Chip select

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An example SPI with a master and three slave select lines. Note that all four chips share the SCK, MISO, and MOSI lines but each slave has its own slave select.

Chip select (CS) or slave select (SS) is the name of a control line in digital electronics used to select one chip out of several connected to the same computer bus usually utilizing the three-state logic.

One bus that uses the chip/slave select is the Serial Peripheral Interface Bus.

[edit] In English

When an engineer needs to connect several devices to the same set of input wires (ie, a computer bus), but retain the ability to send data or commands to each device independently of the others on the bus, they can use a chip select. The chip select is a command pin on most ICs which connects the input pins on the device to the internal circuitry of that device.

When the chip select pin is held in the inactive state, the chip or device is "deaf," and pays no heed to changes in the state of its input pins. When the chip select pin is held in the active state, the chip or device assumes that any input changes it "hears" are meant for it, and responds as if it is the only chip on the bus.

In short, the chip select is an input-enable switch. "ON" means the device responds to changes on its input pins (such as data or address information for a RAM device), while "OFF" tells the device to ignore the outside world.

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