SS Pin Functionality in Master Mode

In Master mode, the Slave Select Disable bit in Control Register B (SSD bit in SPIn.CTRLB) controls how the SPI uses the SS pin.

If SSD in SPIn.CTRLB is ‘0’, the SPI can use the SS pin to transition from Master to Slave mode. This allows multiple SPI masters on the same SPI bus.
If SSD in SPIn.CTRLB is ‘0’, and the SS pin is configured as an output pin, it can be used as a regular I/O pin or by other peripheral modules, and will not affect the SPI system.
If SSD in SPIn.CTRLB is ‘1’, the SPI does not use the SS pin, and it can be used as a regular I/O pin, or by other peripheral modules.

If the SSD bit in SPIn.CTRLB is ‘0’, and the SS is configured as an input pin, the SS pin must be held high to ensure master SPI operation. A low level will be interpreted as another master is trying to take control of the bus. This will switch the SPI into Slave mode, and the hardware of the SPI will perform the following actions:

1.The Master bit in the SPI Control A Register (MASTER in SPIn.CTRLA) is cleared, and the SPI system becomes a slave. The direction of the SPI pins will be switched when conditions in Table 1 are met.
2.The Interrupt Flag in the Interrupt Flags register (IF in SPIn.INTFLAGS) will be set. If the interrupt is enabled and the global interrupts are enabled, the interrupt routine will be executed.

Table 1. Overview of the SS Pin Functionality when the SSD Bit in SPIn.CTRLB is ‘`0`’
SS Configuration	SS Pin-Level	Description
Input	High	Master activated (selected)
Input	Low	Master deactivated, switched to Slave mode
Output	High	Master activated (selected)
Output	Low	Master activated (selected)

Note: If the device is in Master mode and it cannot be ensured that the SS pin will stay high between two transmissions, the status of the Master bit (the MASTER bit in SPIn.CTRLA) has to be checked before a new byte is written. After the Master bit has been cleared by a low level on the SS line, it must be set by the application to re-enable the SPI Master mode.

SS Pin Functionality in Master Mode - Multi-Master Support