I am trying to use a Xilinx Evaluation board to configure and read from AD7794 (using EVAL-AD7794). But a strange problem trapped me for a long time.
Firstly, configure the Mode Register, to configure AD7794 at continuous mode, and fADC=470;
Secondly, Configure the Configuration Register, to configure the input at bipolar mode, Gain=4, External Reference1, Channel 0;
Thirdly, configure the IO register, to configure current source Value=210 uA.
Then I read the ID register upon Dout_RDY becomes low, and repeat this process.
Some times, After downloading the FPGA coding, I can read out the ADC data, and it is correct.
But other times, the read out ID data is all 0X000000. A problem occurs that the Dout_RDY pin did not go high on the last read clock. It goes high on some more clock signal later. (As the attached picture shows)
What's the possible reason for this problem?