Hi Xinling,
Yes, the default mode for reading back data is RDATAC. The way you describe reading the data should be fine. The only concern with an SCLK that is too slow (with too much delay between 8-bit bursts) is that you may not finish reading all of your 8-channel data before the next /DRDY.
The MMBO is used to provide both power supplies as well as the SPI communication signals. Can you please confirm that these signals are being routed properly to the ADS1299? Check to make sure that you have JP21 and JP23 configured correctly on the ADS1299 daughterboard and that you are probing /CS, /DOUT, and SCLK on the appropriate J3 pins.
Regards,
