This could probably go in the embedded forum but since it is a bit more generic question and this subfourm gets more notice I figured I would post it here.

I am building a project involving an RTCC. The project will be powered via a wall wart. The project will run on 3V via an on board regulator. But I want a battery backup for the clock in the event of a power failure. I think I can figure out how to monitor for main power failure and put the pic to sleep. That is not the problem.

Where I need help is how to design the battery backup. Can I simply put the battery in parallel with my 3VDC supply? Or do I need to switch it in somehow during the power failure? If so, how do I do that?

 

Sorry something like this, if your 3VDC doesn't draw power (back into your plug pack), you can remove the diode between 3VDC and MCU



Or if

 

Now why didn't I think of that??? So simple. I am doing just this with my solar panel project. I will need two diodes so the pic can monitor power failure and then go to sleep. I need to make sure it is only monitoring the wall wart supply.

 

Now why didn't I think of that??? So simple. I am doing just this with my solar panel project. I will need two diodes so the pic can monitor power failure and then go to sleep. I need to make sure it is only monitoring the wall wart supply.

Put a pair of resistors in series to ground (before the diode on the Wall wart ). Then monitor the voltage at the midpoint of the two resistors.

 

Put a pair of resistors in series to ground (before the diode on the Wall wart ). Then monitor the voltage at the midpoint of the two resistors.

I am dense. Not sure I understand . "Pictures" please.

Oh and go Steelers!

 

The diode solution is pretty much the standard solution.
Don't forget to use a Schottky diode such as 1N5817 for low diode forward voltage.

 

The diode solution is pretty much the standard solution.
Don't forget to use a Schottky diode such as 1N5817 for low diode forward voltage.

Thanks I was going to mention I was going to use Schottkys.

 

I am dense. Not sure I understand . "Pictures" please.

Oh and go Steelers!

Better to do it with a diode drop (same as power supply, then you can simply check for a digital high or low on any 8nput pin.

 

The diode solution is pretty much the standard solution.
Don't forget to use a Schottky diode such as 1N5817 for low diode forward voltage.

Weird. I have been organizing my parts tonight. Had a envelope sitting here that had some previously ordered . I thought I had removed all components and put them away but before I tossed it decieded to check the contents once more. I noticed something inside. It was a bunch of 1n5819s!

 

Better to do it with a diode drop (same as power supply, then you can simply check for a digital high or low on any 8nput pin.



View attachment 138321

I was sort of thinking something like that except through a current limiting diode. I guess I can go without D2 then? I just need D1 so the supply doesn't try to power my battery.

 

I was sort of thinking something like that except through a current limiting diode. I guess I can go without D2 then? I just need D1 so the supply doesn't try to power my battery.

I would keep D2, I wouldn’t want the little battery to lose any current through the power supply if there is any path to ground.

EDIT ...and Go Steelers.

 

This could probably go in the embedded forum but since it is a bit more generic question and this subfourm gets more notice I figured I would post it here.

I am building a project involving an RTCC. The project will be powered via a wall wart. The project will run on 3V via an on board regulator. But I want a battery backup for the clock in the event of a power failure. I think I can figure out how to monitor for main power failure and put the pic to sleep. That is not the problem.

Where I need help is how to design the battery backup. Can I simply put the battery in parallel with my 3VDC supply? Or do I need to switch it in somehow during the power failure? If so, how do I do that?

Hi Spinnaker, are you using a rtc chip or do you use the main mpu?
If you use a chip like MCP795XX series then a battery switchover is embedded.
( this chip is great I no problems easy to program)
Even an interrupt at a given time could be generated.

Picbuster

 

Better to do it with a diode drop (same as power supply, then you can simply check for a digital high or low on any 8nput pin.



View attachment 138321

Wait I just realized something. How is this setup going to keep my battery from running the Pic when the power is on? Won't it deplete the battery over the long term? Don't I need to switch in the battery somehow only during power failure?

 

Would something like this work with logic level mosfets?

 

Wait I just realized something. How is this setup going to keep my battery from running the Pic when the power is on? Won't it deplete the battery over the long term? Don't I need to switch in the battery somehow only during power failure?

No, if the battery is at 3v and your power supply is 3.3 v, then the diode to the battery will be reverse biased and no current will flow from the battery n

 

No, if the battery is at 3v and your power supply is 3.3 v, then the diode to the battery will be reverse biased and no current will flow from the battery n

Didn't think of that. Again so obvious, once someone mentions it.

 

So spinnaker, your battery is needed, in case of failure, just to keep the RTCC alive but dormant. Right?

 

So spinnaker, your battery is needed, in case of failure, just to keep the RTCC alive but dormant. Right?

Just to keep RTCC alive. I plan to sleep the CPU when main power fails.

 

use two diodes one to power from vdd and one to power from Batt via 1K to feed RTC. (Or gate)

Picbuster

 

Cool. I don't even need to use another pin to monitor it.

26.0 HIGH/LOW VOLTAGE DETECT
(HLVD)
The High/Low-Voltage Detect (HLVD) module can be
used to monitor the absolute voltage on VDD or the
HLVDIN pin. This is a programmable circuit that allows
the user to specify both a device voltage trip point and
the direction of change from that point.
If the module detects an excursion past the trip point in
that direction, an interrupt flag is set. If the interrupt is
enabled, the program execution will branch to the interrupt
vector address and the software can then respond
to the interrupt.

When enabled, the HLVD circuitry continues to operate
during Sleep. If the device voltage crosses the trip
point, the HLVDIF bit will be set and the device will
wake-up from Sleep. Device execution will continue
from the interrupt vector address if interrupts have
been globally enabled.