Fez Raptor and real time clock

Hi guys,

I want to use the RTC so, What should I do to enable it? I just found that connector 5 pin 5 is called VBat.
Does it need a battery between this pin and ground?
Is an external circuit required?
What kind of battery do I have to use?


Crespi Juan

The G400 Real Time Clock will require a voltage source of between 1.8V ~ 3.6V, typically a button battery, to be connected to the VDDBU/VBAT pin.


Do you know if the battery will be charged by the g400 circuit?

@ crespi - not sure about that.

I don’t believe so. Normally a button cell is used, and the RTC draws so little current, it lasts for years.

You’d have to check the processor’s datasheet for exact numbers.

Often a supercap can be used as well, and I think it will be charged but I’d want confirmation from GHI before I tried that.

Supercap, resistor and diode will do the trick.


Indeed. I’m going for the supercap.

Thanks guys.

Remember that the Supercap is only designed for short term power off use. This can range from hours to x days. After this time the clock will not be backed up.

The coin cell provides a much longer backup period and can be years depending on the current draw and battery capacity.

Here’s a handy little calculator from Maxim for the supercap.