軟件設計中，FSM(Finite-State Machine)分為3部分：狀態（State)，事件（Event)，動作（Action)。

狀態模式(State Pattern)是行為型(Behavioral)設計模式，將軟件主機端的行為歸類為各個狀態，狀態之間可以互相轉化，每種狀態的行為不相同；統一交給一個Context類型的模塊負責調度各個狀態的跳轉；

硬件設計中的FSM，不僅是一種電路的描述工具，而且也是一種思想方法；數字邏輯本質上都可以歸一化為FSM；RTL描述FSM可以歸類為常用的幾種方法，通常采用三段式的描述；

在我們的驗證環境中，有時也需要一個組件專門負責FSM的建模；例如驗證USB Device DUT時，驗證環境需要模擬USB Host的行為；對于USB協議復雜的狀態機，使用專門的FSM組件模擬，可以減少組件間的耦合；也可以將FSM組件的狀態賦值到virtual interface上，通過波形協助debug；

并不是所有DUT模塊中包含FSM，驗證環境中就需要對應的FSM建模；RTL的硬件電路是cycle級的時序電路，采用FSM可以很好的描述算法運算；而驗證環境都是事務級的基于事件的高級抽象模型，是否需要采用FSM根據驗證環境而定；對于簡單的設計，不需要模擬FSM；對于復雜的標準協議，VIP中都會采用FSM建模來完成，具有高內聚低耦合的好處。

Simple example

本篇對一個示例，分別使用兩種方式來描述：

一個簡單的FSM如下，分為4種狀態；對于狀態的跳轉條件，本篇通過uvm_event類型的事件觸發，驗證環境中的實際情況可以是事件，某一個signal狀態，或者if的判斷等；不同狀態下的Action，僅使用一句display代表，驗證環境中的實際情況可以調用某一個task，對signal的驅動，或者調用其他組件的API等；僅做結構上的演示；

通過randsequence產生激勵sequence，遍歷狀態機跳轉條件；

classclient;

state_machineFSM;
uvm_event_poolevents_pool;
uvm_eventto_idle,to_state_a,to_state_b,to_state_c;

functionnew();
events_pool=uvm_event_pool::get_global_pool();
to_idle=events_pool.get("to_idle");
to_state_a=events_pool.get("to_state_a");
to_state_b=events_pool.get("to_state_b");
to_state_c=events_pool.get("to_state_c");
endfunction

taskrand_simulate();
for(inti=0;i<2;i++)?begin
??????bit?FLAG?=?0;
??????randsequence?(stream)

?????????stream?:?first?second?third?last;
?????????first??:?state_a;
?????????second?:?state_b?{FLAG?=?1;}?|?state_c;
?????????third??:?if?(FLAG?==1)?state_c?else?state_b;
?????????last???:?state_idle;

?????????state_idle:?{`INTERVALTIME;to_idle.trigger();};
?????????state_a???:?{`INTERVALTIME;to_state_a.trigger();};
?????????state_b???:?{`INTERVALTIME;to_state_b.trigger();};
?????????state_c???:?{`INTERVALTIME;to_state_c.trigger();};

??????endsequence
??????end
???endtask
?......
?......

use task

類state_machine包含四個狀態的task；通過request_state_change函數實現狀態跳轉；每進入一個狀態，對應一個線程，當跳出狀態時，注意線程需要disable掉；

classstate_machine;

typedefenum{
IDLE,STATE_A,STATE_B,STATE_C
}state_t;

uvm_event_poolevents_pool;
uvm_eventto_idle,to_state_a,to_state_b,to_state_c;

localstate_tcur_state;

externfunctionnew();
externfunctionvoidstart();
externfunctionvoidrequest_state_change(state_tcur_state);
externtaskdo_idle();
externtaskdo_state_a();
externtaskdo_state_b();
externtaskdo_state_c();

endclass

functionstate_machine::new();
events_pool=uvm_event_pool::get_global_pool();
to_idle=events_pool.get("to_idle");
to_state_a=events_pool.get("to_state_a");
to_state_b=events_pool.get("to_state_b");
to_state_c=events_pool.get("to_state_c");
endfunction

functionvoidstate_machine::start();
cur_state=IDLE;
request_state_change(cur_state);
endfunction

functionvoidstate_machine::request_state_change(state_tcur_state);
case(cur_state)
IDLE:begin
fork
begin
$display("Enter%sstate!",cur_state.name());
do_idle();
end
join_none
return;
end
STATE_A:begin
fork
begin
$display("Enter%sstate!",cur_state.name());
do_state_a();
end
join_none
return;
end
STATE_B:begin
fork
begin
$display("Enter%sstate!",cur_state.name());
do_state_b();
end
join_none
return;
end
STATE_C:begin
fork
begin
$display("Enter%sstate!",cur_state.name());
do_state_c();
end
join_none
return;
end
default:begin
$display("Enterunknowstate!");
$finish;
end
endcase
endfunction

