The NRST pin in the TPS389R0-Q1 features a programmable reset delay time that can be adjusted from 0.2ms to 200ms when using TI_CONTROL register. NRST is an open-drain output that must be pulled up through a 1kΩ to 100kΩ pullup resistor. When the device is powered up and POR is complete, NRST is asserted low until the BIST is complete. After the BIST, NRST remains high (not asserted) until it is triggered by a mappable fault condition. An NRST_MISMATCH fault will be asserted if the NRST pin is pulled to an unexpected state. For example, if the NRST pin is in a high-impedance state (logic high) and is externally pulled low, then an NRST_MISMATCH fault will assert. During an NRST toggle NRST mismatch will be active after 2μs, NRST must exceed 0.6*VDD to be considered in a logic high state. For conditions resulting in a Failsafe mode NRST pin will assert low and stay asserted until a power cycle.

NRST is also mappable to the OVHF and UVHF faults using the FC_LF[n] registers. If a monitored voltage falls or rises outside of the programmed OVHF and UVHF thresholds, then NRST is asserted, driving the NRST pin low. When the monitored voltage comes back into the valid window, a reset delay circuit is enabled that holds NRST low for a specified reset delay period (t_D). Note if NRST is un-mapped from OVHF and UVHF faults while NRST is asserted then NRST deasserts, NRST reasserts when re-mapped assuming the voltage is still outside the valid window

The t_D period is determined by the RST_DLY[2:0] value found in the TI_CONTROL register. When the reset delay has elapsed, the NRST pin goes to a high-impedance state and uses a pullup resistor to hold NRST high. The pullup resistor must be connected to the proper voltage rail to allow other devices to be connected at the correct interface voltage. To ensure proper voltage levels, give some consideration when choosing the pullup resistor values. The pullup resistor value is determined by output logic low voltage (VOL), capacitive loading, and leakage current.

Figure 7-10 TPS389R0-Q1Start up Behavior Reset Pin

Figure 7-11 TPS389R0-Q1Reset Timing diagram for voltage faults