Correct connection of RTC battery backup

NETMF & Gadgeteer
1

I want to connect a battery for RTC backup to an EMX module. I have a few questions.

  1. Will the RTC_VBA pin still pull from the battery when the module is powered up? I.e. does this pin need connected to a 3.3 volt source when the system is powered up?

  2. Do I need some kind of resistor between the battery and RTC_VBA?

  3. Anyone have an example of circuitry for this or even recharging circuitry for a rechargeable coin battery?

Thanks in advance
Martin

2

See how it is connected here http://www.ghielectronics.com/downloads/EMX/EMX_DevSys_sch.pdf

To help you plan better, EMX has internally a diode from 3.3V to VBAT

3

Gus, in that schematic it uses a coin battery for backup.
Can a super cap be used instead? If so, can you recommend a size to use.
Thanks.

4

You can use super cap, the size is factor of how long you want it to run. We used 0.22F before IIRC

5

Thanks, I’ll do some experimenting.

6

Thanks! That helped.

7

Attached is what I ended up with if anyone else is looking.