SystemVerilog - Event

2023-06-25
#SystemVerilog
SystemVerilog

Event是靜態物件，用來同步兩個或更多個運行中的process，一個process會觸發Event，其他process等待(wait)這個event。

Event的宣告方式

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Event evt;
Event evt2 = evt;
Event evt_empty = null;

Event的使用方式:

觸發Event: 用 -> 或 ->> 運算元
等待Event: 用 @ 運算元 或 .triggered

Example:

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module tb();
    event evt;

    initial begin
        #10
        ->evt;
        $display("1 t:%0t event triggered", $time);
    end

    initial begin
        $display("2 t:%0t start to polling event", $time);
        @evt;
        $display("2 t:%0t reveived event trigger", $time);
    end

    initial begin
        $display("3 t:%0t start to polling event", $time);
        @evt;
        $display("3 t:%0t reveived event trigger", $time);
    end
endmodule

Output:

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# 2 t:0 start to polling event
# 3 t:0 start to polling event
# 1 t:10 event triggered
# 3 t:10 reveived event trigger
# 2 t:10 reveived event trigger
#  quit -f

@ 和 .triggered的差別

如果同一時間觸發event和等待event的話會有race condition的問題，如果是使用wait(event.triggered)可以避免發生問題

Example:

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module tb();

    event evt;

    initial begin
        #10
        ->evt;
        $display("1 t:%0t event triggered", $time);
    end

    initial begin
        $display("2 t:%0t start to polling event", $time);
        #10
        @evt;
        // wait(evt.triggered);
        $display("2 t:%0t reveived event trigger", $time);
    end

    initial begin
        $display("3 t:%0t start to polling event", $time);
        #10
        // @evt;
        wait(evt.triggered);
        $display("3 t:%0t reveived event trigger", $time);
    end
endmodule

Output:

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# run 1000
# 2 t:0 start to polling event
# 3 t:0 start to polling event
# 1 t:10 event triggered
# 3 t:10 reveived event trigger
#  quit -f

可以看見使用@evt的話，因為同一時間觸發Event和用@等待event，所以第二個initial block沒有收到event