SDRAM控制问题请教：为何用的器件模型读数据的时候是高阻Z

为何在写的过程 列地址的第一位在最后给写成zzzz了?
见下面的记录
列地址最大255
.....
SDRAM_TEST_tb.mt48lc4m16a2 : at time  215250.0 ns WRITE: Bank = 0 Row =    0, Col = 250, Data = 5959
SDRAM_TEST_tb.mt48lc4m16a2 : at time  215270.0 ns WRITE: Bank = 0 Row =    0, Col = 251, Data = 6868
SDRAM_TEST_tb.mt48lc4m16a2 : at time  215290.0 ns WRITE: Bank = 0 Row =    0, Col = 252, Data = 7777
SDRAM_TEST_tb.mt48lc4m16a2 : at time  215310.0 ns WRITE: Bank = 0 Row =    0, Col = 253, Data = 8686
SDRAM_TEST_tb.mt48lc4m16a2 : at time  215330.0 ns WRITE: Bank = 0 Row =    0, Col = 254, Data = 9595
SDRAM_TEST_tb.mt48lc4m16a2 : at time  215350.0 ns WRITE: Bank = 0 Row =    0, Col = 255, Data = 3434
SDRAM_TEST_tb.mt48lc4m16a2 : at time  215370.0 ns WRITE: Bank = 0 Row =    0, Col =   0, Data = zzzz
SDRAM_TEST_tb.mt48lc4m16a2 : at time  220915.0 ns READ : Bank = 0 Row =    0, Col =   0, Data = zzzz
SDRAM_TEST_tb.mt48lc4m16a2 : at time  220935.0 ns READ : Bank = 0 Row =    0, Col =   1, Data = 8282
SDRAM_TEST_tb.mt48lc4m16a2 : at time  220955.0 ns READ : Bank = 0 Row =    0, Col =   2, Data = 9191
SDRAM_TEST_tb.mt48lc4m16a2 : at time  220975.0 ns READ : Bank = 0 Row =    0, Col =   3, Data = 3030
SDRAM_TEST_tb.mt48lc4m16a2 : at time  220995.0 ns READ : Bank = 0 Row =    0, Col =   4, Data = 4f4f
SDRAM_TEST_tb.mt48lc4m16a2 : at time  221015.0 ns READ : Bank = 0 Row =    0, Col =   5, Data = 5e5e
SDRAM_TEST_tb.mt48lc4m16a2 : at time  221035.0 ns READ : Bank = 0 Row =    0, Col =   6, Data = 6d6d

发现在仿真出来的波形在写与读的开始位置出有点不对劲
见附图
那位用过类似器件模型仿真的说说可能的缘故

仔细看看器件模型Spec，上面应该会有使用说明的