I develop intellectual sensors and primitive controllers. My favorite MCU - MSP430FR57xx family. Now I need to add new features to my controller, RTC included.
I'm not used to a Real Time Clock and relevant issues. Equally I don't need a LPM in my devices. So my 1-st reaction to a task of adding a real time control - use the dedicated RTC chip.
Then I've read about RTC in FR68xx family. It has RTC_C, good. But has not Vbat pin, AFAIK. So, the only way to continue timekeeping after the main power is off - catch the event and enter LPM3.5 mode. All MCU remains powered from a battery (and drains 700 nA).
In such an approach I have to do:
- use power switch to turn to a battery if VCC drops
- watch VCC to catch event
- enter LPM
- after power restoring I have to exit LPM and begin the program (it may be from reset point)
And 700 nA.
Maybe it's stupid, but it seems a little bit complicated to study all the powerful possibilities of the LPM and on-board RTC of a MSP430FR6870 chip - just to use a very very simple functionality - have a real time clock.
On the contrary: I take an RTC (Abracon AB0805 has stunned me with 55 nA drain) and power it from the battery constantly. In our old devices we used DS1307 and it took several years to discharge CR2032. And:
- no switch of power
- no power monitoring
- no LPM
- no special starting procedures
And, as a bonus, I use MSP430FR5857 that has not RTC but satisfies all other demands.
What have I missed in my meditations?