The window watchdog frame consists of two sub frames t_WC followed by t_WO. The host is not expected to drive valid WDI transition during t_WC time. A valid WDI transition during t_WC frame results in early fault condition and the WDO output is asserted. RESET output is asserted if pinout does not offer independent WDO output. The t_WC timer for TPS36-Q1 can be set using an external capacitor connected between CWD pin and GND pin. This feature is available with pinout options A or B. Applications which are space constrained or need timer values which meet offered timer options, can benefit when using pinout options C or D. The TPS36-Q1 offers multiple fixed timer options ranging from 1 msec up-to 100 sec.

The TPS36-Q1 when using capacitance based timer, senses the capacitance value during the power up or after a RESET event. The capacitor is charged and discharged with known internal current source for one cycle to sense the capacitance value. The sensed value is used to arrive at t_WC timer for the watchdog operation. This unique implementation helps reduce the continuous charge and discharge current for the capacitor, thus reducing overall current consumption. Continuous charge and discharge of capacitance creates wider dead time (no watchdog monitor functionality) when capacitor is discharging. The dead time is higher for high value of capacitance. The unique implementation of TPS36-Q1 helps avoid the dead time as the capacitance is not continuously charging or discharging under normal operation. Ensure C_CWD is < 200 x C_CRST for accurate calibration of capacitance. The close time window is decided based on SETx pin combination and the CWD capacitance. Table 8-1 to Table 8-3 highlights the relationship between t_WC in second and CWD capacitance in farad. The t_WC timer is 20% accurate for an ideal capacitor. Accuracy of the capacitance will have additional impact on the t_WC time. Ensure the capacitance meets the recommended operating range. Capacitance outside the recommended range can lead to incorrect operation of the device.

The TPS36-Q1 also offers wide selection of high accuracy fixed t_WC timer options starting from 1 msec to 100 sec including various industry standard values. The TPS36-Q1 fixed time options are ±10% accurate for t_WC ≥ 10 msec. For t_WC < 10 msec, the accuracy is ±20%. t_WC value relevant to application can be identified from the orderable part number. Refer Section 5 to identify mapping of orderable part number to t_WC value.