1 ECE 545 – Introduction to VHDL Dataflow Modeling of Combinational Logic Simple Testbenches ECE 656. Lecture 2

2 ECE 545 – Introduction to VHDL 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

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

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

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

6 ECE 545 – Introduction to VHDL Data-flow VHDL concurrent signal assignment (  ) conditional concurrent signal assignment (when-else) selected concurrent signal assignment (with-select-when) generate scheme for equations (for-generate) Major instructions Concurrent statements

7 ECE 545 – Introduction to VHDL MLU: Block Diagram

8 ECE 545 – Introduction to VHDL 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;

9 ECE 545 – Introduction to VHDL 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);

10 ECE 545 – Introduction to VHDL 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;

11 ECE 545 – Introduction to VHDL Data-flow VHDL concurrent signal assignment (  ) conditional concurrent signal assignment (when-else) selected concurrent signal assignment (with-select-when) generate scheme for equations (for-generate) Major instructions Concurrent statements

12 ECE 545 – Introduction to VHDL For Generate Statement For - Generate label: FOR identifier IN range GENERATE BEGIN {Concurrent Statements} END GENERATE;

13 ECE 545 – Introduction to VHDL PARITY Example

14 ECE 545 – Introduction to VHDL PARITY: Block Diagram

15 ECE 545 – Introduction to VHDL PARITY: Entity Declaration LIBRARY ieee; USE ieee.std_logic_1164.all; ENTITY parity IS PORT( parity_in : IN STD_LOGIC_VECTOR(7 DOWNTO 0); parity_out : OUT STD_LOGIC ); END parity;

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

17 ECE 545 – Introduction to VHDL PARITY: Architecture ARCHITECTURE parity_dataflow OF parity IS SIGNAL xor_out: std_logic_vector (6 downto 1); BEGIN xor_out(1) <= parity_in(0) XOR parity_in(1); xor_out(2) <= xor_out(1) XOR parity_in(2); xor_out(3) <= xor_out(2) XOR parity_in(3); xor_out(4) <= xor_out(3) XOR parity_in(4); xor_out(5) <= xor_out(4) XOR parity_in(5); xor_out(6) <= xor_out(5) XOR parity_in(6); parity_out <= xor_out(6) XOR parity_in(7); END parity_dataflow;

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

19 ECE 545 – Introduction to VHDL PARITY: Architecture ARCHITECTURE parity_dataflow OF parity IS SIGNAL xor_out: STD_LOGIC_VECTOR (7 downto 0); BEGIN xor_out(0) <= parity_in(0); xor_out(1) <= xor_out(0) XOR parity_in(1); xor_out(2) <= xor_out(1) XOR parity_in(2); xor_out(3) <= xor_out(2) XOR parity_in(3); xor_out(4) <= xor_out(3) XOR parity_in(4); xor_out(5) <= xor_out(4) XOR parity_in(5); xor_out(6) <= xor_out(5) XOR parity_in(6); xor_out(7) <= xor_out(6) XOR parity_in(7); parity_out <= xor_out(7); END parity_dataflow;

20 ECE 545 – Introduction to VHDL PARITY: Architecture (2) ARCHITECTURE parity_dataflow OF parity IS SIGNAL xor_out: STD_LOGIC_VECTOR (7 DOWNTO 0); BEGIN xor_out(0) <= parity_in(0); G2: FOR i IN 1 TO 7 GENERATE xor_out(i) <= xor_out(i-1) XOR parity_in(i); end generate G2; parity_out <= xor_out(7); END parity_dataflow;

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

22 ECE 545 – Introduction to VHDL concurrent signal assignment (  ) conditional concurrent signal assignment (when-else) selected concurrent signal assignment (with-select-when) generate scheme for equations (for-generate) Simple rules for beginners For combinational logic, use only concurrent statements

23 ECE 545 – Introduction to VHDL concurrent signal assignment (  ) 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

