30.4.3.6 SPI Select Low Detection

In Client mode, the SPI can wake the CPU when the SPI Select (SS) goes low. When SPI Select Low Detect is enabled in the Control B register (CTRLB.SSDE), a high-to-low transition sets the SPI Select Low interrupt flag in the Interrupt Flag Status and Clear register (INTFLAG.SSL), and the device will wake up if applicable.