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Simple Testbenches Behavioral Modeling of Combinational Logic

This lecture covers the concept of VHDL testbenches for generating stimuli and observing outputs of digital circuits. It explains how testbenches can be used to test different implementations of the same circuit and how they can be easily adapted to different simulation tools. The lecture also provides examples of testbenches and discusses the execution flow of statements within a process.

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Simple Testbenches Behavioral Modeling of Combinational Logic

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  1. ECE 448 Lecture 4 Simple TestbenchesBehavioral Modeling of Combinational Logic ECE 448 – FPGA and ASIC Design with VHDL

  2. Required reading • P. Chu, FPGA Prototyping by VHDL Examples • Section 1.4, Testbench • Section 3.4, Modeling with a process • Section 3.5, Routing circuit with if and case • statements • S. Brown and Z. Vranesic,Fundamentals of Digital Logic with VHDL Design • Chapter 6.6, VHDL for Combinational Circuits ECE 448 – FPGA and ASIC Design with VHDL

  3. Testbenches ECE 448 – FPGA and ASIC Design with VHDL

  4. Testbench Defined • Testbench= VHDL entity that applies stimuli (drives the inputs) to the Design Under Test (DUT) and (optionally) verifies expected outputs. • The results can be viewed in a waveform window or written to a file. • Since Testbench is written in VHDL, it is not restricted to a single simulation tool (portability). • The same Testbench can be easily adapted to test different implementations (i.e. different architectures) of the same design. ECE 448 – FPGA and ASIC Design with VHDL

  5. Simple Testbench Processes Generating Stimuli Design Under Test (DUT) Observed Outputs ECE 448 – FPGA and ASIC Design with VHDL

  6. Possible sources of expected results used for comparison Testbench actual results VHDL Design = ? Representative Inputs Manual Calculations or Reference Software Implementation(C, Java, Matlab ) expected results ECE 448 – FPGA and ASIC Design with VHDL

  7. Testbench The same testbench can be used to test multiple implementations of the same circuit (multiple architectures) testbench design entity . . . . Architecture N Architecture 2 Architecture 1 ECE 448 – FPGA and ASIC Design with VHDL

  8. Testbench Anatomy ENTITYmy_entity_tbIS --TB entity has no ports ENDmy_entity_tb; ARCHITECTUREbehavioralOFtbIS --Local signals and constants COMPONENT TestComp --All Design Under Test component declarations PORT ( ); END COMPONENT; ----------------------------------------------------- BEGIN DUT:TestComp PORT MAP( -- Instantiations of DUTs ); testSequence: PROCESS -- Input stimuli END PROCESS; ENDbehavioral; ECE 448 – FPGA and ASIC Design with VHDL

  9. Testbench for XOR3 (1) LIBRARYieee; USEieee.std_logic_1164.all; ENTITY xor3_tbIS ENDxor3_tb; ARCHITECTUREbehavioralOFxor3_tbIS -- Component declaration of the tested unit COMPONENTxor3 PORT( A : IN STD_LOGIC; B : IN STD_LOGIC; C : IN STD_LOGIC; Result : OUT STD_LOGIC ); END COMPONENT; -- Stimulus signals - signals mapped to the input and inout ports of tested entity SIGNALtest_vector:STD_LOGIC_VECTOR(2 DOWNTO 0); SIGNAL test_result : STD_LOGIC; ECE 448 – FPGA and ASIC Design with VHDL

  10. Testbench for XOR3 (2) BEGIN UUT : xor3 PORT MAP ( A => test_vector(2), B => test_vector(1), C => test_vector(0), Result => test_result); ); Testing: PROCESS BEGIN test_vector<="000"; WAIT FOR 10 ns; test_vector<="001"; WAIT FOR 10 ns; test_vector<="010"; WAIT FOR 10 ns; test_vector<="011"; WAIT FOR 10 ns; test_vector<="100"; WAIT FOR 10 ns; test_vector<="101"; WAIT FOR 10 ns; test_vector<="110"; WAIT FOR 10 ns; test_vector<="111"; WAIT FOR 10 ns; END PROCESS; ENDbehavioral; ECE 448 – FPGA and ASIC Design with VHDL

