand2/clk 1 0, 0 {50 ps} -r 100ps force -freeze sim:/syncand2/a 0 0

Set a clock Force a signal



syncand2.doforce -freeze sim:/syncand2/clk 1 0, 0 {50 ps} -r 100psforce -freeze sim:/syncand2/a 0 0force -freeze sim:/syncand2/b 0 0run 200force -freeze sim:/syncand2/a 1 0force -freeze sim:/syncand2/b 0 0run 100force -freeze sim:/syncand2/a 0 0force -freeze sim:/syncand2/b 1 0run 100force -freeze sim:/syncand2/a 1 0force -freeze sim:/syncand2/b 1 0run 100force -freeze sim:/syncand2/a 0 0force -freeze sim:/syncand2/b 0 0run 200

To run do file from Modelsim command prompt type:

VSIM #> do syncand2.do

Default unit of time is ns.


VHDL DEBUGING NOTES AND TIPS

• Make backups, at least backup copies of working code before you add features to it.

• Correct all undefined (red) signals in Modelsim first.• Check sensitivity list• Check that signals are only modified/driven in one process (or

if outside of a process, modified/driven on only one line)• If problems are with bidirectional lines make sure that

components correctly share line. And if your forcing the line in simulation remember to deposit Z’s before you want to read values from it.

• Ask Questions!

Graduate Computer Architecture I VHDL Structure and Testing Michael Sorensen.

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