Since the RTC only mode is not supported with the AM3358 / TPS65217D PMIC combination, I would like to find another way to have my system turn itself OFF for a pre-determined amount of time and then wake itself up again. Here are some notes that I would like TI to review and comment on.
- The external RTC I am using has an open drain Timer output that I am planning to connect to the PB_IN pin of the PMIC.
- When we activate the power cycle sequence, we would set a countdown timer in the external RTC to something like 10 seconds.
- While the external timer is counting down, the AM3358 would set the PWR_EN line low to turn OFF Power to the system. In this mode, the PMIC would still be receiving power from a system 5V_STBY power rail to the VBAT input using the Mosfet Circuit we discussed previously.
- When the RTC counter terminates, it will Pulse the PB_IN line of the PMIC low – the pulse width of the open drain timer output of our external RTC is about 7msec which is much shorter than the 50msec debounce time mentioned in the PMIC data sheet.
- To ensure that the PMIC receives a valid Wake UP event, I am planning to add a moderate sized capacitor at the PB_IN to ensure that PB_IN stays below 1.2 volts for at least 50msec.
So, please confirm that I am interpreting the data sheet for the PMIC correctly and that TI would expect the PMIC to reliably Wake Up from OFF as long as I hold the PB_IN to less than 1.2 volts for at least 50msec.