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

3 - CSE/ESE 560M – Graduate Computer Architecture I 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.

4 - CSE/ESE 560M – Graduate Computer Architecture I Components Component AND2 entity and2 is Port ( a : in std_logic; b : in std_logic; s : out std_logic); end and2; architecture Behavioral of and2 is Begin s <= a and b; end Behavioral; Component AND4 entity 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;

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

6 - CSE/ESE 560M – Graduate Computer Architecture I VHDL 2 Process Structure entity 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 is signal a_int, z_int : std_logic_vector(31 downto 0); Begin -- D-Flip Flops process (clk) -- <-- process of only clock begin if clk'event and clk = '1' then-- <-- proper way to trigger on a rising clock edge if reset = '1' then-- <-- proper way to create a reset z <= “ ”; a_int <= “ ”; else a_int <= a; z <= z_int; end if; end process; -- Combinational Logic process (a_int) -- <-- process of signals used in c/l but no clock begin -- z_int <= F(a_int); end process; end Behavioral;

7 - CSE/ESE 560M – Graduate Computer Architecture I ALU using the 2 Processes entity 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' then a_int <= a; b_int <= b; ctrl_int <= ctrl; z <= z_int; ovr <= ovr_int; zero <= zero_int; end if; end process;

8 - CSE/ESE 560M – Graduate Computer Architecture I 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" => --SUBTRACT tmp1 <= a_int - b_int; if tmp1 = " " then zero_int <= '0'; end if; when "010" => --bitwise AND tmp1 <= a_int and b_int; when "011" => --bitwise OR tmp1 <= a_int or b_int; when "100" => --bitwise XOR tmp1 <= a_int xor b_int; when "101" => --shift tmp1 <= to_stdlogicvector(to_bitvector(a_int) sll conv_integer(b_int)); when "110" => --less if a_int < b_int then tmp1 <= " "; else tmp1 <= " "; end if; when others => null; end case; end process; end Behavioral;

9 - CSE/ESE 560M – Graduate Computer Architecture I 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.

10 - CSE/ESE 560M – Graduate Computer Architecture I 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 Gate entity 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' then s <= a and b; end if; end process; end Behavioral;

11 - CSE/ESE 560M – Graduate Computer Architecture I Testing (VHDL Test Vectors) library ieee; use ieee.std_logic_1164.all; entity syncand2tb is -- No Ports for VHDL Test Bench end 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;

12 - CSE/ESE 560M – Graduate Computer Architecture I Testing (VHDL Test Vectors) 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

13 - CSE/ESE 560M – Graduate Computer Architecture I 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;

14 - CSE/ESE 560M – Graduate Computer Architecture I Testing (VHDL Test Vectors) Results

15 - CSE/ESE 560M – Graduate Computer Architecture I Testing (Modelsim Do files) To create a Do file use any standard text editor Example: Synchronized And Gate entity 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' then s <= a and b; end if; end process; end Behavioral;

16 - CSE/ESE 560M – Graduate Computer Architecture I Testing (Modelsim Do files) 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 100psforce -freeze sim:/syncand2/a 0 0 Set a clockForce a signal

17 - CSE/ESE 560M – Graduate Computer Architecture I Testing (Modelsim Do files) syncand2.do force -freeze sim:/syncand2/clk 1 0, 0 {50 ps} -r 100ps force -freeze sim:/syncand2/a 0 0 force -freeze sim:/syncand2/b 0 0 run 200 force -freeze sim:/syncand2/a 1 0 force -freeze sim:/syncand2/b 0 0 run 100 force -freeze sim:/syncand2/a 0 0 force -freeze sim:/syncand2/b 1 0 run 100 force -freeze sim:/syncand2/a 1 0 force -freeze sim:/syncand2/b 1 0 run 100 force -freeze sim:/syncand2/a 0 0 force -freeze sim:/syncand2/b 0 0 run 200 To run do file from Modelsim command prompt type: VSIM #> do syncand2.do Default unit of time is ns.

18 - CSE/ESE 560M – Graduate Computer Architecture I 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!