During the address phase of any transaction, O8-O1 is output on SDO.

If SDI is continuously held at Low (0), the device can treat this as a Read operation from Address 0. Address 0 holds the state on IN8-IN1, so in this special case of Read operation the SDO output can be IN8-IN1 in both Address and Read Phases. For applications that are only interested in the state of the digital inputs, and do not want to access other registers for Read/Write, this option can result in a simpler implementation.

Toggles are needed only on nCS and SCLK for reading the state of input pins.