ECEN 468 Advanced Digital System Designdropzone.tamu.edu/~jhu/teaching/ecen468Spring17/468Lec19_Verilog.pdfLecture 19: Logic Design with Verilog ECEN 468 Advanced Digital System Design

Lecture 19 : Logic Design with Ver i log

ECEN 468 Advanced Digital System Design

ECEN 468 Lecture 19

ECEN 468 Lecture 19

Verilog Module

v Description of internal structure/function o  Implicit semantic of time

associated with each data object/signal

o  Implementation is hidden to outside world

v Communicate with outside through ports o Port list is optional

v Achieve hardware encapsulation

module Add_half ( sum, c_out, a, b ); input a, b;

output sum, c_out; wire c_out_bar;

xor (sum, a, b); nand (c_out_bar, a, b); not (c_out, c_out_bar);

endmodule

c_out

a

b sum

c_out_bar

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ECEN 468 Lecture 19

Implicit Structural Description

module Add_half ( sum, c_out, a, b ); input a, b;

output sum, c_out; assign { c_out, sum } = a + b; // Continuous assignment

endmodule

a

b

Add_half sum

c_out

Concatenation

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ECEN 468 Lecture 19

Module Instantiation

v  Accomplished by entering o Module name as a module item within a parent module o Signal identifiers at appropriate ports

v  Module instantiation needs a module identifier v  A module is never declared within another module v  The order of ports in instantiation usually matches the

order in module declaration

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ECEN 468 Lecture 19

Design a Full Adder

sumHA = a ⊕ b c_outHA = a • b sumFA = a ⊕ b ⊕ c_in c_outFA = a • b + b • c_in + a • c_in sumFA = (a ⊕ b) ⊕ c_in c_outFA = (a ⊕ b) • c_in + a • b

a + b

= a(b+b’) + (a+a’)b

= ab + ab’ + a’b

ab + bc + ac

= ab + (a+b)c

= ab + (a⊕ b+ab)c

= ab + a⊕ bc+abc

= ab + (a⊕ b)c

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ECEN 468 Lecture 19

Full Adder ≈ 2 Half Adders

sumHA = a ⊕ b c_outHA = a • b sumFA = (a ⊕ b) ⊕ c_in c_outFA = (a ⊕ b) • c_in + a • b

Add_half

(a⊕b)•c_in a

b

Add_half a⊕b

a•b

c_in (a ⊕ b) ⊕ c_in

(a ⊕ b) • c_in + a • b

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ECEN 468 Lecture 19

Module instance name

Full Adder in Verilog

module Add_full ( sum, c_out, a, b, c_in ); // parent module input a, b, c_in; output c_out, sum; wire w1, w2, w3; Add_half M1 ( w1, w2, a, b ); Add_half M2 ( sum, w3, w1, c_in ); // child module or ( c_out, w2, w3 ); // primitive instantiation

endmodule

Add_half

(a⊕b)•c_in a

b

Add_half a⊕b

a•b

c_in (a ⊕ b) ⊕ c_in

(a ⊕ b) • c_in + a • b

w1

w2

w3

sum

c_out

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ECEN 468 Lecture 19

Verilog Primitives

v  Basic element to build a module, such as nand, nor, buf and not gates

v  Never used stand-alone in design, must be within a module v  Pre-defined or user-defined v  Identifier (instance name) is optional v  Output is at left-most in port list v  Default delay = 0

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ECEN 468 Lecture 19

Delay Assignment

module AOI_4 ( y, x1, x2, x3, x4 ); input x1, x2, x3, x4; output y; wire y1, y2; and #1 ( y1, x1, x2 ); and #1 ( y2, x3, x4 ); nor #1 ( y, y1, y2 );

endmodule

x1

x2

x3

x4

y1

y2

y

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ECEN 468 Lecture 19

Explicit Structural Descriptions

module AOI_4 ( y, x1, x2, x3, x4 ); input x1, x2, x3, x4; output y; wire y1, y2;

and #1 ( y1, x1, x2 ); and #1 ( y2, x3, x4 ); nor #1 ( y, y1, y2 );

endmodule

x1

x2

x3

x4

y1

y2

y

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ECEN 468 Lecture 19

Implicit Structural Description

module nand2_RTL ( y, x1, x2 ); input x1, x2; output y;

assign y = x1 ~& x2;

endmodule

module nand2_RTL ( y, x1, x2 ); input x1, x2; output y; wire y = x1 ~& x2;

endmodule

Explicit continuous assignment Implicit continuous assignment

Continuous assignment –

Static binding between LHS and RHS

No mechanism to eliminate or alter the binding

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ECEN 468 Lecture 19

Multiple Assignments

module multiple_gates ( y1, y2, y3, a1, a2, a3, a4 ); input a1, a2, a3, a4; output y1, y2, y3;

assign y1 = a1 ^ a2, y2 = a2 | a3, y3 = a1 + a4;

endmodule

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ECEN 468 Lecture 19

Structural Connections

v  By order v  By name v  Empty port

module child( a, b, c );

…

endmodule

module parent;

wire u, v, w;

child m1( u, v, w );

child m2( .c(w), .a(u), .b(v) );

child m3( u, , w );

endmodule

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ECEN 468 Lecture 19

Behavioral Descriptions: Data Flow

module comparetor ( lt, gt, eq, A, B ); input A, B; output lt, gt, eq; reg lt, gt, eq; always @ ( A, B ) begin lt = (A < B); gt = (A > B);

eq = (A == B); end

endmodule

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ECEN 468 Lecture 19

Behavioral Descriptions: Algorithm-based

module comparetor ( lt, gt, eq, A, B ); input A, B; output lt, gt, eq; reg lt, gt, eq; always @ ( A, B ) begin lt = 0; gt = 0; eq = 0; if ( A == B ) eq = 1;

else if ( A > B ) gt = 1; else lt = 1;

end endmodule

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ECEN 468 Lecture 19

Description Styles

v  Structural o  Explicit structural

o  Implicit structural •  Explicit continuous assignment •  Implicit continuous assignment

v  Behavioral o  Data flow/RTL o  Algorithm-based

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ECEN 468 Lecture 19

Arrays of Instances

module flop_array(q, in, clk, rst); input [7:0] in; input clk, rst; output [7:0] q; Flip_flop M[7:0] (q, in, clk, rst);

endmodule module pipeline(q, in, clk, rst );

input [7:0] in; input clk, rst; output [7:0] q; wire [23:0] pipe; flop_array M[3:0] ({q, pipe}, {pipe, in}, clk, rst);

endmodule

rst

clk

in[7:0] q[7:0] pipe[7:0] pipe[23:16]

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ECEN 468 Lecture 19

Verilog for Synthesis

module comp(lt,gt,eq,a0,a1,b0,b1);

input a0, a1, b0, b1; output lt, gt, eq; wire w1, w2, w3, w4, w5, w6, w7; or (lt, w1, w2, w3); nor (gt, lt, eq); and (w1, w6, b1); and (w2, w6, w7, b0); and (w3, w7, b0, b1); not (w6, a1); not (w7, a0); xnor (w4, a1, b1); xnor (w5, a0, b0); endmodule

module comp(lt, gt, eq, a, b); input [1:0] a, b; output lt, gt, eq; assign it = ( a < b ); assign gt = ( a > b ); assign eq = ( a == b ); endmodule

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ECEN 468 Lecture 19

Language Conventions

v  Case sensitive v  Instance of a module must be named v  Keywords are lower-case v  Comments start with “//”, or blocked by “/* */”

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