Why Or'd Vcc with backup battery output shorted battery life?

[mackrackit]

A PIC CAN be powered through an I/O pin.

[rsocor01]

Jellis00,

Test your circuit running with the backup battery only. I would use an ampmeter to see if there is any leak going through any of the resistors.

[jellis00]

Jellis00,

Test your circuit running with the backup battery only. I would use an ampmeter to see if there is any leak going through any of the resistors.

Good suggestion. I will try this in the next couple of days and report back.
If the PIC is consuming power from the backup battery via these leakage paths when the Vcc to the PIC is shutdown, I guess Oring the two power sources is not the way to save battery life. Since I have to use pullup resistors with the DS1337 for it I2C interface and the _INTA interface, how can I prevent the PIC from running down the backup battery?? Any suggestions from anyone??

[mackrackit]

Take a look at the schematic here
http://www.picbasic.co.uk/forum/content.php?r=272-USB-SD-LOGGING

Keep in mind the whole system runs at ~3 volts.

[jellis00]

Thanks, Dave! That is a very cool design. I see from it how you or'd the Vcc with the backup battery for clock use and connected all the pull-up resistors to Vcc rather than to the backup battery supply. That eliminates the leakage path that I am currently stuck with by having the pullups connected to the battery. I was worried that the clock would continue to maintain time or work properly if I didn't keep the pullups active when Vcc was shut down, but from your schematic evidently it will keep on trucking when Vcc is shut down. Correct??

One other question I had regarding your circuit is how you get away with only one diode on the Vcc leg of the Or'd circuit? I thought I would have to also put one on the leg of the backup battery supply so that when Vcc is active, that diode protects the backup battery from the Vcc voltage.

Now I guess I have to consider cutting some traces and green wiring the pullups to Vcc.... a lot cheaper than a PCB re-do

Thanks again for all your help. Now that you have seen my full schematic in this post, can you take a look at my EMAIL routine in the post I made to your ConnectOne example code thread to see if you see any obvious reasons it isn't working in my application.

[mackrackit]

I thought I would have to also put one on the leg of the backup battery supply so that when Vcc is active, that diode protects the backup battery from the Vcc voltage.

I might be doing it wrong, but with VCC being 3 volts I have not had any problems.

evidently it will keep on trucking when Vcc is shut down. Correct??

Correct, I had a part that got pushed to the back of the bench for around six months (no external power) and the RTC was still alive when I fired it up.

can you take a look at my EMAIL routine

Yes, I am looking at it.

[BobK]

Hi John,

When I sent you my DS1337 schematic last year I only showed the two diodes connected to the Vcc of the 1337. I had the resistors connected to the PIC's Vcc for the very reason that you are experiencing now.

BobK

[rsocor01]

A PIC CAN be powered through an I/O pin.

What do you mean? Are you saying that the Vss and Vdd pins are not necessary or can be omitted?

Robert

[mackrackit]

Vss and Vdd pins are not necessary or can be omitted?

No, I am saying that it can.
Say you have a MCU with an I/O sourced to different power supplies. If the I/O is high and the power is cut to the MCU the MCU will not be able to reset. The voltage on the I/O will keep the MCU running or in a near run state. The I/O can even be a RS232 input.

For a test, have the MCU keep a LED high (lighted), apply 5 volts to an input. Turn the power off to the MCU while keeping the input at 5 volts. Look at the LED.

[Heckler]

Vss and Vdd pins are not necessary or can be omitted?

YEP!! you can power PIC's through the I/O pins... anyway take a look here...
http://www.t4f.org/en/projects/open-rfid-tag/57

I would never have thought it possible... I am sure there are current limitations, though.

I ran across this on one of my favorite web sites... www.hackaday.com