5509A和AIC23用mic口对噪声进行采集出现的问题

i2c_status = I2C_write( analog_aduio_path_control_MIC,//pointer to data array
2, //length of data to be transmitted
1, //master or slaver
CODEC_ADDR, //slave address to transmit to
1, //transfer mode of operation
30000 //time out for bus busy
);

for(i=0;i<INPUTNUMBER;i++)
{
while(!MCBSP_xrdy(hMcbsp)){};
aic23data = MCBSP_read16(hMcbsp);
for(j=3000;j>0;j--);
while(!MCBSP_rrdy(hMcbsp)){};
xx[i]=aic23data;

以上程序在调试的时候，mic口不接任何麦克风，但是aic23data的值一直在改变，请问是为什么？理论上来说是不变的。