  11. VHDL Design Styles dataflow VHDL Design Styles structural behavioral Concurrent statements Components and interconnects Sequential statements • Testbenches ECE 448 – FPGA and ASIC Design with VHDL

  12. Process without Sensitivity List and its use in Testbenches ECE 448 – FPGA and ASIC Design with VHDL

  13. What is a PROCESS? • A process is a sequence of instructions referred to as sequential statements. The keyword PROCESS • A process can be given a unique name using an optional LABEL • This is followed by the keyword PROCESS • The keyword BEGIN is used to indicate the start of the process • All statements within the process are executed SEQUENTIALLY. Hence, order of statements is important. • A process must end with the keywords END PROCESS. Testing: PROCESS BEGIN test_vector<=“00”; WAIT FOR 10 ns; test_vector<=“01”; WAIT FOR 10 ns; test_vector<=“10”; WAIT FOR 10 ns; test_vector<=“11”; WAIT FOR 10 ns; END PROCESS; ECE 448 – FPGA and ASIC Design with VHDL

  14. The execution of statements continues sequentially till the last statement in the process. After execution of the last statement, the control is again passed to the beginning of the process. Testing: PROCESS BEGIN test_vector<=“00”; WAIT FOR 10 ns; test_vector<=“01”; WAIT FOR 10 ns; test_vector<=“10”; WAIT FOR 10 ns; test_vector<=“11”; WAIT FOR 10 ns; END PROCESS; Execution of statements in a PROCESS Order of execution Program control is passed to the first statement after BEGIN ECE 448 – FPGA and ASIC Design with VHDL

  15. The last statement in the PROCESS is a WAIT instead of WAIT FOR 10 ns. This will cause the PROCESS to suspend indefinitely when the WAIT statement is executed. This form of WAIT can be used in a process included in a testbench when all possible combinations of inputs have been tested or a non-periodical signal has to be generated. Testing: PROCESS BEGIN test_vector<=“00”; WAIT FOR 10 ns; test_vector<=“01”; WAIT FOR 10 ns; test_vector<=“10”; WAIT FOR 10 ns; test_vector<=“11”; WAIT; END PROCESS; PROCESS with a WAIT Statement Order of execution Program execution stops here ECE 448 – FPGA and ASIC Design with VHDL

  16. WAIT FOR vs. WAIT 0 0 1 1 2 2 3 3 WAIT FOR: waveform will keep repeating itself forever … WAIT: waveform will keep its state after the last wait instruction. … ECE 448 – FPGA and ASIC Design with VHDL

  17. Specifying time in VHDL ECE 448 – FPGA and ASIC Design with VHDL

  18. Time values (physical literals) - Examples unit of time most commonly used in simulation 7 ns 1 min min 10.65 us 10.65 fs Numeric value Space (required) Unit of time ECE 448 – FPGA and ASIC Design with VHDL

  19. Units of time Unit Definition Base Unit fs femtoseconds (10-15 seconds) Derived Units ps picoseconds (10-12 seconds) ns nanoseconds (10-9 seconds) us microseconds (10-6 seconds) ms miliseconds (10-3 seconds) sec seconds min minutes (60 seconds) hr hours (3600 seconds) ECE 448 – FPGA and ASIC Design with VHDL

  20. Simple Testbenches ECE 448 – FPGA and ASIC Design with VHDL

  21. Generating selected values of one input • SIGNAL test_vector : STD_LOGIC_VECTOR(2 downto 0); • BEGIN • ....... • testing: PROCESS • BEGIN • test_vector <= "000"; • WAIT FOR 10 ns; • test_vector <= "001"; • WAIT FOR 10 ns; • test_vector <= "010"; • WAIT FOR 10 ns; • test_vector <= "011"; • WAIT FOR 10 ns; • test_vector <= "100"; • WAIT FOR 10 ns; • END PROCESS; • ........ • END behavioral; ECE 448 – FPGA and ASIC Design with VHDL

