slaveselect

Slave select, often written as slaveselect, is a control signal used in the Serial Peripheral Interface (SPI) to activate a specific slave device on the bus. Each SPI slave typically has its own slave-select (SS) or chip-select (CS) line. The master pulls the corresponding SS line active (commonly low) to enable a chosen slave to participate in data exchange, while all other slaves remain inactive.

In hardware terms, SS is separate from the clock (SCK), master output (MOSI), and master input (MISO) lines. An SS line can be driven by the master or, in some architectures, by hardware logic. Some microcontrollers require the SS pin to be configured in a particular way (e.g., as an input) to maintain master mode and prevent unintended slave operation when no device is selected.

Operation involves the master asserting the SS line of the target slave, then transferring data over MOSI/MISO synchronized by SCK. Only the slave with its SS line active responds; other slaves typically ignore the bus. After the transfer, the master deasserts the SS line to deselect the device. In systems with multiple slaves, each device should have its own dedicated SS line; using a single shared line is unusual and can lead to contention unless managed with additional circuitry.

Software commonly controls slave select via GPIOs, offering selective enabling and disabling around each SPI transaction. Proper timing, idle states, and deassertion practices help prevent bus contention and data corruption.

See also: SPI, Chip select, MISO, MOSI, SCK.