The Universal Verification Methodology (UVM) provides powerful field utility macros for accessing and manipulating data within register models. While these macros simplify verification, situations arise where you need to selectively disable access to individual fields. This article explores techniques for disabling single fields within UVM field utility macros, addressing common challenges and providing practical solutions.
Why Disable Single Fields?
Disabling a single field within a UVM register model might be necessary for several reasons:
- Simplified Verification: During specific test scenarios, you might want to ignore certain fields to focus on others, simplifying debugging and improving test efficiency. For instance, you may only be interested in verifying the functionality of a subset of the register's fields.
- Conditional Access: The need to disable a field might be driven by the state of the system or specific test conditions. This allows for more flexible and robust verification environments.
- Fault Injection: Disabling a field can be part of a targeted fault injection strategy, mimicking potential hardware failures for comprehensive verification.
- Abstraction: In some cases, disabling a field can offer a higher level of abstraction, hiding implementation details not relevant to a particular testbench.
Methods for Disabling Single Fields
Several methods exist for selectively disabling access to individual fields using UVM field utility macros:
1. Conditional Logic within the Access Methods
The simplest approach involves using conditional logic within the get/set methods of your UVM register model. This lets you conditionally enable or disable access based on predefined conditions or flags.
class my_register extends uvm_reg;
rand bit [7:0] field_a;
rand bit [15:0] field_b;
bit disable_field_b;
function new(string name = "my_register");
super.new(name);
field_a = new();
field_b = new();
disable_field_b = 0; // Initialize to enabled
endfunction
virtual function void set_field_b(input bit [15:0] value);
if (!disable_field_b)
field_b = value;
else
$display("Field 'field_b' is disabled!");
endfunction
virtual function bit [15:0] get_field_b();
if (!disable_field_b)
return field_b;
else
$display("Field 'field_b' is disabled!");
return '0; // Return a default value
endfunction
// ... other fields and methods ...
endclass
This example disables field_b access based on the disable_field_b flag. Remember to handle the disabled case appropriately, perhaps by logging a message or returning a default value.
2. Using a Wrapper Class
A more structured approach involves creating a wrapper class that encapsulates the register and provides controlled access to its fields. This offers better encapsulation and maintainability.
class my_register_wrapper;
my_register reg_inst;
bit disable_field_b;
function new(my_register reg);
reg_inst = reg;
disable_field_b = 0;
endfunction
function bit [15:0] get_field_b();
if (!disable_field_b)
return reg_inst.field_b;
else
$display("Field 'field_b' is disabled!");
return '0;
endfunction
// ... similar for set_field_b and other fields ...
endclass
The my_register_wrapper class controls access to my_register fields, providing a single point to manage field disabling.
3. Modifying the Register Model Directly (Advanced)
For more complex scenarios, you might directly modify the UVM register model's definition. This involves modifying the field's access attributes or using more advanced techniques like virtual interfaces. However, this is generally less preferred as it alters the core register model structure. This method requires a thorough understanding of UVM and should be applied carefully.
Handling Disabled Fields
Regardless of your chosen method, consider how to handle the situation when a field is disabled. Logging a message indicating the disabled state is essential for debugging and tracing. Returning a default value might be suitable in some cases. The appropriate approach depends on the context and desired behavior.
Choosing the Right Method
The best approach for disabling single fields depends on the complexity of your verification environment and the specific requirements. For simple cases, conditional logic within the access methods is often sufficient. For larger, more complex register models, a wrapper class offers better encapsulation and maintainability. Modifying the register model directly should be a last resort, considered only after exhausting simpler alternatives. Always prioritize clear, well-documented code to improve readability and ease of maintenance.