taskstate_machine::do_idle();
state_tcur_state;
$display("IDLE:nothingtodo!
");
fork:disable_fork
begin
to_state_a.wait_trigger();
//$display("dosomething!
");
cur_state=STATE_A;
end
join_any
request_state_change(cur_state);
endtask

taskstate_machine::do_state_a();
state_tcur_state;
$display("STATE_A:dosomething!
");
fork:disable_fork
begin
to_state_b.wait_trigger();
//$display("dosomething!
");
cur_state=STATE_B;
end
begin
to_state_c.wait_trigger();
//$display("dosomething!
");
cur_state=STATE_C;
end
join_any
disablefork;
request_state_change(cur_state);
endtask

taskstate_machine::do_state_b();
state_tcur_state;
$display("STATE_B:dosomething!
");
fork:disable_fork
begin
to_state_c.wait_trigger();
//$display("dosomething!
");
cur_state=STATE_C;
end
begin
to_idle.wait_trigger();
//$display("dosomething!
");
cur_state=IDLE;
end
join_any
disablefork;
request_state_change(cur_state);
endtask

taskstate_machine::do_state_c();
state_tcur_state;
$display("STATE_C:dosomething!
");
fork:disable_fork
begin
to_state_b.wait_trigger();
//$display("dosomething!
");
cur_state=STATE_B;
end
begin
to_idle.wait_trigger();
//$display("dosomething!
");
cur_state=IDLE;
end
join_any
disablefork;
request_state_change(cur_state);
endtask

use Sate Pattern

采用狀態模式的設計，每個狀態繼承于virtual class state，實現各自的do_something和request_state_change；state_machine通過宏REGISTER_STATE創建各個state實例；state_machine中forvever執行；狀態模式和策略模式的實現類似，都是使用OOP的組合 + 多態實現；

virtualclassstate;

state_machineFSM;
uvm_event_poolevents_pool;
uvm_eventto_idle,to_state_a,to_state_b,to_state_c;

functionnew();
events_pool=uvm_event_pool::get_global_pool();
to_idle=events_pool.get("to_idle");
to_state_a=events_pool.get("to_state_a");
to_state_b=events_pool.get("to_state_b");
to_state_c=events_pool.get("to_state_c");
endfunction
purevirtualtaskdo_something();
purevirtualtaskrequest_state_change();
endclass

classstate_idleextendsstate;

taskdo_something();
$display("STATE_IDLE:nothingtodo!
");
endtask

taskrequest_state_change();
state_tcur_state;
fork:disable_fork
begin
to_state_a.wait_trigger();
//$display("dosomething!
");
cur_state=STATE_A;
end
join_any
FSM.set_state(cur_state);
endtask
endclass

classstate_aextendsstate;

taskdo_something();
$display("STATE_A:dosomething!
");
endtask

taskrequest_state_change();
state_tcur_state;
fork:disable_fork
begin
to_state_b.wait_trigger();
//$display("dosomething!
");
cur_state=STATE_B;
end
begin
to_state_c.wait_trigger();
//$display("dosomething!
");
cur_state=STATE_C;
end
join_any
disablefork;
FSM.set_state(cur_state);
endtask
endclass

classstate_bextendsstate;

taskdo_something();
$display("STATE_B:dosomething!
");
endtask

taskrequest_state_change();
state_tcur_state;
fork:disable_fork
begin
to_state_c.wait_trigger();
//$display("dosomething!
");
cur_state=STATE_C;
end
begin
to_idle.wait_trigger();
//$display("dosomething!
");
cur_state=STATE_IDLE;
end
join_any
disablefork;
FSM.set_state(cur_state);
endtask
endclass

classstate_cextendsstate;

taskdo_something();
$display("STATE_C:dosomething!
");
endtask

taskrequest_state_change();
state_tcur_state;
fork:disable_fork
begin
to_state_b.wait_trigger();
//$display("dosomething!
");
cur_state=STATE_B;
end
begin
to_idle.wait_trigger();
//$display("dosomething!
");
cur_state=STATE_IDLE;
end
join_any
disablefork;
FSM.set_state(cur_state);
endtask
endclass

classstate_machine;

localstatestate_m;
statestate_pool[state_t];

functionvoidset_state(state_tstate);
$display("Enter%sstate!",state.name());
state_m=state_pool[state];
endfunction


functionvoidfsm_init();
`REGISTER_STATE(IDLE,idle)
`REGISTER_STATE(A,a)
`REGISTER_STATE(B,b)
`REGISTER_STATE(C,c)
this.set_state(STATE_IDLE);
endfunction

taskrun();
fsm_init();
foreverbegin
state_m.do_something();
state_m.request_state_change();
end
endtask
endclass

note

state_machine中還可以加入reset stop函數控制FSM的更多行為；封裝更多API供其他模塊調用；加入assertion做基于cycle的條件判斷；加入covergroup收集功能覆蓋率；

審核編輯：劉清