By using SPI Slave Interface you can use a single or multiple slave devices which are connected to master SPI Host adapter device.
The slave device is not capable of generating the serial clock (SCK) signal and thus can not get active on its own. The slave just sends and receives data if the master generates the necessary serial clock signal. The slave device receives the clock and chip select from the master spi usb device.
In slave mode, the SS pin is always an input. When SS is held low, the SPI is activated and MISO becomes output if configured so by the user. All other pins are inputs. When SS is driven high, all pins are inputs, and the SPI is passive, which means that it will not receive incoming data.
In slave mode, the SPI logic will be reset once the SS pin is passed high. If the SS pin is passed high during a transmission, the SPI will stop sending and receiving and both data received and data sent must be considered as lost.
When the SPI is configured as a slave, the transmission starts with the falling edge of the SS line. This activates the SPI of the slave and the MSB of the byte stored in its data register (SPDR) is output on the MISO line.