Chap. 6 Dataflow Modeling

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Dataflow Modeling

Continuous Assignments Delays Expressions, Operators and Operands Operator Types Examples Summary

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Continuous Assignments - I

Assign a logic value to a wire/net Syntax

Continuous_assign::= assign [drive_strength] [delay] list_of_assignments;

List_of_net_assignments::=net_assignment{, net_assignment}

Net_assignment::=net_lvalue = expression Default drive_strength: strong1 or strong0 Delay: propagation time from inputs to output

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Continuous Assignments - II

Constraints LHS of assignment (=) must be scalar net or vecto

r net (rather than reg or vector reg) Once the value of RHS expression changes, the v

alue of assigned wire also changes accordingly The expression of RHS can be reg, wire or functio

n Delay controls the update time of LHS when the v

alue of RHS has changed like gate delay

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An Example of Continuous Assignments

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Implicit Continuous Assignment Perform a wire assignment when declaring

the wire wire out;

assign out = in1 & in2; (equals the following) wire out = in1 & in2;

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Implicit Net Declaration

Perform assignment for an un-declared wire wire i1, i2;

assign out = i1 & i2; // wire out has not been

// declared

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Dataflow Modeling

Continuous Assignments Delays Expressions, Operators and Operands Operator Types Examples Summary

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Delays

Regular Assignment Delay Implicit Continuous Assignment Delay Net Declaration Delay

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Regular Assignment Delay

assign #10 out = in1 & in2;

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Implicit Continuous Assignment Delay wire #10 out = in1 & in2; (equals the

following) wire out;

assign #10 out = in1 & in2;

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Net Declaration Delay

wire #10 out;

assign out = in1 & in2; (equals the following) wire out;

assign #10 out = in1 & in2;

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Dataflow Modeling

Continuous Assignments Delays Expressions, Operators and Operands Operator Types Examples Summary

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Expressions

Combine operator and operand to output a result a^b addr1[20:17] + addr2[20:17] in1 | in2

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Operands

Data type － constants, integers, real, nets, registers, times, bit-select, part-select, memory or function calls

Integer count, final_count; final_count = count + 1; real a, b, c; c = a – b; reg [15:0] reg1, reg2; reg [3:0] reg_out; reg_out = reg1[3:0] ^ reg2[3:0]; reg ret_value; ret_value = calculate_parity(A, B);

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Operators

Perform an operation on operands d1 && d2 // && operates on operands d1 and d2 !a[0] B1>>1

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Dataflow Modeling

Continuous Assignments Delays Expressions, Operators and Operands Operator Types Examples Summary

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Operator Classes

Arithmetic Logical Relational Equality Bitwise Reduction Shift Concatenation Conditional

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Operation Types - I

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Operator Types - II

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Arithmetic Operators - I

Binary Operator (+, -, *, /, **, %) A = 4’b0011; B = 4’b0100;

D = 6; E = 4;

A * B

D / E

A + B

B – A

F = E ** F;

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Arithmetic Operators - II

Binary Operator (+, -, *, /, **, %) in1 = 4’b101x; in2 = 4’b1010; sum = in1 + in2; // sum is 4’bx 13 % 3 16 % 4 -7 % 2 7 % -2

Unary Operator (+, -) -4 +5

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Logical Operators

&&(logic-and), ||(logic-or), !(logic-not) A = 3; B = 0;

A && B

A || B

!A

!B A = 2’0x; B = 2’b10;

A && B ( a == 2) && (b == 3)

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Relational Operators

>, <, <=, >= A = 4, B = 3

X = 4’b1010, Y = 4’b1101, Z = 4’b1xxx

A <= B

A > B

Y >= X

Y < Z

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Equality Operators - I

Logic Equality (==, !=) Event Equality (===, !==)

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Equality Operators - II

A = 4, B = 3 X = 4’b1010, Y = 4’b1101 Z = 4’b1xxz, M = 4’b1xxz, N = 4’b1xxx A == B // 0 X != Y // 1 X == Z // x Z === M // 1 Z === N // 0 M !=== N // 1

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Bitwise Operators - I

~(Negation), & (and), | (or), ^ (xor), ^~ (xnor)

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Bitwise Operators - II

X = 4’b1010, Y = 4’b1101, Z = 4’b10x1 ~X // 4’b0101 X & Y // 4’b1000 X | Y // 4’b1111 X ^ Y // 4’b0111 X ^~ Y // 4’b1000 X & Z // 4’b10x0 X = 4’b1010, Y = 4’b0000 X | Y // 4’b1010 X || Y // 1

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Reduction Operator

&, ~&, |, ~|, ^, ~^ X = 4’b1010

&X // 1’b0

|X // 1’b1

^X // 1’b0, can be used to count even parity

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Shift Operator

>>(right shift), <<(left shift), >>>(arithmetic right shift), <<<

X = 4’b1100 Y = X >> 1; // 4’b0110 Y = X << 1; // 4’b1000 Y = X << 2; // 4’b0000 Integer a, b, c; a = 0; b = -10; c = a + (b >>> 3);

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Concatenation Operator

{, } A = 1’b1, B = 2’b00, C = 2’b10, D = 3’b110

Y = { B, C }

Y = { A, B, C, D, 3’b001 }

Y = { A, B[0], C[1] }

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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}}

Y = {4{A}, 2{B}}

Y = {4{A}, 2{B}, C}

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Condition_expr ? ture_expr : false_expr;

assign addr_bus = drive_enable ? Addr_out : 36’bz; assign out = control ? in1 : in0; Assign out = ( A == 3 ) ? ( control ? x : y ) : ( control ? m : n );

Conditional Operator

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Operator Precedence

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Dataflow Modeling

Continuous Assignments Delays Expressions, Operators and Operands Operator Types Examples Summary

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Design 4-to-1 Multiplexer

Using logic expression Using conditional operator

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Using Logic Expression

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Using Conditional Operator

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Design 4-bit Full Adder

Using addition (+) and concatenation ({, }) Carry look ahead

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Using Addition and Concatenation Operator (DataFlow Modeling)

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Carry Look Ahead Full Adder - I

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Carry Look Ahead Full Adder - II

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4-bit ripple carry counter

4-bit Ripple Carry Counter

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Negative Triggered D Flip-Flop with Clear

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4-bit Ripple Carry Counter in Verilog

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T Flip-Flop in Verilog

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D Flip-Flop in Verilog

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Testbench for 4-bit Ripple Counter - I

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Testbench for 4-bit Ripple Counter - I

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Simulation Result

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Dataflow Modeling

Continuous Assignments Delays Expressions, Operators and Operands Operator Types Examples Summary

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Summary

Continuous Assignment (expression, operator and operand)

Define delays in continuous assignment Various operators in Verilog

Arithmetic, logical, relational, equality, bitwise, reduction, shift, concatenation, replication, conditional

Conditional operator is equivalent to “if-then-else” statement