We measured the current drawn from VDD_SNVS_IN (3.0V coin cell CR2032) and got the following measurements:
Multimeter: Fluke 8846A:
Supply (3.3V) Off, RTC in Reset (not adjusted): +209uA
Supply (3.3V) On, RTC adjusted, CPU running: +11uA (because coin cell voltage is about 3.2V and LDORTC is 3.0V)
Supply (3.3V) On, Sleep Mode (suspend to RAM): +16uA
Supply (3.3V) Off, RTC adjusted: +439uA (this should be lower than 25uA)
In the IMX6ULCEC.pdf Rev. C
-> Current consumption 4uA + 1uA + 1.5uA = 6.5uA
Why did we get 439uA current drawn from the coin cell in power-off mode? This reduces backup time rapidly to some days/weeks.
The SNVS is clocked from an external 32kHz crystal (not the 40kHz ring oscillator).
We using Linux Kernel 4.3.0, started from u-boot. Could there be an configuration mistake (e.g. tamper detection enabled) or is this an hardware issue? I don't think we need tamper detection, but may be it's activated somewhere.