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Data f low Modeling of Combinational Logic Simple Testbenches

ECE 545 Lecture 3b. Data f low Modeling of Combinational Logic Simple Testbenches. Resources. Volnei A. Pedroni , Circuit Design with VHDL Chapter 5, Concurrent Code (without 5.5) Chapter 4.1, Operators Sundar Rajan, Essential VHDL: RTL Synthesis Done Right

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Data f low Modeling of Combinational Logic Simple Testbenches

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  1. ECE 545 Lecture 3b Dataflow Modeling of Combinational LogicSimple Testbenches ECE 545 – Introduction to VHDL

  2. Resources • Volnei A. Pedroni,Circuit Design with VHDL • Chapter 5, Concurrent Code (without 5.5) • Chapter 4.1, Operators • Sundar Rajan, Essential VHDL: RTL Synthesis • Done Right • Chapter 3, Gates, Decoders and Encoders • (see errata at http://www.vahana.com/bugs.htm) ECE 545 – Introduction to VHDL

  3. Register Transfer Level (RTL) Design Description Combinational Logic Combinational Logic Today’s Topic … Registers ECE 545 – Introduction to VHDL

  4. Describing Combinational Logic Using Dataflow Design Style ECE 545 – Introduction to VHDL

  5. VHDL Design Styles dataflow VHDL Design Styles structural behavioral Components and interconnects Concurrent statements Sequential statements • Registers • State machines • Test benches ECE 545 – Introduction to VHDL

  6. Data-flow VHDL Major instructions 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 545 – Introduction to VHDL

  7. MLU: Block Diagram ECE 545 – Introduction to VHDL

  8. MLU: Entity Declaration LIBRARY ieee; USE ieee.std_logic_1164.all; ENTITY mlu IS PORT( NEG_A : IN STD_LOGIC; NEG_B : IN STD_LOGIC; NEG_Y : IN STD_LOGIC; A : IN STD_LOGIC; B : IN STD_LOGIC; L1 : IN STD_LOGIC; L0 : IN STD_LOGIC; Y : OUT STD_LOGIC ); END mlu; ECE 545 – Introduction to VHDL

  9. MLU: Architecture Declarative Section ARCHITECTURE mlu_dataflow OF mlu IS SIGNAL A1 : STD_LOGIC; SIGNAL B1 : STD_LOGIC; SIGNAL Y1 : STD_LOGIC; SIGNAL MUX_0 : STD_LOGIC; SIGNAL MUX_1 : STD_LOGIC; SIGNAL MUX_2 : STD_LOGIC; SIGNAL MUX_3 : STD_LOGIC; SIGNAL L:STD_LOGIC_VECTOR(1 DOWNTO 0); ECE 545 – Introduction to VHDL

  10. MLU - Architecture Body BEGIN A1<=NOT A WHEN (NEG_A='1') ELSE A; B1<=NOT B WHEN (NEG_B='1') ELSE B; Y<=NOT Y1 WHEN (NEG_Y='1') ELSE Y1; MUX_0<=A1 AND B1; MUX_1<=A1 OR B1; MUX_2<=A1 XOR B1; MUX_3<=A1 XNOR B1; L <= L1 & L0; with (L) select Y1<=MUX_0 WHEN "00", MUX_1 WHEN "01", MUX_2 WHEN "10", MUX_3 WHEN OTHERS; END mlu_dataflow; ECE 545 – Introduction to VHDL

  11. Data-flow VHDL Major instructions 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 545 – Introduction to VHDL

  12. For Generate Statement For - Generate label:FORidentifier IN rangeGENERATE BEGIN {Concurrent Statements} END GENERATE; ECE 545 – Introduction to VHDL

  13. PARITY Example ECE 545 – Introduction to VHDL

  14. PARITY: Block Diagram ECE 545 – Introduction to VHDL

  15. PARITY: Entity Declaration LIBRARYieee; USEieee.std_logic_1164.all; ENTITYparityIS PORT( parity_in : IN STD_LOGIC_VECTOR(7 DOWNTO 0); parity_out : OUT STD_LOGIC ); ENDparity; ECE 545 – Introduction to VHDL

  16. PARITY: Block Diagram xor_out(1) xor_out(2) xor_out(3) xor_out(4) xor_out(5) xor_out(6) ECE 545 – Introduction to VHDL

  17. PARITY: Architecture ARCHITECTUREparity_dataflowOFparityIS SIGNALxor_out: std_logic_vector (6 downto 1); BEGIN xor_out(1) <= parity_in(0) XORparity_in(1); xor_out(2) <= xor_out(1) XORparity_in(2); xor_out(3) <= xor_out(2) XORparity_in(3); xor_out(4) <= xor_out(3) XORparity_in(4); xor_out(5) <= xor_out(4) XORparity_in(5); xor_out(6) <= xor_out(5) XORparity_in(6); parity_out <= xor_out(6) XORparity_in(7); ENDparity_dataflow; ECE 545 – Introduction to VHDL

