ECEN ECEN475 Introduction to VLSI System Design Verilog HDL

ECEN HDLs  Hardware Description Languages  Widely used in logic design  Verilog and VHDL  Describe hardware using code  Document logic functions  Simulate logic before building  Synthesize code into gates and layout  Requires a library of standard cells

ECEN Module ports Module name Verilog keywords Taste of Verilog module Add_half ( sum, c_out, a, b ); inputa, b; outputsum, c_out; wire c_out_bar; xor (sum, a, b); nand (c_out_bar, a, b); not (c_out, c_out_bar); endmodule Declaration of port modes Declaration of internal signal Instantiation of primitive gates c_out a b sum c_out_bar

ECEN Taste of Verilog module Add_full ( sum, c_out, a, b, c_in ); inputa, b, c_in; outputsum, c_out; Adder_half (s1, c1, a, b); Adder_half (sum, c2, s1, c_in); OR (c_out, c1, c2); endmodule

ECEN Behavioral Description module Add_half ( sum, c_out, a, b ); inputa, b; outputsum, c_out; reg sum, c_out; ( a or b ) begin sum = a ^ b;// Exclusive or c_out = a & b;// And end endmodule a b Add_half sum c_out

ECEN Continuous Assignment module Add_half ( sum, c_out, a, b ); inputa, b; outputsum, c_out; assign sum = a ^ b;// Exclusive or assign c_out = a & b;// And endmodule

ECEN Example of Flip-flop module Flip_flop ( q, data_in, clk, rst ); input data_in, clk, rst; output q; reg q; ( posedge clk ) begin if ( rst == 1) q = 0; else q = data_in; end endmodule data_in q rst clk Declaration of synchronous behavior Procedural statement

ECEN Gate Delay  and (yout, x1, x2);// default, zero gate delay  and #3 (yout, x1, x2);// 3 units delay for all transitions  and #(2,3) G1(yout, x1, x2); // rising, falling delay  and #(2,3) G1(yout, x1, x2), G2(yout2, x3, x4); // Multiple instances  and (yout, x1, x2);// default, zero gate delay  and #3 (yout, x1, x2);// 3 units delay for all transitions  and #(2,3) G1(yout, x1, x2); // rising, falling delay  and #(2,3) G1(yout, x1, x2), G2(yout2, x3, x4); // Multiple instances

ECEN Time Scales  Time scale directive: ‘ timescale /  time_unit -> physical unit of measure, time scale of delay  time_precision -> time resolution/minimum step size during simulation  time_unit  time_precision Unit/precisionDelay specification Simulator time step Delay value in simulation 1ns / 100ps#40.1ns4.0ns 100ns / ns#41ns400ns 10ns / 100ps# ns46.3ns

ECEN Net Delay … wire #2 y_tran; and #3 (y_tran, x1, x2); buf #1 (buf_out, y_tran); and #3 (y_inertial, x1, x2); … wire #2 y_tran; and #3 (y_tran, x1, x2); buf #1 (buf_out, y_tran); and #3 (y_inertial, x1, x2); … x1 x2 y_tran y_inertial buf_out x1 x2 y_inertial y_tran buf_out

ECEN Structural vs. Behavioral Descriptions module my_module(…); … assign …; // continuous assignment and (…); // instantiation of primitive adder_16 M(…); // instantiation of module begin … end initial begin … end endmodule Structural, no order Behavior, in order in each procedure

ECEN Behavioral Statements initial | always single_statement; | begin block_of_statements; end initial | always single_statement; | begin block_of_statements; end  initial  Activated from t sim = 0  Executed once  Initialize a simulation  always  Activated from t sim = 0  Executed cyclically  Continue till simulation terminates

ECEN Example of Behavioral Statement module clock1 ( clk ); parameter half_cycle = 50; parameter max_time = 1000; output clk; reg clk; initial clk = 0; always begin #half_cycle clk = ~clk; end initial #max_time $finish; endmodule module clock1 ( clk ); parameter half_cycle = 50; parameter max_time = 1000; output clk; reg clk; initial clk = 0; always begin #half_cycle clk = ~clk; end initial #max_time $finish; endmodule clk t sim

ECEN Assignment  Continuous assignment  Values are assigned to net variables due to some input variable changes  “assign …=… “  Procedural assignment  Values are assigned to register variables when certain statement is executed in a behavioral description  Procedural assignment, “=“

