AVCC, GND left Open....

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Even though datasheet says that AVCC, GND pins can not be left open, What if they are left open? I found ATmega works fine.

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Well, the voltages on the pins will probably NOT meet logic standards. For AVcc, you will see this on the port that has the ADC as AVcc also sets the logic high for those pins. The effect of a missing ground is a little less clear, but I would expect some pins to not pull down to a full logic low.

Also, you simply cannot tell what will happen inside. It is very likely that some peripheral devices (like ADC or EEPROM) could operate "strangely". Some of this strange operation could very well not occur until the chip experiences temperature extremes, or lower input voltages, or especially higher load currents (such as direct drive LEDs).

I am willing to bet (a lot) that "works fine" is simply an illusion.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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There was a donkey once that was trained by it's owner not to eat. It worked fine for a while then the donkey died.

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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yellowboy_75 wrote:
Even though datasheet says that AVCC, GND pins can not be left open, What if they are left open? I found ATmega works fine.

As ka7ehk has told above device working finely not always working device. You must follow all requirements of datasheet to eliminate finding known troubles and mistakes.

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Quote:
As ka7ehk has told above device working finely not always working device. You must follow all requirements of datasheet to eliminate finding known troubles and mistakes.

OP is regarding "what if left open?" assuming ADC is not used. I understand that we(donkey) must follow the datasheet(owner)!!!! And die ;)

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From the datasheet (of the mega32):

Quote:
AVCC is the supply voltage pin for Port A and the A/D Converter. It should be externally connected to VCC, even if the ADC is not used.

What more do you need said of it?

Regards,
Steve A.

The Board helps those that help themselves.

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What if?

It depends on the conditions.

Depending on conditions, maybe it works, maybe it does not, maybe it breaks silently, maybe it catches fire.

Same thing with cats. If you put a cat in a box even if you are told not to put the cat in the box, it may die if the box has no air holes or it may like it if there are mice in the box.

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Koshchi wrote:
From the datasheet (of the mega32):
Quote:
AVCC is the supply voltage pin for Port A and the A/D Converter. It should be externally connected to VCC, even if the ADC is not used.

What more do you need said of it?

Refer Figure 2 (Block Diagram) of ATmega32 datasheet.
AVCC is shown connected to "MUX & ADC". How will it interface port pin's logic levels?

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This thread has become animal friendly!!

Quote:
Same thing with cats. If you put a cat in a box even if you are told not to put the cat in the box, it may die if the box has no air holes or it may like it if there are mice in the box.

Quote:
There was a donkey once that was trained by it's owner not to eat. It worked fine for a while then the donkey died.

Quote:
OP is regarding "what if left open?" assuming ADC is not used. I understand that we(donkey) must follow the datasheet(owner)!!!! And die Wink

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Does it take so much effort to connect those two pins?

Or have you made a PCB design error and now that thousands of them have been made you are wondering whether to fix it or not?

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Quote:

Does it take so much effort to connect those two pins?

My thoughts exactly - how difficult is it to run a GND or Vcc wire to the unconnected pins?

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You can put the question in another context: Assume
you have a broken solder joint in the GND or VCC line
or even both. Can you predict what will happen,
if you know the wiring of the other pins ?

I think it might happen, that the CPU is powered from other pins via the input protection diodes.

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Yes the port may be powered from the inputs, if there are some outputs driving the pins. If not, then the port may not be powered.

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clawson wrote:
Quote:

Does it take so much effort to connect those two pins?

My thoughts exactly - how difficult is it to run a GND or Vcc wire to the unconnected pins?

Oddly coincidental..

I just yesterday emailed (second try) a vendor who produces an AVR trainer board using megas, which has GND, AVCC, and AREF pins floating. I hadn't received any response to my previous email a couple weeks ago.

Their docs for the old and new versions of the board show these pins open, and I can confirm that the old version definitely IS open.

Very odd.

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Very odd indeed--UNLESS...

... the "trainer board" happened to be a Mega8 design. That model did (and AFAIK still does) have a "bug" where Vcc and AVcc are tied together internally.

So that could explain everything still working. (It is hard to envision an AVR trainer board with A/D that doesn't have some provision to use A/D and abandons the whole port.) It still violates the good practice for trouble-free operation to have >>all<< Vcc & Gnd hooked up and properly decoupled.

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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The one I'm looking at definitely is using the ADC.

It "works", if you don't look at it too closely.

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AFAIK, all Gnd pins are internally connected inside chip. While all port pin currents are small, ATmega really works fine.
But in any case, please feed your donkey! :)

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The device specification assumes that all the power and ground pin pins are connected and properly decoupled, and designers depend on devices meeting their published specification. If any connections are missing you don't really know how the device will actually behave in your circuit.

Leon

Leon Heller G1HSM

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dbvanhorn wrote:
Oddly coincidental..

I just yesterday emailed (second try) a vendor who produces an AVR trainer board using megas, which has GND, AVCC, and AREF pins floating.


what vendor and board is this?

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Well, still no response from them.

It's Deccan Robotics, the AVR-LIP and AVR-LIP-II.

I spoke to one company that reps their board, they queried Deccan on it, and were told that it was left that way because there were so many options that the end user needed to configure it.

I don't buy that explanation for several reasons.

First, GND is not an option. Period.

Second, there are no pads that would support user configuration.

Third, there is not one word in the documentation
that indicates this is the case.

The board comes with a number of example programs that use the analog section, so one would expect the board to be properly configured for this, or to have jumpers, and some text explaining how to set it up.

IMHO, they made a mistake. I'm dissapointed that they don't just own up to it. It's not a big deal to solder on a cap, and two jumpers. When I found the problem on my board, I contacted them, and have had no response to date. I can't swear that the problem is in their AVR-LIPII boards, but the schematic indicates that it is.

Other than that, it's a good board, and I like that it comes with lots of example code and a complete (though significantly outdated) WINAVR environment. All the examples that I tried worked properly, so in that respect it was a good student board. But, leaving AVCC and AREF, and GND open will cause wierd problems which is exactly what a student does NOT need.

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Well, odd coincidence no longer, the OPs profile says he's with Deccan, and I got an email from Deccan over the weekend on this subject.

They still maintain it was a decision made a while back, and that they expected that students would see the disconnected pins in the schematic and know what to do... Their examples make use of the ADC, and make no mention of any configuring needing to be done.

I saw the pins "open" on the schematic when I first got the board, but I didn't believe my eyes. I thought it must be a difference in schematic style, or that they had connected the pins by naming the pads in the PCB footprint. It never occurred to me that they were seriously leaving those pins open.

All that aside, the GND and AVCC pins must be connected, no matter WHAT you're doing with the device. If they'd simply done the standard inductor and cap on AVCC, and 0.1uF on AREF, and connected the second ground pin, that would have served just fine for all the examples I've seen.

It's unfortunate, but hopefully they will make their users aware of the problem, and this thread will hopefully serve the ones who didn't get the word.

It's a nice board all in all, once you add two jumpers and a cap.

I upgraded mine to a M644, and changed the xtal and caps for 20 MHz, which is a pretty sweet uptick in performance.