1. National Taiwan University Verilog HDL Overview Prof. An-Yeu Wu Date:2002/05/17 For NTUEE Undergraduate VLSI Design Course

2. A. Y. Wupp. 2 OUTLINE Introduction Basics of the Verilog Language Gate-level modeling Data-flow modeling Behavioral modeling Simulation and test bench

3. A. Y. Wupp. 3 What is Verilog HDL ? Hardware Description Language Mixed level modeling Behavioral —Algorithmic ( like high level language) —Register transfer (Synthesizable) Structural —Gate (AND, OR ……) —Switch (PMOS, NOMS, JFET ……) Single language for design and simulation Built-in primitives and logic functions User-defined primitives Built-in data types High-level programming constructs

4. A. Y. Wupp. 4 OUTLINE Introduction Basics of the Verilog Language Overview of Verilog Module Identifier & Keywords Logic Values Data Types Numbers & Negative Numbers Gate-level modeling Data-flow modeling Behavioral modeling Simulation and test bench

5. A. Y. Wupp. 5 Overview of Verilog Module Test bench

6. A. Y. Wupp. 6 Basic unit --Module module module_name (port_name); port declaration data type declaration module functionality or structure endmodule

7. A. Y. Wupp. 7 Latch module Latch ( Q,Q_b,D,D_b,clk ); //port declaration output Q,Q_b; input D,D_b,clk ; reg Q,Q_b; always@(D or D_b or clk or Q_b or Q) begin Q=(D&clk)|(~clk&Q); Q_b=D_b&clk|~clk&Q_b; end endmodule Q=D ․ clk+ clk_b ․ Q Q_b=D_b ․ clk+clk_b ․ Q_b

8. A. Y. Wupp. 8 Register / Flip-flop module D_register(q,d1,clk); input d1,clk; output q; wire d1_b, clk_b; assign d1_b=~d1; assign clk_b=~clk; dff dff1(d,d_b,d1,d1_b,clk_b), dff2(q_b,q,d_b,d,clk); endmodule dff ( Q,Q_b,D,D_b,clk ); Sub-module

9. A. Y. Wupp. 9 Instances module adder (in1,in2,cin,sum,cout);....... endmodule module adder8(....) ; adder add1(a,b,1’b0,s1,c1), add2(.in1(a2),.in2(b2),.cin(c1),.sum(s2),.cout(c2)) ;..... endmodule Mapping port positions Mapping names

10. A. Y. Wupp. 10 Identifier & Keywords Identifier User-provided names for Verilog objects in the descriptions Legal characters are “a-z”, “A-Z”, “0-9”, “_”, and “$” First character has to be a letter or an “_” —Example: Count, _R2D2, FIVE$ Keywords Predefined identifiers to define the language constructs All keywords are defined in lower case Cannot be used as identifiers —Example:initial, assign, module, always….

11. A. Y. Wupp. 11 Logic Values 0:logic 0 / false 1:logic 1 / true X:unknown logic value Z:high-impedance

12. A. Y. Wupp. 12 Data Types Nets ( wire or reg (in combinational always block) Connects between structural elements Must be continuously driven by —Continuous assignment (assign) —Module or gate instantiation (output ports) Default initial value for a wire is “Z” Registers ( reg (in sequential always block) ) Represent abstract data storage elements Updated at an edge trigger event and holds its value until another edge trigger event happens Default initial value for a wire is “X”

13. A. Y. Wupp. 13 Examples reg a; // a scalar register wand w; // a scalar net of type “wire and” reg [3:0] v; // a 4-bit vector register/wire from msb to lsb reg [7:0] m, n; // two 8-bit registers/wire tri [15:0] busa; // a 16-bit tri-state bus wire [0:31] w1, w2; // Two 32-bit wires with msb being the 0 bit, not recommended

14. A. Y. Wupp. 14 Net Types The most common and important net types wire and tri —for standard interconnection wires supply 1 and supply 0 Other wire types wand, wor, triand, and trior —for multiple drivers that are wired-anded and wired-ored tri0 and tri1 —pull down and pull up trireg —for net with capacitive storage —If all drivers at z, previous value is retained

15. A. Y. Wupp. 15 Register Types reg any size, unsigned Integer (not synthesizable) integet a,b; // declaration 32-bit signed (2’s complement) Time (not synthesizable) 64-bit unsigned, behaves like a 64-bit reg $display(“At %t, value=%d”,$time,val_now) real, realtime (not synthesizable) real c,d; //declaration 64-bit real number Defaults to an initial value of 0

