Printable View
I'm using the DS1307 in an automotive application. In the datasheet the RTC backup is a 3V coin battery. Since the vehicle as a battery would it make sense to use it in place of the coin battery? If so all I should need to do is swap out the battery for a voltage regulator?
I would still use a coin cell.
The voltage regulator will use far more current than the RTC itself and if the car battery gets disconnected then the RTC will stop.
My concern is more space related. The coin cell holder eats up valuable board space.
How about using an electrolytic backup instead of the coin cell. Thus, even if the battery is disconnected for a few minutes, the RTC still works. You could work out the value of the capacitor based on the amount of time you need it to hold while the car battery is disconnected. Of course, you still need to charge the cap while the battery is connected.
Jerson
I’m not worried about losing the time when the battery is removed. If you remove the battery in your personal vehicle, you have to reset all the clocks. People are used to this fact, I think.
A voltage Regulator is probably overkill. A 3.3v Zener in series with a Resistor should suffice. Zeners are noisy, so a small Capacitor across it would keep the RTC happy, but you might not even need it. Actually, if you made it something like a 1uF, it probably would have enough charge to keep the RTC Backed-Up for an hour or more.
"A 3.3v Zener in series with a Resistor should suffice."
In series?
You should want to put it in parallel? A 3.3v Zener directly across a 12v car Battery is something you will only try once - Don't hold the Zener in your fingers when you do it!
So to recap:
12V (battery) --> Resistor --> Zener K--> 1uF Cap --> DS1307
Both the Zener Anode and Cap get tied to Common.
Yes, I think you've got it...
Oh sure one up me with a picture. ;)
Yeah thats what I had, we are both on the same page. :)