2.1 Chip Select (CS)

The AT25128B/AT25256B is selected when the Chip Select (CS) pin is low. When the device is not selected, data will not be accepted via the Serial Data Input (SI) pin, and the Serial Output (SO) pin will remain in a high‑impedance state.

To ensure robust operation, the CS pin should follow V_CC upon power-up. It is, therefore, recommended to connect CS to V_CC using a pull-up resistor (less than or equal to 10 kΩ). After power-up, a low level on CS is required prior to any sequence being initiated.