Generic Payload in UVM !!!

Generic payload is the default transport vehicle for TLM2  blocking and non blocking transport interface. Generic payload is a transaction class derived from uvm_sequence_item which enables it to be generated in sequences and transported to drivers through sequencers. Generic payload can be used for any memory mapped bus based system. Generic payload has property like m_address , m_command, m_byte_enables etc. Generic payload class has access methods for each of the property which is virtual enabling it to be used in the class that extends generic payload class.

It would be good idea to explore the generic payload transaction class before writing your own transaction class for memory mapped bus interface.

Base class library specific to product on top of RVM/VMM/OVM/UVM !!!

RVM/VMM/OVM/UVM provides rich set of features that can be used to develop a sophisticated test bench. Extending your Driven/monitor/scoreboard/sequence directly from the methodology may not be an ideal solution with respect to product specific requirement. The ideal way is have a set of base class extending from the methodology class which adds specific requirement which will be product line specific. Having company/product specific base class helps in better reuse and isolates the users from the changes introduced  due to new releases in the methodology. When ever specific base class is changed it can be qualified with the supported version of tool,methodology and VIP. This approach also helps to incrementally move towards the latest releases of the methodology ensuring current development effort is not stalled.

Layering sequences in UVM !!!

We come across different types of layering requirement for different  protocols. Basic ones are as follows.

1. One to one mapping  --  One higher level protocol frame is mapped to the payload of one lower level protocol frame.
2. One to many mapping -- One higher level protocol frame is mapped to the payload of many lower level protocol frames.
3. Many to one mapping  -- Many higher  level protocol frames are mapped to the payload of single lower level protocol frame.
4. Many to many mapping  -- Many higher level protocol frames are mapped to the payload of many lower level protocol frames. 

What ever be the layering scenario , the basic principle to generate a layering transaction is to randomize the higher level sequence and use byte_pack to convert it to a byte stream and package the bytes in the lower level sequence.

System verilog 2012 features !!!

I recently came across a paper presented at DVcon 2012 which summarized the proposed features of system verilog 2012 standard. The feature that  first grabbed my attention was multiple inheritance  support in system verilog similar to Java. This feature will ease the test bench development effort in future. The current methodology based on system verilog will potentially leverage this feature when it is available. Other feature which is also a good addition is the soft constraints which allow the constraints to be overridden without creating a conflict , currently we have to turn of the conflicting constraints using constraint mode to override a constraint. another interesting addition is the unique constraint to generate unique values across a list of variable or array elements , currently most of the users use custom logic or algorithm to generate unique values.

Callback in UVM !!!

Callbacks are empty virtual methods that are embedded in the user components at strategic  points to allow the user to make customization which allows better reuse.

In UVM this can be achieved in two ways. 

1. Simply add  empty virtual methods in the component  say in a driver  and invoke the virtual methods at appropriate locations , test writer or  the user extends the driver class implements the virtual methods and uses  set instance override to accomplish the functionality.
2. Create a class which extends form uvm_callback class and it  implements the virtual methods.  Use `uvm_do_callbacks() to place the virtual methods at strategic points in the component say a driver. Now the test writer or user extends the callback class and implements the virtual methods and registers or associate the extended callback class with the instance of the component say the driver using add to accomplish the functionality.

Adding user defined phase using uvm_phase !!!

In addition to the predefined phases available in uvm , the user has the option to add his own phase to a component. This is typically done by extending the uvm_phase class the constructor needs to call super.new which has three arguments 

1. name of the phase task or function
2. top down or bottom up phase
3. task or function

The call_task  or call_func and get_type_name need to be implemented to complete the addition of new phase.

Example

class custom_phase extends uvm_phase;

   function new();

      super.new(“custom”,1,1);

   endfunction

   task call_task  ( uvm_component parent);

    

     my_comp_type comp;

      

     if ( $cast(comp,parent) )

             comp.custom_phase();

    

   endtask

   virtual function string get_type_name();

      return “custom”;

   endfunction

endclass

Usage of uvm_resource_db & uvm_config_db !!!

Both uvm_config_db and uvm_resource_db share the same underlying database to store and retrieve information. Infact you can write a value to the database using uvm_config_db ::set() method and retrieve the information using uvm_resource_db::read_by_name().  The recommended method is to use uvm_config_db when hierarchical   based access is required.  When you want to share object and access it from different location without using the hierarchy you can use uvm_resource_db.

You can set a resource to the resource db using the uvm_resource_db::set()  method

Example

uvm_resource_db# (int）::set("enable","*",1,this);

To retrieve the information from the resource db you can use uvm_resource_db::read_by_name() method

Example

   Bit success;

   Success=uvm_resource_db#（int)::read_by_name("enable",get_full_name(),value,this);

   If(success==1’b0)
      `uvm_error("ERROR","cannot locate the resource ");