Vbat & RTC problems

[Curro]

I did the first pre-production with K60 micro.

some boards are ok.

In some other I have this problem:

I set date/time at the first power up. All ok.

I power down board.

At the next power up date/time remains with values i set at the first power up (RTC module stopped).

Some considerations:

- 32K oscillator is ok

- Battery voltage ok (3V)

- Board Vcc 3.3V

- Vbat pin voltage is ok

- tested board with 100nF and 10uF between Vbat pin and GND

- 2 BAT54 diodes to decouple 3V from battery and 3.3V from board regulator

- If I supply RTC module with external 3.3V supply instead of 3V battery it's all ok

- K60P144M100SF2 doc gives 1.71V as min. RTC battery supply voltage. Using battery I have 3V-(BAT54 drop)=2.8V

- On K60P144M100SF2 I found the min. value of VRFVBAT (VBAT voltage required to retain the VBAT register file) as TBD

Any ideas on how to solve the problem?

Regards,

Corrado

[egoodii]

First off, if you have Rev 0 chips (0M33Z) I will assume you have implemented the 100ohm workaround for e2666 latchup.  Insertion of coin cells has an inherently fast Vbat ramp.  As for the oscillator, the impedance of this whole circuit is VERY HIGH, so I might suspect layout leakage/noise/capacitance, AND I would try different crystals to check their ability to start-up (and hold).