How to power RTC during sleep?

Razo · Joined: 02 Jan 2009 Posts: 6

OK this isn't a CCS question. But I would like to know what your thoughts are on it.
I'm building a project where the controller is doing some some data collection and will spend most of its time in sleep mode, but when it does do the collection I need to timestamp it. So I'm using a ds1307 RTC as thats what I have to hand and I've used them before.
My question is... Should I...
a) Power the RTC perm using the main battery to keep the clock running, thus keeping the backup battery only for battery changes?
or
b) Power the RTC with a I/O pin only when I need to ask it the time and cut its power when the pic goes to sleep?

The backup battery will drain faster with option b but will save the main batteries powering it when I'm not calling it. I'm only going to be calling the RTC a handful of times a day once the project is complete.

I would love to hear your thoughts.

Razo Smile

FvM · Joined: 27 Aug 2008 Posts: 2337 Location: Germany

The questions sounds confuse. The RTC needs a permanent power to keep the time, but it's current consumption
is below 1 uA, so it can operate 10 years and more with a lithium coin cell.

DS1307 has a dual supply, so the battery isn't discharged during processor activities.

bungee- · Joined: 27 Jun 2007 Posts: 206

@razo: I really don't understand what are you trying to say with option a and b. I have one project that is battery operated. I used same RTC and I connected backup battery for time keeping and the other one was main battery. I totally forgot about that project and after few years everything was still working, RTC was off for a few seconds but that is not unexpected for a longer period of time.

So again what is your dilemma here.

Razo · Joined: 02 Jan 2009 Posts: 6

Ok so i posted that late last night so maybe i wasn't clear.
In my project i have 2 batteries. One powering the pic, lcd and atm the RTC
and also on the rtc I have a 3v coin cell with keeps the time while the main power is off. What i'm asking is, when i don't need to get the time from the RTC and the microcontroller is sleeping should i still power the RTC using the main battery or let it keep its time just on the coin cell.

bkamen · Posted: Tue Nov 17, 2009 12:10 pm

Figure out which mode requires the least current.

or

Decide if you want the 3V coin to last longer because maybe the main battery is more easily replenished or replaced.

Then go with that.

That's a design thing you have to ultimately decide.

-Ben
_________________
Dazed and confused? I don't think so. Just "plain lost" will do. :D

FvM · Joined: 27 Aug 2008 Posts: 2337 Location: Germany

The meaning of your a) versus b) alternatives gets understandable now.

I don't know, what's your total circuit standby current (without DS1307). But DS1307 has a considerably higher Vcc standby
current (50 - 100 uA) than it's Vbat idle current, I fear, it may be too high. So it's possibly meaningful to use variant b).
A single supply RTC (e.g. DS1337) has a much lower standby current.