When the SPI is configured as a Master (MSTR in SPCR is set), the user can determine the direction of the SS pin.
If SS is configured as an output, the pin is a general output pin that does not affect the SPI system. Typically, the pin will be driving the SS pin of the SPI slave.
If SS is configured as an input, it must be held high to ensure master SPI operation. If the SS pin is driven low by peripheral circuitry when the SPI is configured as a master with the SS pin defined as an input, the SPI system interprets this as another master selecting the SPI as a slave and starting to send data to it. To avoid bus contention, the SPI system takes the following actions:
Thus, when interrupt-driven SPI transmission is used in Master mode, and there exists a possibility that SS is driven low, the interrupt should always check that the MSTR bit is still set. If the MSTR bit has been cleared by a slave select, it must be set by the user to re-enable SPI Master mode.