6.1 Chip Select (CS) Hold Timing

SPI uses the CS or Serial Select (SS) line to select a device to communicate with. It is important to hold the line before beginning communication so the target device is ready. If insufficient time is provided, the device may not operate correctly.

An MCU acting as an SPI target may be used as an example. When inactive, this MCU remains asleep to save power. When the CS line is asserted, an interrupt is generated to wake the MCU and load data into the SPI shift register. If provided insufficient time, the shift register will have old or invalid data loaded, as the MCU could not perform the required actions before communication.