Question about Audio Codec Usage in DE 2 Board

Hello Everyone,

I had a problem with ADC inside Audio Codec inside DE 2 board. I have power down every other components inside Audio Codec in Register R6 except ADC. But I get something like this in the signal tap. I have attached a screenshot in the attachment. Should I expect that ADCDAT should output "ZERO" if nothing is powered on? I am confused about the output from AUD_ADCDAT.

I attached some code for I2C controller for Audio Codec here:

always @(posedge END) begin
// ROM= 16'h1e00;
 ROM= 16'h0c0b;      //power down
 ROM= 16'h0ec2;      //master
 ROM= 16'h0814;      //sound select -- make it MIC -> ADC -> DAC -> LinOut
 
 ROM= 16'h1000;   //mclk
 
 ROM= 16'h0017;   //
 ROM= 16'h0217;   //
 ROM= {8'h04,1'b0,vol};   //
 ROM= {8'h06,1'b0,vol};      //sound vol
 
 //ROM= 16'h1e00;   //reset 
 ROM= 16'h1201;//active
 DATA_A=ROM;
end

There are just a few changes to Register compared to Altera Provided DE_I2sound sample. Can someone explain why I am having output from AUD_ADCDAT.

Any idea is welcome here.

Thanks

Chen Huang