  18. PARITY: Block Diagram (2) xor_out(1) xor_out(0) xor_out(2) xor_out(3) xor_out(4) xor_out(5) xor_out(6) xor_out(7) ECE 545 – Introduction to VHDL

  19. PARITY: Architecture ARCHITECTUREparity_dataflowOFparityIS SIGNALxor_out: STD_LOGIC_VECTOR (7 downto 0); BEGIN xor_out(0) <= parity_in(0); xor_out(1) <= xor_out(0) XORparity_in(1); xor_out(2) <= xor_out(1) XORparity_in(2); xor_out(3) <= xor_out(2) XORparity_in(3); xor_out(4) <= xor_out(3) XORparity_in(4); xor_out(5) <= xor_out(4) XORparity_in(5); xor_out(6) <= xor_out(5) XORparity_in(6); xor_out(7) <= xor_out(6) XORparity_in(7); parity_out <= xor_out(7); ENDparity_dataflow; ECE 545 – Introduction to VHDL

  20. PARITY: Architecture (2) ARCHITECTUREparity_dataflowOFparityIS SIGNALxor_out: STD_LOGIC_VECTOR (7 DOWNTO 0); BEGIN xor_out(0) <= parity_in(0); G2: FOR i IN1TO7GENERATE xor_out(i) <= xor_out(i-1) XORparity_in(i); end generate G2; parity_out <= xor_out(7); ENDparity_dataflow; ECE 545 – Introduction to VHDL

  21. Combinational Logic Synthesis for Beginners ECE 545 – Introduction to VHDL

  22. 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 545 – Introduction to VHDL

  23. 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 545 – Introduction to VHDL

  24. 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 545 – Introduction to VHDL

  25. Left vs. right side of the assignment Left side <= <= when-else with-select <= Right side • Expressions including: • Internal signals (defined • in a given architecture) • Ports of the mode • - in • - inout • - buffer • Internal signals (defined • in a given architecture) • Ports of the mode • - out • - inout • - buffer ECE 545 – Introduction to VHDL

  26. Arithmetic operations Synthesizable arithmetic operations: • Addition, + • Subtraction, - • Comparisons, >, >=, <, <= • Multiplication, * • Division by a power of 2, /2**6(equivalent to right shift) • Shifts by a constant, SHL, SHR ECE 545 – Introduction to VHDL

  27. Arithmetic operations The result of synthesis of an arithmetic operation is a - combinational circuit - without pipelining. The exact internal architecture used (and thus delay and area of the circuit) may depend on the timing constraints specified during synthesis (e.g., the requested maximum clock frequency). ECE 545 – Introduction to VHDL

  28. Operations on Unsigned Numbers For operations on unsigned numbers USE ieee.std_logic_unsigned.all and signals (inputs/outputs) of the type STD_LOGIC_VECTOR OR USE ieee.std_logic_arith.all and signals (inputs/outputs) of the type UNSIGNED ECE 545 – Introduction to VHDL

  29. Operations on Signed Numbers For operations on signed numbers USE ieee.std_logic_signed.all and signals (inputs/outputs) of the type STD_LOGIC_VECTOR OR USE ieee.std_logic_arith.all and signals (inputs/outputs) of the type SIGNED ECE 545 – Introduction to VHDL

  30. Signed and Unsigned Types Behave exactly like STD_LOGIC_VECTOR plus, they determine whether a given vector should be treated as a signed or unsigned number. Require USE ieee.std_logic_arith.all; ECE 545 – Introduction to VHDL

  31. Addition of Unsigned Numbers LIBRARY ieee ; USE ieee.std_logic_1164.all ; USE ieee.std_logic_unsigned.all ; ENTITY adder16 IS PORT ( Cin : IN STD_LOGIC ; X, Y : IN STD_LOGIC_VECTOR(15 DOWNTO 0) ; S : OUT STD_LOGIC_VECTOR(15 DOWNTO 0) ; Cout : OUT STD_LOGIC ) ; END adder16 ; ARCHITECTURE Behavior OF adder16 IS SIGNAL Sum : STD_LOGIC_VECTOR(16 DOWNTO 0) ; BEGIN Sum <= ('0' & X) + Y + Cin ; S <= Sum(15 DOWNTO 0) ; Cout <= Sum(16) ; END Behavior ; ECE 545 – Introduction to VHDL

  32. Addition of Unsigned Numbers LIBRARY ieee ; USE ieee.std_logic_1164.all ; USE ieee.std_logic_arith.all ; ENTITY adder16 IS PORT ( Cin : IN STD_LOGIC ; X, Y : IN UNSIGNED(15 DOWNTO 0) ; S : OUT UNSIGNED(15 DOWNTO 0) ; Cout : OUT STD_LOGIC ) ; END adder16 ; ARCHITECTURE Behavior OF adder16 IS SIGNAL Sum : UNSIGNED(16 DOWNTO 0) ; BEGIN Sum <= ('0' & X) + Y + Cin ; S <= Sum(15 DOWNTO 0) ; Cout <= Sum(16) ; END Behavior ; ECE 545 – Introduction to VHDL

