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Verilog HDL Basic Syntax

Verilog HDL Basic Syntax. Four valued logic system in Verilog. 0: logic low, false , ground 1: logic high , true , power unknown High impedance ， tri-state. Identifier. Begin with A-Z, a-z or _, and letter/number/$/_ follow Which is true or false?

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Verilog HDL Basic Syntax

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  1. Verilog HDL Basic Syntax

  2. Four valued logic system in Verilog 0: logic low, false, ground 1: logic high, true, power unknown High impedance，tri-state

  3. Identifier • Begin with A-Z, a-z or _, and letter/number/$/_ follow • Which is true or false? • _aZ89$ $a908d 8a9b is987 • __ada_7Z_ a*b_net n@238 _bus3 • Case sensitive • AB and ab are different identifiers • Maximum length: 1023 letters • Key words (reserved words): lowercase • module endmodule begin end • assign initial always • if else case endcase • for forever while repeat • reg wire integer parameter • posedgenegedge • input output inout or default

  4. Constant • Syntax: <size>’<base><value> size: size of bits, decimal, default: 32 base: 1. b (binary) 2. o(octonary) 3. d (decimal) 4. h(hexadecimal) default: decimal case insensitive value: any valid number in the selected number base, “_” is available. Note: When value is larger than size, MSB (Most significant bit) will be truncated to meet the size of bits.

  5. Constant • 12 unsized decimal (zero-extended to 32 bits) • ’83a unsized hexadecimal (zero-extended to 32 bits) • 8’b1100_0001 8-bit binary • 16’hff01 16-bit hexadecimal • 3’b1010_1101 3-bit number, truncated to 3’b101

  6. Variable type • Net type • Register type • Parameter type

  7. Variable type – Net type • Net type • default type: wire • default size: 1 bit • Note: wire type is the most common.

  8. Variable type – Register type • Register type • Only be used in always and initial block • regdefault size: 1 bit • Note: reg type is the most common in register type.

  9. Variable Declaration • Net declaration • <net_type> [range][delay] <net_name>[, net_name]; • Register declaration • <reg_type> [range] <reg_name>[, reg_name]; • Note: • rang: [MSB:LSB] • <>: required • []: optional

  10. Variable Declaration • Example: wire w; // 1 bit net type wire [15:0] w1, w2; // two 32-bit wire variables, LSB is bit0 wire [0:15] w3, w4; // two 32-bit wire variables, LSB is bit15 reg 1; // 1-bit reg type reg [3:0] v; // 4-bit reg variable reg [7:0] m, n; // two 8-bit reg variable

  11. Register Array • Syntax: reg_type [MSB:LSB] <memory_name> [first_addr:last_addr]; • [MSB:LSB]: to define the bits of register word • [first_addr:last_addr]: to define the depth of register • Example: • reg [15:0] MEM[0:1023]; // 1K x 16bit register

  12. Register Array - addressing • Register Array Element can be addressed by array index. mem_name[array_index] • Multidimensional array is not supported in Verilog. • Register Word only. • Bit in Register Word is not supported.

  13. Register Array - addressing • Example module mems; reg [8: 1] mem_a [0: 255]; // the definition of mem_a reg [8: 1] mem_word; // temporary variable: mem_ word . . . initial begin $displayb( mem_a[5]); // to display the 6th Register Word mem_word = mem_a[5]; // to save the 6th Register Word $displayb( mem_word[8]); // to display MSB in the 6th Register Word end endmodule

  14. How to choose right data type? • Input port Must be net type, but can be drived by net/register, (in1, in2 signal in the diagram) • Output port Can be register type, but only can drive net, (O signal in the diagram) A in1 Y O B in2

  15. How to choose right data type? module DUT (O, in1, in2); output O; input in1, in2; regin1, in2; wire O; always @(in1 or in2) O=in1+in2; endmodule • Find errors in the program.

  16. Parameter • Define a variable constant, be often used in the definition of delay, size, etc. • Can define multiple parameters, separated by commas. • Local definition, valid in current module only. • Recommendation: Upper case in parameter, lower case in variable.

