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I guess I can get a tiny series and use UART but I was hoping I could create a circuit so I only use one microcontroller.

As for the backup battery, it will be the small round ones.

I don't quite understand what you mean by the ground pins are not connected. The only ground not connected from the diagram is pin 14 which I haven't finished.

I will check, when I get home, how much current is flowing from the battery when both main and battery source are connected.

As for the battery, I guess I will be using 2 of the 3 V round batteries.

EDIT: Okay, I just realized what you were talking about for AVCC and AGND. I never knew these pins were so important because I never used in any of my projects and never ran into problems with it. What exactly is the disadvantage if you don't use them? And, I'm guessing you just directly connect the AVCC and AGND without any components connected to them. Also, my microcontroller is still programmable and works fine from what I'm seeing.

Disadvantage? It is a miracle it even works at all.

Feels like I know nothing about electronics even after finishing my second year, lmao.

Feels like I know nothing about electronics even after finishing my second year, lmao.

Thank you everyone.

I've been programming for some 30+ years. Sometimes it feels like I don't know anything about it.

Sometimes it sounds like it too.

Right. I'll be looking out for your next slip, and then it is going to come down hard on you..

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Re the

I guess I can get a tiny series and use UART but I was hoping I could create a circuit so I only use one microcontroller.

I gotta ask. If you are making an AVR design that may need to run from battery why on earth would you use a mega8 and not a mega88PA?

(the P in that stands for PicoPower)

3v round batteries - that's hardly a precise description! Are we talking about button cells like a CR3025 or similar? Or a cylindrical cell like a 16450 ? with engineering, you want to be precise and read the datasheet.

On a recent design I had a 5V LDO and a 3V battery as my power sources. I dioded the two supplies together (anode 1 connected to 5V, anode 2 connected to 3V, both cathodes tied to my AVR's supply voltage. During normal operation the device was powered through the diode by 5V. But when the 5V dies the battery kicks in.

I don't see any good reason to pay extra for a Maxim RTC for most applications - doing it onboard the microcontroller is pretty easy.

Re the circuit I just plowed though the external disk with all the photos, and found it. Dual supply circuit, battery backup when main supply goes out. With sense on when that happens (on PORTD2 in this case), so that you can take all low-power consumptions you can/need. Sorry, no part numbers for the zeners and transistor.

[EDIT: Uploaded correct picture, with the 1K resistor between transistor and GND... Now, about that shaded pull-up, I really don't know for sure, but probably as an alternative to an internal pull-up.)

??? OK, I'll bite--what does that statement have to do with anything said before that?

If power draw is important, why did you start with an older-generation AVR model that is going to be much less power efficient than a replacement e.g. ATmega88PA?

...but other than that, I am a total noob with microcontrollers.

Oh, that was because I was going to have one dedicated ATtiny that had RTC constantly running after the ATmega8 turned off until the other people showed me the RTC IC.

That is an interesting design, I'll definitely try this out. But, it seems pretty much like what I was doing before except using a lower voltage battery.

Welcome to the hobby!!!

No - you're using a resistor for some reason that I cannot explain. My suggestion is to use a diode so that your power supply does not back feed your battery. This is making the assumption that your battery's voltage is lower than your power supply's voltage. As others have mentioned, there aren't a lot of 5V batteries out there...

I'm looking at digikey and there is ATMEGA88PA-PU-ND and ATMEGA88PA-PN-ND (this one is more expensive).

I've been using 2N3904 NPN transistors and they heat up so quickly!

The details on different package codes are readily available in the data sheet for the device. I'm too lazy to look it up for you.

Also, is it wise to just use one diode and let the motor

I happened to have the datasheet open so did but my copy (8161D–AVR–10/09) does not seem to know about the -PN variant only the -PU.

(as always the model numbers are explained in the "Ordering Information" chapter towards the end).

BTW I just thought: why is this thread in General Electronics - it's surely on topic for AVR Forum? Should I move it?

First use of the word "motor" in the whole thread, and still you sound like it is obvious what you are doing. This seems to turn into the usual guessing game, so I'm out.

I'm confused about what you said

Let me try to explain: We've had almost two full pages discussing mainly a battery backup system and an RTC. Then suddenly you pull up a motor like a rabbit out of a hat. There has been no earlier mention of it, but you say "the motor", with formulations that clearly indicate that you believe we know about what kind of motor, how you have currently connected it and the alternative you discuss. We don't. How can we?!

This is typical for many posts here at AVRfreaks. The poster assumes that us others can see what he has on his bench, and how he has wired it up. We can't. If we are really nice we have patience, and ask nicely about it to get the details. It then often spins on with many posts before we start to get a picture of what it is all about. This is what I call "the usual guessing game".

Today I don't have that patience, so I'll leave it to others to play TUGG with you.

Before you click the submit button for a post, do this exercise: Read through your post but imagine that you are the presumptive reader of it, and think about what he knows about your problem.

Finally, I often repeat here at AVRfreaks: The quality of the answers you get is highly correlated with the quality of the question you ask.

Think about it..

Everything above in all friendliness, and as an advice on how you can get better answers here at 'freaks and keep people interested in your questions and problems. Personally I lost interest right about "the motor".

The difference was the temperature of operation and I used 6V power source, 100 Ohms with an LED, and the base of the transistor was connected to one of the pins of the ATmega8.

Also, is it wise to just use one diode and let the motor get most of the 2A current from a DC adapter? From my very limited understanding of using motors in a circuit design, I would treat the motor as an inductor am I right? Or should I limit the current with a resistor?

Sorry to be a nit-pick, but a DC adapter is not a constant current source. It is a constant voltage source, and every load pulls the amount of current it needs at the rated voltage. If you have a 6V 2A rated power supply, it will provide 6V to any load from 0 to 2A, and pulling more than 2A will make output drop or it to shut down.