Answer: We share the DOUT and RDY pins to reduce the pin count. Sharing DOUT and RDY has some other benefits also. If isolation is used between the ADC and the uC, having a shared DOUT/RDY line reduces the number of isolation channels needed. To know when a conversion is ready, /CS can be taken low and the DOUT/RDY pin can be monitored. Alternatively, the RDY bit in the status register can be monitored.