  17. Parameter • Example module mod1( out, in1, in2); . . . parameter cycle = 20, p1 = 8, setup = cycle/2 – p1; // rule number 2 . . . wire [p1: 0] w1; // use parameter p1 . . . endmodule

  18. Verilog Operator Priority high low

  19. Verilog Operator module bitwise (); reg [3: 0] rega, regb, regc; reg [3: 0] num; initial begin rega = 4'b1001; regb = 4'b1010; end initial begin #10 num = rega & 0; // num = ? #20 num = rega & regb; // num = ? #30 num = rega | regb; // num = ? end endmodule

  20. Verilog Operator module logical (); parameter five = 5; regans; reg [3: 0] rega, regb, regc; initial begin rega = 4‘b0011; // logic value“1” end initial begin #10 ans = rega && 0; // ans = ? #20 ans = rega || 0; // ans = ? #30 ans = rega && five; // ans = ? end endmodule

  21. Verilog Operator >> logic shift right << logic shift left module shift (); reg [9: 0] num, num1; reg [7: 0] rega, regb; initial rega = 8'b00001100; initial begin #10 num <= rega << 5 ; // num = 01_1000_0000 #10 regb <= rega << 5 ; // regb = 1000_0000 #20 num <= rega >> 3; // num = 00_0000_0001 #20 regb <= rega >> 3 ; // regb = 0000_0001 #30 num <= 10'b11_1111_0000; #40 rega <= num << 2; // rega= 1100_0000 #40 num1 <= num << 2; // num1=11_1100_0000 #50 rega <= num >> 2; // rega= 1111_1100 #50 num1 <= num >> 2; // num1=00_1111_1100 end endmodule

  22. Verilog Operator module concatenation; reg [7: 0] rega, regb, regc, regd; reg [7: 0] new; initial begin rega = 8'b0000_0011; regb = 8'b0000_0100; regc = 8'b0001_1000; regd = 8'b1110_0000; end initial begin #10 new = {regc[ 4: 3], regd[ 7: 5], regb[ 2], rega[ 1: 0]}; end endmodule { } concatenate operation

  23. Verilog Operator • { {} } replicate operator • Syntax: {integer{}} • Function: copy the value that is in the inner {}, the integer indicates the times that copies. • Note: bits must be assigned when concatenate and replicate, or errors will occur. • Wrong Examples: a[7:0] = {4{ ´b10}}; b[7:0] = {2{ 5}}; c[3:0] = {3´b011, ´b0};

  24. Verilog Operator module replicate (); reg [3: 0] rega; reg [1: 0] regb, regc; reg [7: 0] bus; initial begin rega = 4’b1001; regb = 2'b11; regc = 2'b00; end initial begin #10 bus <= {4{ regb}}; // bus = 11111111 // regb is replicated 4 times. #20 bus <= { {2{ regb}}, {2{ regc}} }; // bus = 11110000. regc and regb are each // replicated, and the resulting vectors // are concatenated together #30 bus <= { {4{ rega[1]}}, rega }; // bus = 00001001. rega is sign-extended #40 $finish; end endmodule

  25. Verilog Program Structure Every module begins with a key word “module”, and ends with a key word “endmodule”. module_name The description of logic and function implementation is put in the internal of module.

  26. Verilog Program Structure module gates(a,b,led); input a,b; output [5:0] led; wire [5:0] z; //Combinational logic style assign z[5]=a&b; // AND assign z[4]=~(a&b); // NAND assign z[3]=a|b; // OR assign z[2]=~(a|b); // NOR assign z[1]=a^b; // XOR assign z[0]=a~^b; // XNOR assign led=~z; endmodule

  27. Module Implementation • Implementation name is required. • Positional Mapping • Named Mapping

  28. Module Implementation Positional Mapping module comp (o1, o2, i1, i2); output o1, o2; input i1, i2; . . . endmodule module test; comp c1 (Q, R, J, K); // Positional mapping comp c2 (.i2(K), .o1(Q), .o2(R), .i1(J)); // Named mapping endmodule Named Mapping： .internal signal（external signal）

  29. Always Statement • Function: to do something over and over again. • Level control sensitive events always@(A or B) • Edge trigger control always@(negedge Clock) • Combination always@(Reset or posedge Clock)

