Power loss - Brown out?

I have made a clock (using PIC16F870 & DS1302 RTC) which works fine.

However, when power is lost to the circuit, the clock looses the time and start's up in its 'set the time' routine. Now I have found a way to attach a backup battery to the DS1302, but I do not know how to get the PIC to skip the 'set the time' routine. From what I can gather this can be done with brown out detection. But I don't understand how that works - or how you set a different start point for the program to run (ie not go through the 'set the time' routine when power comes back)

Any and all help really appreciated!

At the start of your code, check to see if PCON.0 = 0. If it does, a brownout has occurred. Set PCON.0 to 1 and jump past the Set Time routine.

However, PCON.0 must have been previously set to 1 in your code, probably within the Time Set routine.

Another approach (that I use on the Xicor X1226) is to read the clock on startup. If is a good value, skip the Set Time routine.

Hope this helps,
Paul

that seems simple enough! thanks!

A coin cell feeding the RTC and a diode between the RTC VDD and the PIC VDD works well.

For the code jump add a button. At startup if the time needs reset for some reason hold the button.

Quote:

---

Originally Posted by mackrackit

A coin cell feeding the RTC and a diode between the RTC VDD and the PIC VDD works well.

For the code jump add a button. At startup if the time needs reset for some reason hold the button.

---

Thanks mackrackit for this advice - I've got the battery working and added a rest button. I've also been looking into the diode you mentioned - it seems that this is actually essential to prevent the accidental charging of the battery, and hence a possible explosion! What diode would I use here how is it connected? I don't understand your comment about connecting it between the two power sources - is it not simply in series between the ds1302 and the battery?

The data sheet says:
"UL recognized to ensure against reverse charging current when used in conjunction with a
lithium battery."

Does that mean I can trust the d1302 to protect the battery?

Thanks again,
O.

I forgot the DS1302 has two supplies.
I would connect the DS1302 as shown here but use a diode with the battery, and have the trickle charger turned off. I do not have enough experience with the DS1302 to say about trusting it. http://www.maxim-ic.com/images/appno.../3449Fig02.pdf

A 1N914 woks for the diode.
Here is what I was getting at when I posted before. The whole system runs on 3.3 volts.
Attachment 4880