Welcome back to VLSI 360, your trusted resource for VLSI design, verification, and SystemVerilog tutorials! Today, we’re tackling a fascinating problem: generating two infinite repeating sequences, 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 2 0 3 0 4 0 5 ... and 0 0 1 0 0 2 0 0 3 0 0 4 … 0 0 9 0 0 1 0 0 2 …, using SystemVerilog constraints in dynamic arrays. This exercise is ideal for mastering randomization, constraints, and array manipulation, skills commonly tested in VLSI verification interviews or academic projects. We’ll explore the provided working code, explain its logic, and share tips to help you excel. Let’s dive in!
Question: Write SystemVerilog Constraints to Generate Sequences 0 1 0 2 0...0 8 0 9 0 1 0 2... and 0 0 1 0 0 2 0 0 3 0 0 4 …0 0 9 0 0 1 0 0 2…
We need to generate two specific sequences in dynamic arrays using SystemVerilog constraints, representing patterns that repeat infinitely:
Sequence for array a: 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 2 0 3 0 4 0 5 ...
- Pattern: Alternates between 0 and a number from 1 to 9, cycling back to 1 after 9, implying an infinite sequence.
- Example: 0 (index 0), 1 (index 1), 0 (index 2), 2 (index 3), ..., 0 9 0 1 ....
Sequence for array b: 0 0 1 0 0 2 0 0 3 0 0 4 … 0 0 9 0 0 1 0 0 2 …
- Pattern: Every third element is a number from 1 to 9, cycling back to 1 after 9, with 0 in between, implying an infinite sequence.
- Example: 0 (index 0), 0 (index 1), 1 (index 2), 0 (index 3), 0 (index 4), 2 (index 5), ...
Since the sequences are infinite, we’ll implement them in finite arrays of 60 elements to capture the repeating pattern, as specified. The provided code is verified to work, so let’s analyze it.
SystemVerilog Constraints for Sequences 0 1 0 2 ... and 0 0 1 ...
Below is the working SystemVerilog code to generate the specified sequences using a class-based approach with dynamic arrays and constraints.
class sample;
rand int a[]; // Dynamic array for sequence 0 1 0 2 0 3 ...
rand int b[]; // Dynamic array for sequence 0 0 1 0 0 2 ...
constraint unique_vals {
a.size inside {60}; // Array a has 60 elements
b.size inside {60}; // Array b has 60 elements
foreach (a[i]) {
if (i % 2 == 0) a[i] == 0; // Even indices are 0
else a[i] == (((1+i)/2)-1) % 9 + 1; // Odd indices are 1, 2, ..., 9 (repeating)
}
foreach (b[i]) {
if (i % 3 == 2) b[i] == ((i/3) % 9) + 1; // Every third element is 1, 2, ..., 9 (repeating)
else b[i] == 0; // Other indices are 0
}
}
endclass
module top;
sample s;
initial begin
s = new();
assert (s.randomize());
$display("arr[a]=:%p", s.a);
$display("arr[b]=:%p", s.b);
$display("******************************************");
end
endmoduleHow It Works
Let’s break down the code to understand how it generates the sequences 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 2 0 3 0 4 0 5 ... and 0 0 1 0 0 2 0 0 3 0 0 4 … 0 0 9 0 0 1 0 0 2 ….
Class Definition (sample):
- The sample class encapsulates the logic for both sequences.
- Dynamic Arrays: rand int a[] and rand int b[] are 1D dynamic arrays to store the sequences. The rand keyword enables randomization with constraints.
Size Constraint (unique_vals):
- a.size inside {60}: Ensures array a has exactly 60 elements, capturing multiple cycles of the pattern.
- b.size inside {60}: Ensures array b has exactly 60 elements, sufficient to show the repeating nature.
- The size of 60 is chosen to demonstrate the infinite repetition (e.g., 0 1 0 2 ... 0 9 0 1 cycles every 18 elements in a).
Value Constraint for Array a:
- foreach (a[i]): Iterates over each element in array a.
- if (i % 2 == 0) a[i] == 0: Sets even-indexed elements (0, 2, 4, ...) to 0.
- else a[i] == (((1+i)/2)-1) % 9 + 1: For odd indices (1, 3, 5, ...), generates values 1, 2, ..., 9 (repeating).
- Formula Explanation: For i = 1, ((1+1)/2)-1 = 0, 0 % 9 + 1 = 1; for i = 3, ((1+3)/2)-1 = 1, 1 % 9 + 1 = 2; ..., for i = 19, ((1+19)/2)-1 = 9, 9 % 9 + 1 = 1. The modulo % 9 ensures the sequence cycles from 1 to 9, mimicking the infinite pattern.
Value Constraint for Array b:
- foreach (b[i]): Iterates over each element in array b.
- if (i % 3 == 2) b[i] == ((i/3) % 9) + 1: Sets every third element (indices 2, 5, 8, ...) to 1, 2, ..., 9 (repeating).
- Formula Explanation: For i = 2, (2/3) = 0, 0 % 9 + 1 = 1; for i = 5, (5/3) = 1, 1 % 9 + 1 = 2; ..., for i = 29, (29/3) = 9, 9 % 9 + 1 = 1. The modulo % 9 ensures cycling from 1 to 9.
- else b[i] == 0: Sets other indices (0, 1, 3, 4, 6, 7, ...) to 0, matching the infinite sequence structure.
Top Module (top):
- Instantiates the sample class.
- Calls randomize() to apply the constraints and generate the sequences.
- Displays arrays a and b using $display.
Output
Running the code produces
arr[a]=:{0,1,0,2,0,3,0,4,0,5,0,6,0,7,0,8,0,9,0,1,0,2,0,3,0,4,0,5,...}
arr[b]=:{0,0,1,0,0,2,0,0,3,0,0,4,0,0,5,0,0,6,0,0,7,0,0,8,0,0,9,0,0,1,...}
******************************************This output matches the required sequences: 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 2 0 3 0 4 0 5 ... for array a and 0 0 1 0 0 2 0 0 3 0 0 4 … 0 0 9 0 0 1 0 0 2 … for array b, with 60 elements each, capturing the infinite repeating nature.
Tips for Interviews
This problem is an excellent fit for VLSI verification interviews, as it tests:
- Constraint writing for periodic, infinite-like sequence patterns.
- Dynamic array manipulation.
- Mathematical logic for sequence generation using modulo arithmetic.
- Randomization techniques in SystemVerilog.
Potential follow-up questions include:
- Can you generate these sequences using loops instead of constraints?
- How would you verify the correctness of the sequences programmatically?
- Can you modify the pattern to cycle through a different range (e.g., 1 to 5 instead of 1 to 9)?
- How would you handle larger arrays to show more cycles of the infinite sequence?
Conclusion
Generating infinite-repeating sequences like 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 2 0 3 0 4 0 5 ... and 0 0 1 0 0 2 0 0 3 0 0 4 … 0 0 9 0 0 1 0 0 2 … using SystemVerilog constraints is a powerful way to practice randomization, dynamic arrays, and class-based programming. The provided code is verified to work and is ideal for VLSI verification interviews or academic projects. Experiment with it by adjusting the array size or adding verification checks to deepen your skills.
Keep following VLSI 360 for more SystemVerilog tutorials and VLSI insights! Drop a comment if you have questions or want another topic covered. Happy coding!
0 Comments