  22. Generating all values of one input SIGNAL test_vector : STD_LOGIC_VECTOR(3 downto 0):="0000"; BEGIN ....... testing: PROCESS BEGIN WAIT FOR 10 ns; test_vector <= test_vector + 1; end process TESTING; ........ END behavioral; ECE 448 – FPGA and ASIC Design with VHDL

  23. Arithmetic Operators in VHDL (1) To use basic arithmetic operations involving std_logic_vectors you need to include the following library packages: LIBRARY ieee; USE ieee.std_logic_1164.all; USE ieee.std_logic_unsigned.all; or USE ieee.std_logic_signed.all; ECE 448 – FPGA and ASIC Design with VHDL

  24. Arithmetic Operators in VHDL (2) You can use standard +, - operators to perform addition and subtraction: signal A : STD_LOGIC_VECTOR(3 downto 0); signal B : STD_LOGIC_VECTOR(3 downto 0); signal C : STD_LOGIC_VECTOR(3 downto 0); …… C <= A + B; ECE 448 – FPGA and ASIC Design with VHDL

  25. Different ways of performing the same operation signal count: std_logic_vector(7 downto 0); You can use: count <= count + “00000001”; or count <= count + 1; or count <= count + ‘1’; ECE 448 – FPGA and ASIC Design with VHDL

  26. Different declarations for the same operator Declarations in the package ieee.std_logic_unsigned: function “+” ( L: std_logic_vector; R:std_logic_vector) return std_logic_vector; function “+” ( L: std_logic_vector; R: integer) return std_logic_vector; function “+” ( L: std_logic_vector; R:std_logic) return std_logic_vector; ECE 448 – FPGA and ASIC Design with VHDL

  27. Operator overloading • Operator overloading allows different argument types for a given operation (function) • The VHDL tools resolve which of these functions to select based on the types of the inputs • This selection is transparent to the user as long as the function has been defined for the given argument types. ECE 448 – FPGA and ASIC Design with VHDL

  28. Generating all possible values of two inputs • SIGNAL test_ab : STD_LOGIC_VECTOR(1 downto 0); • SIGNAL test_sel : STD_LOGIC_VECTOR(1 downto 0); • BEGIN • ....... • double_loop: PROCESS • BEGIN • test_ab <="00"; • test_sel <="00"; • for I in 0 to 3 loop • for J in 0 to 3 loop • wait for 10 ns; • test_ab <= test_ab + 1; • end loop; • test_sel <= test_sel + 1; • end loop; • END PROCESS; • ........ • END behavioral; ECE 448 – FPGA and ASIC Design with VHDL

  29. Generating periodical signals, such as clocks CONSTANT clk1_period : TIME := 20 ns; CONSTANT clk2_period : TIME := 200 ns; SIGNAL clk1 : STD_LOGIC; SIGNAL clk2 : STD_LOGIC := ‘0’; BEGIN ....... clk1_generator: PROCESS clk1 <= ‘0’; WAIT FOR clk1_period/2; clk1 <= ‘1’; WAIT FOR clk1_period/2; END PROCESS; clk2 <= not clk2after clk2_period/2; ....... END behavioral; ECE 448 – FPGA and ASIC Design with VHDL

  30. Generating one-time signals, such as resets CONSTANT reset1_width : TIME := 100 ns; CONSTANT reset2_width : TIME := 150 ns; SIGNAL reset1 : STD_LOGIC; SIGNAL reset2 : STD_LOGIC := ‘1’; BEGIN ....... reset1_generator: PROCESS reset1 <= ‘1’; WAIT FOR reset_width; reset1 <= ‘0’; WAIT; END PROCESS; reset2_generator: PROCESS WAIT FOR reset_width; reset2 <= ‘0’; WAIT; END PROCESS; ....... END behavioral; ECE 448 – FPGA and ASIC Design with VHDL

