INTRODUCTION TO VERILOG HDL Presented by m.vinoth

What is verilog? Verilog is a HDL- hardware description language to design the digital system. VHDL is other hardware description language. Virtually every chip (FPGA, ASIC, etc.) is designed in part using one of these two languages Verilog was introduced in 1985 by Gateway Design System Corporation

verilog IEEE is the latest Verilog HDL standard Verilog is case sensitive (Keywords are in lowercase) The Verilog is both a behavioral and a structure language

Difference between VERILOG and VHDL Verilog is similar to c- language. VHDL is similar to Ada- (ada is a structured, statically typed, imperative, wide-spectrum, and object-oriented high- level computer programming language, extended from Pascal ) structuredstatically typedimperative wide-spectrumobject-orientedhigh- levelcomputerprogramming languagePascal Many feel that Verilog is easier to learn and use than VHDL.

Elements of verilog-logic values  0: zero, logic low, false, ground  1: one, logic high, power X: unknown Z: high impedance, unconnected, tri-state

Elements of verilog- data type Nets –Nets are physical connections between devices –Many types of nets, but all we care about is wire. Declaring a net wire [ ] ; Range is specified as [MSb:LSb]. Default is one bit wide Registers –Implicit storage-holds its value until a new value is assigned to it. –Register type is denoted by reg. Declaring a register reg [ ] ;  Parameters are not variables, they are constants.

Verilog Primitives Basic logic gates only –and –or –not –buf –xor –nand –nor –xnor –bufif1, bufif0 –notif1, notif0

Numbers in Verilog ’ –8’h ax = 1010xxxx –12’o 3zx7 = 011zzzxxx111 No of bits No of bits Binary  b or B Octal  o or O Decimal  d or D Hexadecimal  h or H Binary  b or B Octal  o or O Decimal  d or D Hexadecimal  h or H Consecutive chars 0-f, x, z Consecutive chars 0-f, x, z

Logical Operators &&  logical AND ||  logical OR !  logical NOT Operands evaluated to ONE bit value: 0, 1 or x Result is ONE bit value: 0, 1 or x A = 1;A && B  1 && 0  0 B = 0;A || !B  1 || 1  1 C = x;C || B  x || 0  x but C&&B=0

Bitwise Operators (i) &  bitwise AND |  bitwise OR ~  bitwise NOT ^  bitwise XOR ~^ or ^~  bitwise XNOR Operation on bit by bit basis

Bitwise Operators (ii) c = ~a; c = a & b; a = 4’b1010; b = 4’b1100; a = 4’b1010; b = 2’b11; c = a ^ b;

shift, Conditional Operator >>  shift right <<  shift left a = 4’b1010; d = a >> 2;// d = 0010,c = a << 1;// c = 0100 cond_expr ? true_expr : false_expr A B Y sel Y = (sel)? A : B; 0 1

keywords Note : All keywords are defined in lower case Examples : module, endmodule input, output, inout reg, integer, real, time not, and, nand, or, nor, xor parameter begin, end fork, join specify, endspecify

keywords module – fundamental building block for Verilog designs Used to construct design hierarchy Cannot be nested endmodule – ends a module – not a statement => no “;” Module Declaration module module_name (module_port, module_port, …); Example: module full_adder (A, B, c_in, c_out, S);

Verilog keywords Input Declaration Scalar input list of input identifiers; Example: input A, B, c_in; Vector input[range] list of input identifiers; Example: input[15:0] A, B, data; Output Declaration Scalar Example: output c_out, OV, MINUS; Vector Example: output[7:0] ACC, REG_IN, data_out;

Types verilog coding Behavioral  Procedural code, similar to C programming  Little structural detail (except module interconnect) Dataflow  Specifies transfer of data between registers  Some structural information is available (RTL)  Sometimes similar to behavior Structural (gate,switch)  Interconnection of simple components  Purely structural

Hierarchical Design Top Level Module Top Level Module Sub-Module 1 Sub-Module 1 Sub-Module 2 Sub-Module 2 Basic Module 3 Basic Module 3 Basic Module 2 Basic Module 2 Basic Module 1 Basic Module 1 Full Adder Half Adder E.g.

Module f in1 in2 inN out1 out2 outM my_module module my_module(out1,.., inN); output out1,.., outM; input in1,.., inN;.. // declarations.. // description of f (maybe.. // sequential) endmodule Everything you write in Verilog must be inside a module exception: compiler directives

VHDL coding for AND gate --The IEEE standard 1164 package, declares std_logic, etc. library IEEE; use IEEE.std_logic_1164.all; use IEEE.std_logic_arith.all; use IEEE.std_logic_unsigned.all; Entity Declarations entity andgate is Port( A : in std_logic; B : in std_logic; Y : out std_logic ); end andgate; architecture Behavioral of andgate is begin Y<= A and B ; end Behavioral;

VERILOG coding for all gate module gates(a, b, y1, y2, y3, y4, y5, y6, y7); input [3:0] a, b; output [3:0] y1, y2, y3, y4, y5, y6, y7; /* Seven different logic gates acting on four bit busses */ assign y1= ~a; // NOT gate assign y2= a & b; // AND gate assign y3= a | b; // OR gate assign y4= ~(a & b); // NAND gate assign y5= ~(a | b); // NOR gate assign y6= a ^ b; // XOR gate assign y7= ~(a ^ b); // XNOR gate endmodule

Example: Half Adder module half_adder(S, C, A, B); output S, C; input A, B; wire S, C, A, B; assign S = A ^ B; assign C = A & B; endmodule Half Adder Half Adder A B S C A B S C

