Yes this question is related to MCP4131 which is a 8 pin digital potentiometer. I have never seen/heard/used multiplexed SPI before so am trying to clear some confusions. The datasheet shows connection as shown below:
Now lets look at the timing diagrams which is where the confusion lies. They are found starting from page 44:
The first and third waveforms show SDI and SDO being driven at the same time. Certainly the MCP4131 cannot output and input signal at the same time since in SPI a signal is latched at a specific edge of SCK. In this case it is the rising edge. So do I understand correctly that for MCP4131 only the second waveform is applicable since the first and third do not make sense with multiplexed SPI?
With multiplexed SPI and a high voltage mode on CS this looks like a mysterious device. Why make life harder by squeezing so much functionality in such a small package?