  31. Typical error SIGNAL test_vector : STD_LOGIC_VECTOR(2 downto 0); SIGNAL reset : STD_LOGIC; BEGIN ....... generator1: PROCESS reset <= ‘1’; WAIT FOR 100 ns reset <= ‘0’; test_vector <="000"; WAIT; END PROCESS; generator2: PROCESS WAIT FOR 200 ns test_vector <="001"; WAIT FOR 600 ns test_vector <="011"; END PROCESS; ....... END behavioral; ECE 448 – FPGA and ASIC Design with VHDL

  32. Combinational Logic Synthesis for Beginners ECE 448 – FPGA and ASIC Design with VHDL

  33. Simple rules for beginners For combinational logic, use only concurrent statements • concurrent signal assignment () • conditional concurrent signal assignment • (when-else) • selected concurrent signal assignment • (with-select-when) • generate scheme for equations • (for-generate) ECE 448 – FPGA and ASIC Design with VHDL

  34. Simple rules for beginners For circuits composed of - simple logic operations (logic gates) - simple arithmetic operations (addition, subtraction, multiplication) - shifts/rotations by a constant use • concurrent signal assignment () ECE 448 – FPGA and ASIC Design with VHDL

  35. Simple rules for beginners For circuits composed of - multiplexers - decoders, encoders - tri-state buffers use • conditional concurrent signal assignment • (when-else) • selected concurrent signal assignment • (with-select-when) ECE 448 – FPGA and ASIC Design with VHDL

  36. Combinational Logic Synthesis for Intermediates ECE 448 – FPGA and ASIC Design with VHDL

  37. List of signals to which the process is sensitive. Whenever there is an event on any of the signals in the sensitivity list, the process fires. Every time the process fires, it will run in its entirety. WAIT statements are NOT ALLOWED in a processes with SENSITIVITY LIST. label: process (sensitivity list) declaration part begin statement part end process; PROCESS with a SENSITIVITY LIST ECE 448 – FPGA and ASIC Design with VHDL

  38. Anatomy of a Process OPTIONAL [label:]PROCESS[(sensitivity list)] [declaration part] BEGIN statement part ENDPROCESS [label]; ECE 448 – FPGA and ASIC Design with VHDL

  39. Processes in VHDL • Processes Describe Sequential Behavior • Processes in VHDL Are Very Powerful Statements • Allow to define an arbitrary behavior that may be difficult to represent by a real circuit • Not every process can be synthesized • Use Processes with Caution in the Code to Be Synthesized • Use Processes Freely in Testbenches ECE 448 – FPGA and ASIC Design with VHDL

  40. Describing combinational logic using processes LIBRARY ieee ; USE ieee.std_logic_1164.all ; ENTITY dec2to4 IS PORT ( w : IN STD_LOGIC_VECTOR(1 DOWNTO 0) ; En : IN STD_LOGIC ; y : OUT STD_LOGIC_VECTOR(0 TO 3) ) ; END dec2to4 ; ARCHITECTURE Behavior OF dec2to4 IS BEGIN PROCESS ( w, En ) BEGIN IF En = '1' THEN CASE w IS WHEN "00" => y <= "1000" ; WHEN "01" => y <= "0100" ; WHEN "10" => y <= "0010" ; WHEN OTHERS => y <= "0001" ; END CASE ; ELSE y <= "0000" ; END IF ; END PROCESS ; END Behavior ; ECE 448 – FPGA and ASIC Design with VHDL

