UVM中，component的task phase是消耗仿真时间的，各个components的task phase之间需要完成同步。只有在所有components的相同task phase结束之后，才能进入下一个task phase。
UVM中通过objection mechanism中的raise/drop objection来控制phase的开始和停止。
当仿真器进入某个消耗时间的task phase时，首先会收集所有component提起（raise）的objection,然后按顺序执行所有component各自的task phase，直到所有component提起的objection都被撤销（drop）该phase的执行才算结束。
//driver中提起objection
task driver::main_phase(uvm_phase phase);phase.raise_objection(this); .....phase.drop_objection(this);endtask//monitor中提起objection
task monitor::main_phase(uvm_phase phase); phase.raise_objection(this); .....phase.drop_objection(this);endtask例如在上述代码中，仿真器收集driver和monitor各自提起的objection，按顺序执行省略代码，直到driver和monitor都撤销objection才算main_phase执行结束。
如果某个component的相应task phase没有被提起，则永远也不会执行消耗仿真时间的语句。同样，如果才task phase被提起后不被撤销，则该phase永远也不会结束。
在前面的代码中，main phase具有一个uvm_phase参数，这就是为具体实现raise/drop objection而设置的，这样每个component才能通过phase.raise_objection来提起相应的objection
UVM中的run_phase和其他task phase是并行关系，如果其他task phase中的objection被提起，run_phase也会执行，直到所有objection被撤销，run_phase才会结束。
下面通过几个例子展示objection的用法：
示例1：不加raise objection
class my_driver extends uvm_driver;
`uvm_component_utils(my_driver) function new(string name = "my_driver", uvm_component parent =null); super.new(name, parent); `uvm_info("my_driver", "new is called", UVM_LOW); endfunction extern virtual task main_phase(uvm_phase phase);endclass
task my_driver::main_phase(uvm_phasephase); `uvm_info("my_driver", "main_phase is called",UVM_LOW); @(posedge top_tb.clk); `uvm_info("my_driver", "data is drived", UVM_LOW);endtask输出：
UVM_INFO my_driver.sv(8) @ 0: uvm_test_top[my_driver] new is calledUVM_INFO my_driver.sv(15) @ 0: uvm_test_top[my_driver] main_phase is called没有输出"data is drived"
示例2：raise objection没有加在仿真时间0时刻
class my_driver extends uvm_driver;
`uvm_component_utils(my_driver) function new(string name = "my_driver", uvm_component parent =null); super.new(name, parent); `uvm_info("my_driver", "new is called", UVM_LOW); endfunction extern virtual task main_phase(uvm_phase phase);endclass
task my_driver::main_phase(uvm_phasephase); @(posedge top_tb.clk); phase.raise_objection(this); `uvm_info("my_driver", "main_phase is called",UVM_LOW); @(posedge top_tb.clk); `uvm_info("my_driver", "data is drived", UVM_LOW); phase.drop_objection(this);endtask输出：
UVM_INFO my_driver.sv(6) @ 0: uvm_test_top[my_driver] new is called没有输出 "main_phase is called"和"data is drived"
示例3：不加drop objection
class my_driver extends uvm_driver; `uvm_component_utils(my_driver) function new(string name = "my_driver", uvm_component parent =null); super.new(name, parent); `uvm_info("my_driver", "new is called", UVM_LOW); endfunction extern virtual task main_phase(uvm_phase phase);endclass
task my_driver::main_phase(uvm_phasephase); phase.raise_objection(this); `uvm_info("my_driver", "main_phase is called",UVM_LOW); while(1) begin @(posedge top_tb.clk); `uvm_info("my_driver", "data is drived", UVM_LOW); //phase.drop_objection(this); endendtask输出：
UVM_INFO my_driver.sv(8) @ 0: uvm_test_top[my_driver] new is calledUVM_INFO my_driver.sv(15) @ 0: uvm_test_top[my_driver] main_phase is called
UVM_INFO my_driver.sv(16) @ 125840500000:uvm_test_top [my_driver] data is drivedUVM_INFO my_driver.sv(16) @ 125840700000:uvm_test_top [my_driver] data is drivedUVM_INFO my_driver.sv(16) @ 125840900000:uvm_test_top [my_driver] data is drived…UVM_INFO my_driver.sv(16) @ 125841100000:uvm_test_top [my_driver] data is drivedUVM_INFO my_driver.sv(16) @ 125841300000:uvm_test_top [my_driver] data is drivedUVM_INFO my_driver.sv(16) @ 125841500000:uvm_test_top [my_driver] data is drivedUVM_INFO my_driver.sv(16) @ 125841700000:uvm_test_top [my_driver] data is drived陷入死循环
示例4：增加drop objection
class my_driver extends uvm_driver;
`uvm_component_utils(my_driver) function new(string name = "my_driver", uvm_component parent =null); super.new(name, parent); `uvm_info("my_driver", "new is called", UVM_LOW); endfunction extern virtual task main_phase(uvm_phase phase);endclass
task my_driver::main_phase(uvm_phasephase); phase.raise_objection(this); `uvm_info("my_driver", "main_phase is called",UVM_LOW); while(1) begin @(posedge top_tb.clk); `uvm_info("my_driver", "data is drived", UVM_LOW); phase.drop_objection(this); endendtask输出：
UVM_INFO my_driver.sv(6) @ 0: uvm_test_top[my_driver] new is called
UVM_INFO my_driver.sv(13) @ 0: uvm_test_top[my_driver] main_phase is calledUVM_INFO my_driver.sv(16) @ 100000:uvm_test_top [my_driver] data is drived正常输出
从上面几个示例可以看出，要想正确地实现对设计的仿真，必须合理地控制objection的提起和撤销，并且raise objection要发生在消耗时间的仿真语句之前。
