有时候，我们需要检查几个时钟周期才能完成的事务。在SVA中，可以用“##”表示时钟周期延迟，如“ a ##2 b”即当a为高电平时，2个时钟周期之后b应为高电平。下面举个简单的例子来说明：

测试代码：

/*******************************************************
作者   :  CrazyBird
文件   :  assert_test2.sv
日期   :  2015-5-7
功能   :  时钟周期延时“##”的测试
********************************************************/
`timescale 1ns/1ps
module  assert_test2(
    output reg  clk,
    output reg  a,
    output reg  b
    );
    
    //  产生时钟
    parameter   PERIOD = 10;
    
    initial
    begin
        clk = 0;
        forever #(PERIOD/2)
            clk = ~clk;  
    end
    
    //  产生激励
    initial
    begin
        a = 1;
        b = 1;
        repeat(20)@(negedge clk)
        begin
            a = {$random()}%2;
            b = {$random()}%2;
        end
        @(negedge clk);
        $stop;
    end
    
    //建立序列
    sequence  s1;
        a ##2 b;
    endsequence
    
    //  建立属性
    property  p1;
        @(posedge clk) s1;
    endproperty

    //  断言属性
    a1 : assert property(p1);

endmodule

    按照代码的描述，其任务是检查信号a在给定的时钟上升沿是否为高电平。若a为低电平，断言直接失败；若a为高电平，则延迟2个时钟周期后检查b是否为高电平。若b为高电平，表示断言成功；若b为低电平，表示断言失败。

    其仿真结果如下所示：

# ** Error: Assertion error.
#    Time: 15 ns Started: 15 ns  Scope: assert_test2.a1 File: D:/electron/modelsim/assert_test2/assert_test2.sv Line: 49
# ** Error: Assertion error.
#    Time: 45 ns Started: 25 ns  Scope: assert_test2.a1 File: D:/electron/modelsim/assert_test2/assert_test2.sv Line: 49
# ** Error: Assertion error.
#    Time: 65 ns Started: 65 ns  Scope: assert_test2.a1 File: D:/electron/modelsim/assert_test2/assert_test2.sv Line: 49
# ** Error: Assertion error.
#    Time: 75 ns Started: 55 ns  Scope: assert_test2.a1 File: D:/electron/modelsim/assert_test2/assert_test2.sv Line: 49
# ** Error: Assertion error.
#    Time: 95 ns Started: 75 ns  Scope: assert_test2.a1 File: D:/electron/modelsim/assert_test2/assert_test2.sv Line: 49
# ** Error: Assertion error.
#    Time: 115 ns Started: 115 ns  Scope: assert_test2.a1 File: D:/electron/modelsim/assert_test2/assert_test2.sv Line: 49
# ** Error: Assertion error.
#    Time: 125 ns Started: 125 ns  Scope: assert_test2.a1 File: D:/electron/modelsim/assert_test2/assert_test2.sv Line: 49
# ** Error: Assertion error.
#    Time: 125 ns Started: 105 ns  Scope: assert_test2.a1 File: D:/electron/modelsim/assert_test2/assert_test2.sv Line: 49
# ** Error: Assertion error.
#    Time: 135 ns Started: 135 ns  Scope: assert_test2.a1 File: D:/electron/modelsim/assert_test2/assert_test2.sv Line: 49
# ** Error: Assertion error.
#    Time: 145 ns Started: 145 ns  Scope: assert_test2.a1 File: D:/electron/modelsim/assert_test2/assert_test2.sv Line: 49
# ** Error: Assertion error.
#    Time: 175 ns Started: 175 ns  Scope: assert_test2.a1 File: D:/electron/modelsim/assert_test2/assert_test2.sv Line: 49
# ** Error: Assertion error.
#    Time: 175 ns Started: 155 ns  Scope: assert_test2.a1 File: D:/electron/modelsim/assert_test2/assert_test2.sv Line: 49
# ** Error: Assertion error.
#    Time: 185 ns Started: 185 ns  Scope: assert_test2.a1 File: D:/electron/modelsim/assert_test2/assert_test2.sv Line: 49
# ** Error: Assertion error.
#    Time: 195 ns Started: 195 ns  Scope: assert_test2.a1 File: D:/electron/modelsim/assert_test2/assert_test2.sv Line: 49

待续~~~