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6.3 Expressions, Operators, and Operands
Dataflow modeling describes the design in terms of expressions instead of
p
rimitive 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
reg ret_value;
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 0 0 0
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) +
x(3).x(2).x(1).A(0)'.B(0)
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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. + 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). 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