Vstby for battery backed internal ram

[hooziewhatsit]

I'm using an MC912DG128A.  I also have a separate RTC that will have a 3.0v battery to maintain its time when the system is powered off.

Can I also connect the battery to the Vstby pin on the HC12 to maintain the contents of the internal RAM?  The datasheet is very fuzzy about the use of that pin.  All it says is the minimum voltage is 1.5v, with a draw of 50uA.  

When the HC12 is powered, does Vstby continue to draw its 50uA?  Or does it internally switch off and run on the 5v supply?

I would assUme that I can connect the battery and be just fine, but want to make sure before I have circuit boards made

Thanks in advance,

-Dave

[kef]

Good question. I would choose 2 diodes with least possible reverse currents and connect cathodes to Vstb, one anode to 3V battery, another anode to 5V. This way you should be able to cut battery current draw when 5V is present. When you get PCBs you may check what current is drawn from battery when 5V diode is not installed. Also try making Service Request, maybe Freescale can explain what happens to IVstby when device is powered.

But I would not choose this old device with fragile Collpits oscillator and required external brown out detector. Parts with no Vstby may have similar stop current like IVstby on 912DG. If this is still to high or you if you need to shut down, external FRAM, EEPROM or battery backed RAM may be better choice.

[hooziewhatsit]

Thanks for the ideas.  I may just tell the client, 'sorry, can't be done'.  There are 56 bytes in the RTC they can use.  I'm really just surprised at how vague the datasheet is about it. 

I could probably use a newer chip, but to be honest, I know very little about the HC12 family and compatibility, and I know this one works with the clients existing code base.  And it's just a low run product.  If there was an updated chip that dropped in with the same specs, I'd use it.

If I end up contacting Freescale directly, I'll post the response here.

Thanks for the quick reply 

-Dave