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Logic Values
0:logic 0 / false
1:logic 1 / true
X:unknown logic value
Z:high-impedance
Data Types
 Nets
 Connects between hardware elements
 Must be continuously driven by
 Continuous assignment (assign)
 Module or gate instantiation (output ports)
 Default initial value for a wire is “Z” (and for a trireg is “x”)
 Registers
 Represent data storage elements
 Retain value until another value is placed on to them
 Similar to “variables” in other high level language
 Different to edge-triggered flip-flop in real ciucuits
 Do not need clock
 Default initial value for a reg is “X”
Examples
reg a; // a scalar register
wand w; // a scalar net of type “wire and”
reg [3:0] v; // a 4-bit vector register from msb to lsb
reg [7:0] m, n; // two 8-bit registers
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
Net Types
 The most common and important net types
wire and tri
 for standard interconnection wires
 wire: single driver e.g. output of “and” gate
 tri: multiple driver e.g. multiplexer
Register Types
 reg
any size, unsigned
 Integer
integet a,b; // declaration
32-bit signed (2’s complement)
 Time
64-bit unsigned, behaves like a 64-bit reg
$display(“At %t, value=%d”,$time,val_now)
 real
real c,d; //declaration
64-bit real number
Defaults to an initial value of 0
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 or unsized
Number Specification (continue)
 Sized numbers
<size>’<base_format><number>
<size> is in decimal and specifies the number of bits
‘<base_format> is: ‘d ‘D ‘h ‘H ‘b ‘B ‘o ‘O
The <number> digits are 0-f, uppercase may be used
Examples:
4’b1111
12’habc
16’d255
Number Specification
Unsized numbers – The <size> is not specified
(default is simulator/compiler specific, >= 32
bits)
Numbers without a base are decimal by default
Examples
100 // Decimal 100, 32 bits by default
’h3a // Binary 111010, 32 bits by default
’bx // Binary X, 32 bits by default
’bz // High-Z (impedance), 32 bits by default
Operators
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); //Ax=x
assign Bx = (1'bx!=1'bz); //Bx=x
assign D0 = (1= =0); //D0=False
assign D1 = (1= =1); //D1=True
assign E0 = (1= = =1'bx); //E0=False
assign E1 = (4'b01xz = = = 4'b01xz);; //E1=True
assign F1 = (4'bxxxx = = = 4'bxxxx); //F1= True
assign x = a ? b : c
//if (a) then x = b else x = c
Concatenation operator
// A=1’b1; B=2’b00, C=2’b10; D=3’b110;
Y={B, C} //Result Y is 4’b0010
Y={A, B, C, D, 3’b001} //Result Y is 11’b10010110001
Y={A, B[0], C[1]} // Result Y is 3’b101
Replication operator
 Reg A;
 Reg [1:0] B, C;
 Reg [2:0] D;
 A=1’b1; B=2’b00, C=2’b10; D=3’b110;
 Y={4{A}} // Result Y is 4’b1111
 Y={4{A}, 2{B}} // Result Y is 8’b11110000
 Y ={4{A}, 2{B}, C} //Result Y is 8’b1111000010
Example – Multiplexer_1
 // Verilog code for Multiplexer implementation using assign
// File name: mux1.v
// by Harsha Perla for http://electrosofts.com
// harsha@electrosofts.com
// Available at http://electrosofts.com/verilog
module mux1( select, d, q );
input [1:0] select;
input [3:0] d;
output q;
wire
q;
wire[1:0] select;
wire[3:0] d;
assign q = d[select];
endmodule
Example – Multiplexer_2
// Verilog code for Multiplexer implementation using always block.
// by Harsha Perla for http://electrosofts.com
// harsha@electrosofts.com
// Available at http://electrosofts.com/verilog
module mux2( select, d, q );
input[1:0] select;
input[3:0] d;
output q;
reg q;
wire[1:0] select;
wire[3:0] d;
always @(d or select)
q = d[select];
endmodule
Example – Multiplexer_3
module mux4_1 (out, in0, in1, in2, in3, sel) ;
output out ;
input in0,in1,in2,in3 ;
input [1:0] sel ;
assign out = (sel == 2'b00) ? in0 :
(sel == 2'b01) ? in1 :
(sel == 2'b10) ? in2 :
(sel == 2'b11) ? in3 :
1'bx ;
endmodule
Homework 2
Design an 1-to-8 Demultiplexer