Problems with Slave Select Control on dsPIC33/PIC24 Family
The SSxOUT does not function as a "SPIx Slave Select Output" and is always in a low state. Why is this?
Slave select signals are not automatically generated. When using the Slave Select mode, the SSx pin, or another GPIO pin, must be used to control the slave's SSx input. This slave select pin is controlled by your application.
There must be a separate control signal for each slave on an SPI bus.
For more information, please refer to the "Master Mode Operation" section of the SPI Peripheral Chapter in the dsPIC33/PIC24 Family Reference Manual.