Clock generation in testbench environment

Clock generation is the key of any design and verification environment. There are many ways to generate clock, some of them are listed below:

Method #1 Using always block and negation operator for 50% duty cycle
initial begin
clk = 0;
end
always
  begin
#5 clk = ~clk;
   end

//----------------------------------------------------------------------------//
Method #2 Using always block, negation operator and parameter for 50% duty cycle
 parameter simulation_cycle = 10;
initial begin
clk = 0;
end
always begin
#(simulation_cycle/2) clk = ~clk;
end

//----------------------------------------------------------------------------//
Method #3 Using forever block and  negation operator for 50% duty cycle
initial begin
clk = 0;
forever begin
#5 clk = ~clk;
end
end

//----------------------------------------------------------------------------//
Method #4 Using forever block, negation operator and parameter for 50% duty cycle
    parameter simulation_cycle = 10;
initial begin
clk = 0;
forever begin
#(simulation_cycle/2) clk = ~clk;
end
end

//------------------------------------------------------------------------------------//
Method #5 using always block, directed low time and high time value for 50% duty cycle
initial begin
clk = 0;
end
always begin
#5 clk = 0;
#5 clk = 1;
end

//------------------------------------------------------------------------------------//
Method #6 using always block, directed low time and high time value and parameter for 50% duty cycle
    parameter simulation_cycle = 10;
initial begin
clk = 0;
end
always begin
#(simulation_cycle/2) clk = 0;
#(simulation_cycle/2) clk = 1;
end

//------------------------------------------------------------------------------------//
 There are different other ways also to generate the clock, but commonly these methods are used.

difference between uvm_resource_db & uvm_config_db

From the usage perspective there is no difference between these two and recommendation is to use uvm_config_db

Both uvm_config_db and uvm_resource_db share the same underlying database to store and retrieve information.

In-fact you can write a value to uvm database using uvm_config_db ::set() method and retrieve the information either using  uvm_config_db ::get() method or using uvm_resource_db::read_by_name().  

The recommended method is to use uvm_config_db when hierarchical 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 ");