The Cext is used to wake the part up from suspend. The Cext pin should be connected to GND with an external cap and Vcc with an external resistor. Firmware needs to write 0 to Cext Register to discharge the cap, and then writes 1 to Cext to disable the open drain output driver. As soon as the voltage at the Cext pin rises above Cext threshold voltage, an interrupt will be generated. The external RC circuit determines how fast/slow the cap is charged up, and as a result, determines how often the interrupts are generated. Note that Cext threshold voltage is a variable ranging from 12 to 30% Vcc, so for a fix RC, you're guaranteed a RANGE of periodical Cext interrupts.