  30. Always Statement module HalfAdder(A, B, Sum, Carry); input A, B; output Sum, Carry; always@(A or B) begin Sum=A^B; Carry=A&B; end endmodule Find error

  31. Task 1: Design an adder module adder (out, a, b, cin); input a, b, cin; output [1:0] out; wire a, b, cin; reghalf_sum; wire half_carry; reg [1: 0] out; always @( a or b or cin) begin half_sum = a ^ b ^ cin ; // OK half_carry = a & b | a & !b & cin | !a & b & cin ; out = {half_carry, half_ sum} ; end endmodule Find error

  32. Obstructive assignment &Non obstructive assignment • Obstructive assignment: = Other statement can not be executed when executing this statement, and after execution, variable value changes immediately. • Non obstructive assignment: <= After execution, variable value can not change immediately. After block execution, variable value will change.

  33. Obstructive assignment &Non obstructive assignment module swap_vals; reg a, b, clk; initial begin a = 0; b = 1; clk= 0; end always #5 clk = ~clk; always @( posedgeclk) begin a <= b; // non obstructive assignment b <= a; // swap the value a and b end endmodule

  34. If branch statement if (statement） begin …… end else if (表达式） begin …… end else if (表达式） begin …… end else begin …… end always #20 if (index > 0) if (rega > regb) result = rega; else result = 0; else if (index == 0) begin $display(" Note : Index is zero"); result = regb; end else $display(" Note : Index is negative");

  35. casebranch statement case <statement> < statement >, < statement >: assign or nop; < statement >, < statement >: assign or nop; default:assign or nop;

  36. case branch statement module compute (result, rega, regb, opcode); input [7: 0] rega, regb; input [2: 0] opcode; output [7: 0] result; reg [7: 0] result; always @( rega or regb or opcode) case (opcode) 3'b000 : result = rega + regb; 3'b001 : result = rega - regb; 3'b010 , 3‘b100 : result = rega / regb; // two conditions, same operation default : begin result = 8'b0000_0000; $display (" no match"); end endcase endmodule

  37. Task 2: Select one signal from two signal module mux2(out,a,b,sl); input a,b,sl; output out; not u1(nsl,sl); and u2(sela,a,nsl); and u3(selb,b,sl); or u4(out,sela,selb); endmodule module mux2(out,a,b,sl); input a,b,sl; output out; reg out; always @ (sl or a or b) if (!sl) out = a; else out = b; endmodule

  38. Task 3: Seven-segment Decoder module bin27seg (data_in ,EN ,data_out ); input [3:0] data_in ; input EN ; output [6:0] data_out ; reg [6:0] data_out ; always @(data_in or EN ) begin data_out = 7'b1111111; if (EN == 1) case (data_in ) endcase end endmodule 4'b0000: data_out = 7'b1000000; // 0 4'b0001: data_out = 7'b1111001; // 1 4'b0010: data_out = 7'b0100100; // 2 4'b0011: data_out = 7'b0110000; // 3 4'b0100: data_out = 7'b0011001; // 4 4'b0101: data_out = 7'b0010010; // 5 4'b0110: data_out = 7'b0000011; // 6 4'b0111: data_out = 7'b1111000; // 7 4'b1000: data_out = 7'b0000000; // 8 4'b1001: data_out = 7'b0011000; // 9 4'b1010: data_out = 7'b0001000; // A 4'b1011: data_out = 7'b0000011; // b 4'b1100: data_out = 7'b0100111; // c 4'b1101: data_out = 7'b0100001; // d 4'b1110: data_out = 7'b0000110; // E 4'b1111: data_out = 7'b0001110; // F default: data_out = 7'b1111111;

  39. Task 4: adder (3 bit) (use concatenate operator{}) module adder3(cout, sum, a, b, cin); input [2:0] a,b; input cin; output cout; output [2:0] sum; assign { cout, sum } = a + b + cin; endmodule

  40. Task 5: Comparator module compare2(eq,a,b); input [1:0] a,b; output eq; assign eq = (a == b)? 1:0 ; endmodule

  41. Thank You

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