chipselect

Chip select is a control signal used to enable a specific integrated circuit on a shared bus or within a memory-mapped address space. Its primary purpose is to ensure that only one device responds to a given address or bus transaction, preventing conflicts when multiple peripherals share the same resources.

The signal is usually multiplexed with other signals in a system. A chip select can be active-low or active-high, with active-low being common in many interfaces. In practice, a master asserts the appropriate chip select for the targeted device, and the device becomes active only when its select line is asserted together with the correct address, data, and control signals. If the chip select is not asserted, the device remains unresponsive, and its outputs typically go to a high-impedance state.

Common implementations include SPI, where each slave device has its own dedicated chip select line controlled by the master, and memory-mapped or I/O peripherals in microcontroller systems, where address decoding drives chip selects corresponding to specific address ranges. In PCI and other bus standards, chip select-like signals enable a device’s memory or I/O space when the address and control conditions are met.

Electrical and timing considerations include ensuring correct polarity, proper setup and hold times, and avoiding conditions where multiple devices are selected simultaneously due to decoding errors. Robust decoding logic and, if necessary, level shifting or tri-state management help maintain bus integrity.

See also: SPI, memory-mapped I/O, address decoding, peripheral interfacing.