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Logic Values

Logic Values. 0:logic 0 / false 1:logic 1 / true X:unknown logic value Z:high-impedance. Strength levels. Data Types. Nets Connects between hardware elements Must be continuously driven by Continuous assignment (assign) Module or gate instantiation (output ports)

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Logic Values

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  1. Logic Values • 0:logic 0 / false • 1:logic 1 / true • X:unknown logic value • Z:high-impedance

  2. Strength levels

  3. 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”

  4. 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

  5. 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 • supply1 and supply0  “x” • strong1 and supply1  “supply1”

  6. Net Types • 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 • Two states: • Driven state: at least one driver drives 0, 1, x • Capacitive state: • all driver have high impedance “z” • Strength: small, medium, large; default is medium

  7. An example for wire, tri0, tri1 • module tritest(); • wire w1, w2, w3, w4; • tri0 t01, t02, t03, t04; • tri1 t11, t12, t13, t14; • assign w1 = 0; • assign t01 = 0; • assign t11 = 0; • assign w2 = 1'bz; • assign t02 = 1'bz; • assign t12 = 1'bz; • assign w3 = 1; • assign t03 = 1; • assign t13 = 1; • Initial • begin • #1;$display(w1, w2, w3, w4); • $display(t01, t02, t03, t04); • $display(t11, t12, t13, t14); • end • endmodule Results: 0 z 1 z 0 0 1 0 0 1 1 1

  8. 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

  9. 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 or unsized

  10. 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

  11. 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 • 6’h3a // Binary 111010 • 1’bx // One-bit X • 32’bz // 32 bits of High-Z (impedance)

  12. Operators

  13. 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

  14. 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/verilogmodule 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

  15. 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; regq; wire[1:0] select; wire[3:0] d; always @(d or select) q = d[select]; endmodule

  16. 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

  17. Exercise • Design an 1-to-8 Demultiplexer (Becareful! you cannot use assign q[select]=d; since only constant is allowed in [.] on rightside of “=“. However, it is ok in the “always” procedure if “select” is declared as “reg” type. )

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