In this method of data retrieval, a command is used to read ADC1 or ADC2 data. When the command is sent, the data are retrieved from the respective ADC data-holding register. Read the data at any time because the command method does not require synchronizing to DRDY. In addition to hardware polling of DRDY or DOUT/DRDY to determine when ADC1 data are ready, the ADC can be software polled by reading bits ADC1 and ADC2 of the status byte. If the ADC1 or ADC2 status bit is 1, the ADC1 or ADC2 data are new since the last ADC1 or ADC2 read operation. If data are read again before the new data are ready, the status bit is 0 and the previous data are returned. ADC2 data can only be read by the command method.
Figure 9-44 shows the read data by command sequence. The output data MSB begins on the first rising edge of SCLK after the command. The output data field can be 4, 5, or 6 bytes long, depending on programming. The data field consists of an optional status byte, four bytes of conversion data, and an optional checksum byte. The ADC2 data block consists of the optional status byte, 3 bytes of data, a fixed-value byte equal to 00h (zero pad byte), and the optional checksum byte. A read data command must be sent for each read operation. The ADC does not respond to commands until the read operation is complete, or terminated by taking CS high.