Hi RVG,
With the START pin hight the ADS1147 will convert continually and the DRDY transition from high to low will signify that new results are available and are ready to be read out of the device. If you never read the results from the device you will see DRDY pulse at the configured data rate. The DRDY pulse width will be a minimum of 3 tosc periods as shown in Figure 2 of the datasheet. This transition is very difficult to capture using an input polling method. It is best to use an interrupt driven method which triggers the interrupt from a high to low transition.
If the START pin is low, the device will be in a powered down state. If a the device was in the middle of a conversion period when the START pin is brought low, the conversion period will end and the results updated to the results register, DRDY will pulse (if already DRDY is low) or will transition from high to low (if communication to the device took place following the previous posting of results). After that, the device will go to sleep until the START pin is brought back high.
Best regards,
Bob B