24 ECE 545 – Introduction to VHDL 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)

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

26 ECE 545 – Introduction to VHDL 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

27 ECE 545 – Introduction to VHDL 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).

28 ECE 545 – Introduction to VHDL 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

29 ECE 545 – Introduction to VHDL 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

30 ECE 545 – Introduction to VHDL 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;

31 ECE 545 – Introduction to VHDL 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 ;

32 ECE 545 – Introduction to VHDL 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 ;

33 ECE 545 – Introduction to VHDL 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 ;

34 ECE 545 – Introduction to VHDL

35 ECE 545 – Introduction to VHDL 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 ;

36 ECE 545 – Introduction to VHDL 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);

37 ECE 545 – Introduction to VHDL 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;

38 ECE 545 – Introduction to VHDL 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 -( ) for INTEGERs and their subtypes for NATURALs and their subtypes

39 ECE 545 – Introduction to VHDL 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.

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

41 ECE 545 – Introduction to VHDL Testbenches

42 ECE 545 – Introduction to VHDL 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;

43 ECE 545 – Introduction to VHDL 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;

44 ECE 545 – Introduction to VHDL 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; Generating all possible values of two inputs

45 ECE 545 – Introduction to VHDL 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 clk2 after clk2_period/2; END behavioral;

46 ECE 545 – Introduction to VHDL 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;

47 ECE 545 – Introduction to VHDL 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;

48 ECE 545 – Introduction to VHDL Wait for vs. Wait Wait for: waveform will keep repeating itself forever Wait : waveform will keep its state after the last wait instruction … …

49 ECE 545 – Introduction to VHDL Asserts & Reports

50 ECE 545 – Introduction to VHDL 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.

51 ECE 545 – Introduction to VHDL Assert - syntax ASSERT condition [REPORT "message" [SEVERITY severity_level ]; The message is written when the condition is FALSE. Severity_level can be: Note, Warning, Error (default), or Failure.

52 ECE 545 – Introduction to VHDL Assert - Examples assert initial_value <= max_value report "initial value too large" severity error; assert packet_length /= 0 report "empty network packet received" severity warning; assert false report "Initialization complete" severity „note”;

53 ECE 545 – Introduction to VHDL Report - syntax REPORT "message" [SEVERITY severity_level ]; The message is always written. Severity_level can be: Note (default), Warning, Error, or Failure.

54 ECE 545 – Introduction to VHDL Report - Examples report "Initialization complete"; report "Current time = " & time'image(now); report "Incorrect branch" severity error;

55 ECE 545 – Introduction to VHDL Generating reports in the message window reports: process(clk_trigger) begin if (clk_trigger = '0' and clk_trigger'EVENT) then case segments is when seg_0 => report time'image(now) & ": 0 is displayed" ; when seg_1 => report time'image(now) & ": 1 is displayed" ; when seg_2 => report time'image(now) & ": 2 is displayed" ; when seg_3 => report time'image(now) & ": 3 is displayed" ; when seg_4 => report time'image(now) & ": 4 is displayed" ; when seg_5 => report time'image(now) & ": 5 is displayed" ; when seg_6 => report time'image(now) & ": 6 is displayed" ; when seg_7 => report time'image(now) & ": 7 is displayed" ; when seg_8 => report time'image(now) & ": 8 is displayed" ; when seg_9 => report time'image(now) & ": 9 is displayed" ; end case; end if; end process;

56 ECE 545 – Introduction to VHDL Anatomy of a Process [label:] process [(sensitivity list)] [declaration part] begin statement part end process;

57 ECE 545 – Introduction to VHDL Sequential Statements (1) If statement else and elsif are optional if boolean expression then statements elsif boolean expression then statements else boolean expression then statements end if;

58 ECE 545 – Introduction to VHDL Sequential Statements (3) Loop Statement Repeats a section of VHDL code for i in range loop statements end loop;

59 ECE 545 – Introduction to VHDL Sequential Statements (2) Case statement Choices have to cover all possible values of the condition Use others to specify all remaining cases case condition is when choice_1 => statements when choice_2 => statements when others => statements end case;