Tài liệu Daflow Modeling part 2 pdf

6.3 Expressions, Operators, and Operands Dataflow modeling describes the design in terms of expressions instead of primitive gates.. Expressions, operators, and operands form the basis o

6.3 Expressions, Operators, and Operands

Dataflow modeling describes the design in terms of expressions instead of primitive gates Expressions, operators, and operands form the basis of

dataflow modeling

6.3.1 Expressions

Expressions are constructs that combine operators and operands to produce a result

// Examples of expressions Combines operands and operators

a ^ b

addr1[20:17] + addr2[20:17]

in1 | in2

6.3.2 Operands

Operands can be any one of the data types defined in Section 3.2, Data

Types Some constructs will take only certain types of operands Operands can be constants, integers, real numbers, nets, registers, times, bit-select (one bit of vector net or a vector register), part-select (selected bits of the vector net or register vector), and memories or function calls (functions are

discussed later)

integer count, final_count;

final_count = count + 1;//count is an integer operand

real a, b, c;

c = a - b; //a and b are real operands

reg [15:0] reg1, reg2;

reg [3:0] reg_out;

reg_out = reg1[3:0] ^ reg2[3:0];//reg1[3:0] and reg2[3:0] are

//part-select register operands

ret_value = calculate_parity(A, B);//calculate_parity is a

//function type operand

6.3.3 Operators

Operators act on the operands to produce desired results Verilog provides

various types of operators Operator types are discussed in detail in Section 6.4, Operator Types

d1 && d2 // && is an operator on operands d1 and d2

!a[0] // ! is an operator on operand a[0]

B >> 1 // >> is an operator on operands B and 1

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6.7 Exercises

1: A full subtractor has three 1-bit inputs x, y, and z (previous borrow)

and two 1-bit outputs D (difference) and B (borrow) The logic

equations for D and B are as follows:

D = x'.y'.z + x'.y.z' + x.y'.z' + x.y.z

B = x'.y + x'.z +y.z

Write the full Verilog description for the full subtractor module,

including I/O ports (Remember that + in logic equations corresponds

to a logical or operator (||) in dataflow) Instantiate the subtractor

inside a stimulus block and test all eight possible combinations of x,

y, and z given in the following truth table

x y z B D

0 0 1 1 1

0 1 0 1 1

0 1 1 1 0

1 0 0 0 1

1 0 1 0 0

1 1 0 0 0

1 1 1 1 1

2: A magnitude comparator checks if one number is greater than or

equal to or less than another number A 4-bit magnitude comparator takes two 4-bit numbers, A and B, as input We write the bits in A and B as follows The leftmost bit is the most significant bit

A = A(3) A(2) A(1) A(0)

B = B(3) B(2) B(1) B(0)

The magnitude can be compared by comparing the numbers bit by bit, starting with the most significant bit If any bit mismatches, the number with bit 0 is the lower number To realize this functionality in logic equations, let us define an intermediate variable Notice that the function below is an xnor function

x(i) = A(i).B(i) + A(i)'.B(i)'

The three outputs of the magnitude comparator are A_gt_B, A_lt_B, A_eq_B They are defined with the following logic equations:

A_gt_B = A(3).B(3)' + x(3).A(2).B(2)' + x(3).x(2).A(1).B(1)' +

x(3).x(2).x(1).A(0).B(0)'

A_lt_B = A(3)'.B(3) + x(3).A(2)'.B(2) + x(3).x(2).A(1)'.B(1) +

A_eq_B = x(3).x(2).x(1).x(0)

Write the Verilog description of the module magnitude_comparator Instantiate the magnitude comparator inside the stimulus module and try out a few combinations of A and B

3: A synchronous counter can be designed by using master-slave JK

flipflops Design a 4-bit synchronous counter Circuit diagrams for the synchronous counter and the JK flipflop are given below The clear signal is active low Data gets latched on the positive edge of clock, and the output of the flipflop appears on the negative edge of clock Counting is disabled when count_enable signal is low Write the dataflow description for the synchronous counter Write a

stimulus file that exercises clear and count_enable Display the

output count Q[3:0]

Figure 6-5 Master-Slave JK-flipflop

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