I am wondering about the RTC section of the i.MX7 and power supply in combination with a PF3000 PMIC and if I understood everything correctly.
Based on this I want to decide wether the i.MX7 internal RTC is suited or if an external RTC is the way to go.
The PF3000 / i.MX7 provide NVCC voltage rails for e.g. the RTC. These allow power states with most of the system disabled, including many power rails, clocks, cores etc. If the systems power is cut the NVCC voltage rails should not be powered anymore. Some kind of backup power supply for the RTC is needed.
On pin 36 (LICELL) a Lithium Cell may be connected to the system. The PF3000 provides an internal LiCell charger. Registers allow to enable/disable the charger and to set the batterys/caps voltage. All SNVS regions are powered by either an internal LDO (if the systems input voltage is above a certain threshold) or the battery.
Coin cell power mode (all power rails etc turned off) current consumption: up to 7uA based on '5.3 - current consumption' in the PF3000 datasheet. This is probably the current consumption of the PF3000 itsself?
It is said the current will increase when the voltage drops, up to 50uA. Are there any graphs or additional info on how the current increases?
I cannot find information on the i.MX7D current consumption when only the SNVS rails are powered. Only information I could find is in another thread: NXP Community
Based on this, around 500uA at 3V are consume. For a 4 Month backup (from PCB manufacturing until system delivery), assuming constant voltage and current, a 1.4 Ah battery is necessary, (without self discharge) (correct?).
As an alternative, a supercap storing 1.4Ah*3V = 4,2 Wh = 15 kWs could be used. For a voltage drop of 2V7 .. 1V8, this means a 7500F capacitor (!!!) is required, so a capacitor is out of question. Or did I miscalculate here?
Dedicated RTCs require about 50nA supply current, what is just 1/10000 of the mx7 requirement. I should certainly go with a dedicated RTC, if the assumptions above are correct and if the mx7 + pf3000 RTC power backup really consumes about 500uA continously when powered off.