Three Wire SPI connection question

As an optimal solution, you would build the output enable control for the bidrectional pin into your SPI interface. It can switch the direction after 16 clocks depending on the R/W bit.If you don't want to do this, you have two options:

-operate the SDIO line with a pull-up resistor and an open drain driver. You have to write 0xFF to the SPI bus during read phases

-implement an additional R/W control signal that enables the driver and set it to "read" in the middle of a SPI read transaction. This should work, because the SOPC SPI interface is using 8-Bit entities.