16. A. Y. Wupp. 16 Numbers & Negative Numbers Constant numbers are integer or real constants. Integer constants are written as “width ‘radix value” The radix indicates the type of number Decimal (d or D) Hex (h or H) Octal (o or O) Binary (b or B) A number may be sized (two’s complement) or unsized Ex: h12_unsized = ‘h12; h12_sized = 6’h12; Ex: PA = -12, PB = -’D12, PC= -32’D12;

17. A. Y. Wupp. 17 Operators

18. A. Y. Wupp. 18 Example assign A1 = (3+2) %2; A1 = 1 assign A2 = 4 >> 1; assign A4 = 1 >> 2; A2 = 2 A4 = 4 assign Ax = (1= =1'bx); assign Bx = (1'bx!=1'bz); assign D0 = (1= =0); assign D1 = (1= =1); assign E0 = (1= = =1'bx); E0 = 0; False assign E1 = 4'b01xz = = = 4'b01xz; E1 = 1; True assign F1 = (4'bxxxx = = = 4'bxxxx); F1 = 1; True assign x = a ? b : c if (a) then x = b else x = c

19. A. Y. Wupp. 19 OUTLINE Introduction Basics of the Verilog Language Gate-level modeling Data-flow modeling Behavioral modeling Simulation and test bench

20. A. Y. Wupp. 20 Gate-level modeling(1) The following gates are built-in types in the simulator and, nand, nor, or, xor, xnor First terminal is output, followed by inputs and a1 (out1, in1, in2); nand a2 (out2, in21, in22, in23, in24); buf, not One or more outputs first, followed by one input not N1 (OUT1, OUT2, OUT3, OUT4, INA); buf B1 (BO1, BIN);

21. A. Y. Wupp. 21 Gate-level modeling(2) bufif0, bufif1, notif0, notif1: three-state drivers Output terminal first, then input, then control bufif1 BF1 (OUTA,INA,CTRLA); pullup, pulldown Put 1 or 0 on all terminals pullup PUP (PWRA, PWRB, PWRC); Instance names are optional ex: not (QBAR, Q)

22. A. Y. Wupp. 22 Example module MUX4x1 (Z, D0, D1, D2, D3, S0, S1); output Z; input D0, D1, D2, D3, S0, S1; and (T0, D0, S0BAR, S1BAR); (T1, D1, S0BAR, S1), (T2, D2, S0, S1BAR), (T3, D3, S0, S1); not (S0BAR, S0), (S1BAR, S1); nor (Z, T0, T1, T2, T3); endmodule 4 X 1 multiplexer circuit

23. A. Y. Wupp. 23 OUTLINE Introduction Basics of the Verilog Language Gate-level modeling Data-flow modeling Behavioral modeling Simulation and test bench

24. A. Y. Wupp. 24 Data-flow modeling(1) Models behavior of combinational logic Assign a value to a net using continuous assignment Examples: wire [3:0] Z, PRESET, CLEAR; assign Z = PRESET & CLEAR; wire COUT, CIN; wire [3:0] SUM, A, B; assign {COUT, SUM} = A + B + CIN;

25. A. Y. Wupp. 25 Data-flow modeling(2) Left-hand side (target) expression can be: Single net (ex: Z) Part-select (ex: SUM[2:0]) Bit-select (ex: Z[1]) Concatenation of both (ex: {COUT, SUM[3:0]}) Expression on right-hand side is evaluated whenever any operand value changes Note: Concatenation example {A,1’b0} -> x2 {A,2’b0} -> x4 wire [7:0]A,B; wire [16:0]C; assign C={A,B};

26. A. Y. Wupp. 26 Delay Delay between assignment of right-hand side to left-hand side assign #6 ASK = QUIET || LATE; //Continuous delay Netdelay wire #5 ARB; // Any change to ARB is delayed 5 time units before it takes effect If value changes before it has a chance to propagate, latest value change will be applied – Inertial delay

27. A. Y. Wupp. 27 OUTLINE Introduction Basics of the Verilog Language Gate-level modeling Data-flow modeling Behavioral modeling Simulation and test bench

28. A. Y. Wupp. 28 Behavioral modeling Procedural blocks: initial block: executes only once always block:executes in a loop Block execution is triggered based on user- specified conditions always @ (posedge clk) ……… All procedural blocks are automatically activated at time 0 All procedural blocks are executed concurrently reg is the main data type that is manipulated within a sequential block It holds its value until assigned a new value

