Operation
The SPI bus can operate with a single master device and with one or more slave devices.
If a single slave device is used, the SS pin may be fixed to logic low if the slave permits it. Some slaves require the falling edge (high→low transition) of the chip select to initiate an action such as the Maxim MAX1242 ADC, which starts conversion on said transition. With multiple slave devices, an independent SS signal is required from the master for each slave device.
Most slave devices have tri-state outputs so their MISO signal becomes high impedance (disconnected) when the device is not selected. Devices without tri-state outputs can't share SPI bus segments with other devices; only one such slave could talk to the master, and only its chip select could be activated.
Read more about this topic: Serial Peripheral Interface Bus
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