ECEN Example module mux4_PCA(a, b, c, d, select, y_out); input a, b, c, d; input [1:0] select; output y_out; reg y_out; or a or b or c or d) begin if (select == 0) y_out=a; else if (select == 1) y_out=b; else if (select == 2) y_out=c; else if (select == 3) y_out=d; else y_out=1’bx; end endmodule module mux4_PCA(a, b, c, d, select, y_out); input a, b, c, d; input [1:0] select; output y_out; reg y_out; or a or b or c or d) begin if (select == 0) y_out=a; else if (select == 1) y_out=b; else if (select == 2) y_out=c; else if (select == 3) y_out=d; else y_out=1’bx; end endmodule Value of ‘a’ is assigned to y_out at this time

ECEN Blocking and Non-blocking Assignment initial begin a = 1; b = 0; a = b; // a = 0; b = a; // b = 0; end initial begin a = 1; b = 0; a <= b; // a = 0; b <= a; // b = 1; end  Blocking assignment “=“  Statement order matters  A statement has to be executed before next statement  Non-blocking assignment “<=“  Concurrent assignment  If there are multiple non- blocking assignments to same variable in same behavior, latter overwrites previous

ECEN Delay Control Operator (#) initial begin #0in1 = 0; in2 = 1; #10 in3 = 1; #40 in4 = 0; in5 = 1; #60 in3 = 0; end initial begin #0in1 = 0; in2 = 1; #10 in3 = 1; #40 in4 = 0; in5 = 1; #60 in3 = 0; end

ECEN Event Control Operator ( eventA or eventB ) begin … end ( eventA or eventB ) begin … end  Event -> identifier or expression  When is reached  Activity flow is suspended  The event is monitored  Other processes keep going  posedge: 0->1, 0->x, x->1  negedge: 1->0, 1->x, x->0

ECEN Intra-assignment Delay: Blocking Assignment // B = 0 at time 0 // B = 1 at time 4 … #5 A = B; // A = 1 C = D; … A = #5 B; // A = 0 C = D; … A B; C = D; … A B; C= D; … // B = 0 at time 0 // B = 1 at time 4 … #5 A = B; // A = 1 C = D; … A = #5 B; // A = 0 C = D; … A B; C = D; … A B; C= D; …  If timing control on LHS  Blocking delay  RHS evaluated at  Assignment at  If timing control on RHS  Intra-assignment delay  RHS evaluated immediately  Assignment at

ECEN Activity Flow Control ( if … else ) if ( A == B ) P = d; if ( B < C ); if ( a >= b ) begin … end if ( A < B ) P = d; else P = k; if ( A > B ) P = d; else if ( A < B ) P = k; else P = Q; if ( A == B ) P = d; if ( B < C ); if ( a >= b ) begin … end if ( A < B ) P = d; else P = k; if ( A > B ) P = d; else if ( A < B ) P = k; else P = Q;  Syntax: if ( expression ) statement [ else statement ]  Value of expression  0, x or z => false  Non-zero number => true

ECEN Conditional Operator ( ? … : ) ( posedge clock ) yout = ( sel ) ? a + b : a – b; ( posedge clock ) yout = ( sel ) ? a + b : a – b; Conditional operator can be applied in either continuous assignments or behavioral descriptions

ECEN The case Statement module mux4 ( a, b, c, d, select, yout ); input a, b, c, d; input [1:0] select; output yout; reg yout; a or b or c or d or select ) begin case ( select ) 0: yout = a; 1: yout = b; 2: yout = c; 3: yout = d; default yout = 1`bx; endcase endmodule module mux4 ( a, b, c, d, select, yout ); input a, b, c, d; input [1:0] select; output yout; reg yout; a or b or c or d or select ) begin case ( select ) 0: yout = a; 1: yout = b; 2: yout = c; 3: yout = d; default yout = 1`bx; endcase endmodule  Case items are examined in order  Exact match between case expression and case item  casez – treats “z” as don’t cares  casex – treats both “x” and “z” as don’t cares

ECEN Switch Level NAND Gate module nand_2 ( Y, A, B ); output Y; input A, B; supply0 GND; supply1 PWR; wire w; pmos ( Y, PWR, A ); pmos ( Y, PWR, B ); nmos ( Y, w, A ); nmos ( w, GND, B ); endmodule module nand_2 ( Y, A, B ); output Y; input A, B; supply0 GND; supply1 PWR; wire w; pmos ( Y, PWR, A ); pmos ( Y, PWR, B ); nmos ( Y, w, A ); nmos ( w, GND, B ); endmodule Y V dd A A B B