  41. Describing combinational logic using processes LIBRARY ieee ; USE ieee.std_logic_1164.all ; ENTITY seg7 IS PORT ( bcd : IN STD_LOGIC_VECTOR(3 DOWNTO 0) ; leds : OUT STD_LOGIC_VECTOR(1 TO 7) ) ; END seg7 ; ARCHITECTURE Behavior OF seg7 IS BEGIN PROCESS ( bcd ) BEGIN CASE bcd IS -- abcdefg WHEN "0000" => leds <= "1111110" ; WHEN "0001" => leds <= "0110000" ; WHEN "0010" => leds <= "1101101" ; WHEN "0011" => leds <= "1111001" ; WHEN "0100" => leds <= "0110011" ; WHEN "0101" => leds <= "1011011" ; WHEN "0110" => leds <= "1011111" ; WHEN "0111" => leds <= "1110000" ; WHEN "1000" => leds <= "1111111" ; WHEN "1001" => leds <= "1110011" ; WHEN OTHERS => leds <= "-------" ; END CASE ; END PROCESS ; END Behavior ; ECE 448 – FPGA and ASIC Design with VHDL

  42. Describing combinational logic using processes LIBRARY ieee ; USE ieee.std_logic_1164.all ; ENTITY compare1 IS PORT ( A, B : IN STD_LOGIC ; AeqB : OUT STD_LOGIC ) ; END compare1 ; ARCHITECTURE Behavior OF compare1 IS BEGIN PROCESS ( A, B ) BEGIN AeqB <= '0' ; IF A = B THEN AeqB <= '1' ; END IF ; END PROCESS ; END Behavior ; ECE 448 – FPGA and ASIC Design with VHDL

  43. Incorrect code for combinational logic- Implied latch (1) LIBRARY ieee ; USE ieee.std_logic_1164.all ; ENTITY implied IS PORT ( A, B : IN STD_LOGIC ; AeqB : OUT STD_LOGIC ) ; END implied ; ARCHITECTURE Behavior OF implied IS BEGIN PROCESS ( A, B ) BEGIN IF A = B THEN AeqB <= '1' ; END IF ; END PROCESS ; END Behavior ; ECE 448 – FPGA and ASIC Design with VHDL

  44. Incorrect code for combinational logic- Implied latch (2) A AeqB B ECE 448 – FPGA and ASIC Design with VHDL

  45. Describing combinational logic using processes Rules that need to be followed: • All inputs to the combinational circuit should be included • in the sensitivity list • No other signals should be included • in the sensitivity list • None of the statements within the process • should be sensitive to rising or falling edges • All possible cases need to be covered in the internal • IF and CASE statements in order to avoid • implied latches ECE 448 – FPGA and ASIC Design with VHDL

  46. Covering all cases in the IF statement Using ELSE IF A = B THEN AeqB <= '1' ; ELSE AeqB <= '0' ; Using default values AeqB <= '0' ; IF A = B THEN AeqB <= '1' ; ECE 448 – FPGA and ASIC Design with VHDL

  47. Covering all cases in the CASE statement Using WHEN OTHERS CASE y IS WHEN S1 => Z <= "10"; WHEN S2 => Z <= "01"; WHEN S3 => Z <= "00"; WHEN OTHERS => Z <= „--"; END CASE; CASE y IS WHEN S1 => Z <= "10"; WHEN S2 => Z <= "01"; WHEN OTHERS => Z <= "00"; END CASE; ECE 448 – FPGA and ASIC Design with VHDL

  48. Covering all cases in the CASE statement Using default values Z <= "00"; CASE y IS WHEN S1 => Z <= "10"; WHEN S2 => Z <= “01"; END CASE; Z <= "00"; CASE y IS WHEN S1 => Z <= "10"; WHEN S2 => Z <= “01"; when others => null; END CASE; ECE 448 – FPGA and ASIC Design with VHDL

  49. Decoders described using behavioral description ECE 448 – FPGA and ASIC Design with VHDL

  50. Decode Table Instruction Active output signals (15 downto 0) (asserted with 1) 8xxx Op1 3xx4 Op2 3xx6 Op3 x – stands for don’t care ECE 448 – FPGA and ASIC Design with VHDL

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