Example: Full Adder module full_adder(sum, cout, in1, in2, cin); output sum, cout; input in1, in2, cin; wire sum, cout, in1, in2, cin; wire I1, I2, I3; half_adder ha1(I1, I2, in1, in2); half_adder ha2(sum, I3, I1, cin); assign cout = I2 || I3; endmodule Instance name Module name Half Adder ha2 Half Adder ha2 A B S C Half Adder 1 ha1 Half Adder 1 ha1 A B S C in1 in2 cin cout sumI1 I2 I3

Example 4-bit adder module add4 (s,c3,ci,a,b) input [3:0] a,b ;// port declarations input ci ; output [3:0] s :// vector output c3 ; wire [2:0] co ; add a0 (co[0], s[0], a[0], b[0], ci) ; add a1 (co[1], s[1], a[1], b[1], co[0]) ; add a2 (co[2], s[2], a[2], b[2], co[1]) ; add a3 (c3, s[3], a[3], b[3], co[2]) ; endmodule a0a1a2a3 c3ci Simpler than VHDL Only Syntactical Difference

Assignments Continuous assignments assign values to nets (vector and scalar) –They are triggered whenever simulation causes the value of the right-hand side to change –Keyword “assign” e.g. assign out = in1 & in2; Procedural assignments drive values onto registers (vector and scalar) –They Occur within procedures such as always and initial –They are triggered when the flow of execution reaches them (like in C) –Blocking and Non-Blocking procedural assignments

Assignments (cont.) Procedural Assignments –Blocking assignment statement (= operator) acts much like in traditional programming languages. The whole statement is done before control passes on to the next statement –Nonblocking assignment statement (<= operator) evaluates all the right-hand sides for the current time unit and assigns the left-hand sides at the end of the time unit

Delay Control (#) –Expression specifies the time duration between initially encountering the statement and when the statement actually executes. –Delay in Procedural Assignments Inter-Statement Delay Intra-Statement Delay –For example: Inter-Statement Delay #10 A = A + 1; Intra-Statement Delay A = #10 A + 1; Delay based Timing Control

Example: Half Adder, 2nd Implementation Assuming: XOR: 2 t.u. delay AND: 1 t.u. delay module half_adder(S, C, A, B); output S, C; input A, B; wire S, C, A, B; xor #2 (S, A, B); and #1 (C, A, B); endmodule A B S C

Combinational Circuit Design Outputs are functions of inputs Examples –MUX –decoder –priority encoder –adder comb. circuits inputs Outputs

Procedural Statements: if if (expr1) true_stmt1; else if (expr2) true_stmt2;.. else def_stmt; E.g. 4-to-1 mux: module mux4_1(out, in, sel); output out; input [3:0] in; input [1:0] sel; reg out; wire [3:0] in; wire [1:0] sel; or sel) if (sel == 0) out = in[0]; else if (sel == 1) out = in[1]; else if (sel == 2) out = in[2]; else out = in[3]; endmodule

Procedural Statements: case case (expr) item_1,.., item_n: stmt1; item_n+1,.., item_m: stmt2;.. default:def_stmt; endcase E.g. 4-to-1 mux: module mux4_1(out, in, sel); output out; input [3:0] in; input [1:0] sel; reg out; wire [3:0] in; wire [1:0] sel; or sel) case (sel) 0: out = in[0]; 1: out = in[1]; 2: out = in[2]; 3: out = in[3]; endcase endmodule

Decoder 3-to 8 decoder with an enable control module decoder(o,enb_,sel) ; output [7:0] o ; input enb_ ; input [2:0] sel ; reg [7:0] o ; (enb_ or sel) if(enb_) o = 8'b1111_1111 ; else case(sel) 3'b000 : o = 8'b1111_1110 ; 3'b001 : o = 8'b1111_1101 ; 3'b010 : o = 8'b1111_1011 ; 3'b011 : o = 8'b1111_0111 ; 3'b100 : o = 8'b1110_1111 ; 3'b101 : o = 8'b1101_1111 ; 3'b110 : o = 8'b1011_1111 ; 3'b111 : o = 8'b0111_1111 ; default : o = 8'bx ; endcase endmodule

Sequential Circuit Design –a feedback path –the state of the sequential circuits –the state transition l synchronous circuits l asynchronous circuits Examples- D latch D flip-flop register Memory elements Combinational circuit Inputs Outputs

d-Latch,flip-flop module latch (G, D, Q); input G, D; output Q; reg Q; or D) begin if (G) Q <= D; end endmodule module dff(Q, D, Clk); output Q; input D, Clk; reg Q; wire D, Clk; Clk) Q = D; endmodule

Jk flip-flop module jkff(J, K, clk, Q); input J, K, clk; output Q; reg Q; reg Qm; clk) if(J == 1 && K == 0) Qm <= 1; else if(J == 0 && K == 1) Qm <= 0; else if(J == 1 && K == 1) Qm <= ~Qm; assign Q <= Qm; endmodule

A counter which runs through counts 0, 1, 2, 4, 9, 10, 5, 6, 8, 7, 0, … Sequence counter module CntSeq(clk, reset, state); parameter n = 4; input clk, reset; output [n-1:0]state; reg1:0]state; [n- integer k; // clk) if(reset) state = 0; else begin case (state) 4'b0000:state = 4'b0001; //0 -> 1 4'b0001:state = 4'b0010; //1 -> 2 4'b0010:state = 4'b0100; //2 -> 4 4'b0100:state = 4'b1001; //4 -> 9 4'b1001:state = 4'b1010; //9 -> 10 4'b1010:state = 4'b0101; //10-> 5 4'b0101:state = 4'b0110; //5 -> 6 4'b0110:state = 4'b1000; //6 -> 8 4'b1000:state = 4'b0111; //8 -> 7 default:state = 4'b0000; endcase end endmodule