  33. Addition of Signed Numbers (1) LIBRARY ieee ; USE ieee.std_logic_1164.all ; USE ieee.std_logic_signed.all ; ENTITY adder16 IS PORT ( Cin : IN STD_LOGIC ; X, Y : IN STD_LOGIC_VECTOR(15 DOWNTO 0) ; S : OUT STD_LOGIC_VECTOR(15 DOWNTO 0) ; Cout, Overflow : OUT STD_LOGIC ) ; END adder16 ; ARCHITECTURE Behavior OF adder16 IS SIGNAL Sum : STD_LOGIC_VECTOR(16 DOWNTO 0) ; BEGIN Sum <= ('0' & X) + Y + Cin ; S <= Sum(15 DOWNTO 0) ; Cout <= Sum(16) ; Overflow <= Sum(16) XOR X(15) XOR Y(15) XOR Sum(15) ; END Behavior ; ECE 545 – Introduction to VHDL

  34. ECE 545 – Introduction to VHDL

  35. Addition of Signed Numbers (2) LIBRARY ieee ; USE ieee.std_logic_1164.all ; USE ieee.std_logic_arith.all ; ENTITY adder16 IS PORT ( Cin : IN STD_LOGIC ; X, Y : IN SIGNED(15 DOWNTO 0) ; S : OUT SIGNED(15 DOWNTO 0) ; Cout, Overflow : OUT STD_LOGIC ) ; END adder16 ; ARCHITECTURE Behavior OF adder16 IS SIGNAL Sum : SIGNED(16 DOWNTO 0) ; BEGIN Sum <= ('0' & X) + Y + Cin ; S <= Sum(15 DOWNTO 0) ; Cout <= Sum(16) ; Overflow <= Sum(16) XOR X(15) XOR Y(15) XOR Sum(15) ; END Behavior ; ECE 545 – Introduction to VHDL

  36. Multiplication of signed and unsigned numbers (1) LIBRARY ieee; USE ieee.std_logic_1164.all; USE ieee.std_logic_arith.all ; entity multiply is port( a : in STD_LOGIC_VECTOR(15 downto 0); b : in STD_LOGIC_VECTOR(7 downto 0); cu : out STD_LOGIC_VECTOR(23 downto 0); cs : out STD_LOGIC_VECTOR(23 downto 0) ); end multiply; architecture dataflow of multiply is SIGNAL sa: SIGNED(15 downto 0); SIGNAL sb: SIGNED(7 downto 0); SIGNAL sres: SIGNED(23 downto 0); SIGNAL ua: UNSIGNED(15 downto 0); SIGNAL ub: UNSIGNED(7 downto 0); SIGNAL ures: UNSIGNED(23 downto 0); ECE 545 – Introduction to VHDL

  37. Multiplication of signed and unsigned numbers (2) begin -- signed multiplication sa <= SIGNED(a); sb <= SIGNED(b); sres <= sa * sb; cs <= STD_LOGIC_VECTOR(sres); -- unsigned multiplication ua <= UNSIGNED(a); ub <= UNSIGNED(b); ures <= ua * ub; cu <= STD_LOGIC_VECTOR(ures); end dataflow; ECE 545 – Introduction to VHDL

  38. Integer Types Operations on signals (variables) of the integer types: INTEGER, NATURAL, and their sybtypes, such as TYPE day_of_month IS RANGE 0 TO 31; are synthesizable in the range -(231-1) .. 231 -1 for INTEGERs and their subtypes 0 .. 231 -1 for NATURALs and their subtypes ECE 545 – Introduction to VHDL

  39. Integer Types Operations on signals (variables) of the integer types: INTEGER, NATURAL, are less flexible and more difficult to control than operations on signals (variables) of the type STD_LOGIC_VECTOR UNSIGNED SIGNED, and thus are recommened to be avoided by beginners. ECE 545 – Introduction to VHDL

  40. Addition of Signed Integers ENTITY adder16 IS PORT ( X, Y : IN INTEGER RANGE -32767 TO 32767 ; S : OUT INTEGER RANGE -32767 TO 32767 ) ; END adder16 ; ARCHITECTURE Behavior OF adder16 IS BEGIN S <= X + Y ; END Behavior ; ECE 545 – Introduction to VHDL

  41. Testbenches ECE 545 – Introduction to VHDL

  42. 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 545 – Introduction to VHDL

  43. 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 545 – Introduction to VHDL

  44. 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 545 – Introduction to VHDL

  45. 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 545 – Introduction to VHDL

  46. 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 545 – Introduction to VHDL

  47. 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 545 – Introduction to VHDL

  48. 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 545 – Introduction to VHDL

  49. Asserts & Reports ECE 545 – Introduction to VHDL

  50. Assert Assert is a non-synthesizable statement whose purpose is to write out messages on the screen when problems are found during simulation. Depending on the severity of the problem, The simulator is instructed to continue simulation or halt. ECE 545 – Introduction to VHDL

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