29. A. Y. Wupp. 29 Initial Statement Executes only once at the beginning of simulation initial statements Used for initialization and waveform generation //Initialization: reg [7:0] RAM[0:1023]; reg RIB_REG; initial begin integer INX; RIB_REG =0; for (INX = 0; INX < 1024; INX = INX + 1) RAM[INX] = 0; end group multiple statements

30. A. Y. Wupp. 30 Always Statement(1) Executes continuously; must be used with some form of timing control always (timing_control) always statements CLK = ~CLK // Will loop indefinitely Four forms of event expressions are often used An OR of several identifiers (comb/seq logic) The rising edge of a identifier (for clock signal of a register) The falling edge of a identifier (for clock signal of a register) Delay control (for waveform generator)

31. A. Y. Wupp. 31 Always Statement(2) Any number of initial and always statements may appear within a module Initial and always statements are all executed in parallel

32. A. Y. Wupp. 32 Example module example (D, CURRENT_STATE, Q, NEXT_STATE); input D, CURRENT_STATE; output Q, NEXT_STATE; reg CLK, Q, NEXT_STATE; always #5 CLK = ~CLK; always @(posedge CLK) begin Q =D; end always @(negedge CLK) begin NEXT_STATE = CURRENT_STATE; end endmodule delay-controlled always block clock period = 10 activated when CLK has a 0 -> 1 transition activated when CLK has a 1 -> 0 transition

33. A. Y. Wupp. 33 Procedural Assignments The assignment statements that can be used inside an always or initial block The target must be a register or integer type always @(A or B) // infer wire begin B = A; C = B; end // C=A always @(posedge CLK) // infer flip-flop begin B <= A; C <= B; D <= C; end // Clock skew!!

34. A. Y. Wupp. 34 Conditional Statements if and else if statements if (expression) statements { else if (expression) statements } [ else statements ] if (total < 60) begin grade = C; total_C = total_C + 1; end else if (sum < 75) begin grade = B; total_B = total_B + 1; end else grade = A; case statement case (case_expression) case_item_expression {, case_item_expression }: statements …… [ default: statements ] endcase case (OP_CODE) 2`b10: Z = A + B; 2`b11: Z = A – B; 2`b01: Z = A * B; 2`b00: Z = A / B; default: Z = 2`bx; endcase

35. A. Y. Wupp. 35 Loop Statements Four loop statements are supported The for loop The while loop The repeat loop The forever loop The syntax of loop statements is very similar to that in C language Most of the loop statements are not synthe- sizable in current commercial synthesizers

36. A. Y. Wupp. 36 OUTLINE Introduction Basics of the Verilog Language Gate-level modeling Data-flow modeling Behavioral modeling Simulation and test bench

37. A. Y. Wupp. 37 Simulation and test bench(1) Design, stimulus, control, saving responses, and verification can be completed in a single language Stimulus and control —Use initial procedural block Saving responses —Save on change —Display data Verification —Automatic compares with expected responses

38. A. Y. Wupp. 38 Simulation and test bench(2) The behavior of a design can be simulated by HDL simulators Test benches are given by users as the inputs of the design Some popular simulators —Verilog-XL (Cadence TM, direct-translate simulator) —NC-Verilog (Cadence TM, compiled-code simulator) —VCS (ViewLogic TM, compiled-code simulator)

39. A. Y. Wupp. 39 Supports for Verification Verilog provides a number of system tasks and system functions. $time is a system function that returns the current simulation time. $monitor is a system task that displays the values of the argument list whenever any of the arguments change. Examples: $monitor($time,,out,,a,,b,,sel); $monitor($time,”%b %h %d %o, sig1,sig2,sig3,sig4);

40. A. Y. Wupp. 40 Test Bench module test_bench; data type declaration module instantiation applying stimulus display results endmodule A test bench is a top level module without inputs and outputs Data type declaration Declare storage elements to store the test patterns Module instantiation Instantiate pre-defined modules in current scope Connect their I/O ports to other devices Applying stimulus Describe stimulus by behavior modeling Display results By text output, graphic output, or waveform display tools

41. A. Y. Wupp. 41 Example module testfixture; reg [15:0] a,b,ci; wire [15:0] sum; wire cout; adder adder0(sum,cout,a,b,ci); initial begin a=0;b=0;ci=0; #10 ci=1; #10 ci=0; b=1; #10 ci=1; a=1; #10 $finish; end Initial $monitor ($time,” sum= %b cout= %b a= %b b= %b ci=%b”,sum,cout,a,b,ci); endmodule data type declaration module instantiation applying stimulus display results