Unconnected AVCC pin in ATmega644P

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Probably I'm not the first person to ask this but would like to hear it again. Yes what I ask is not what the datasheet recommends.

 

I use ATmega644PA in 44 QFN and I leave AVcc pin not connected to anything. I dont need ADC functionality.

 

So my question is what happens?

  1. Port A can still work as 'open drain'
  2. Any voltage on Port A cannot be clamped to Vcc through internal diodes

 

Is that all? It should not affect any other functionality of the chip, shouldn't it?

Cheers,
Vignesh

If everything seems to be coming onto your way, then you are probably driving on a wrong lane..

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You MUST connect AVcc.

 

It also powers the digital circuitry for those port pins and there is a strong probability that it will affect other parts of the chip.

 

If you don't use the ADC, you don't need the extra filtering - just connect the AVcc pin directly to Vcc.

 

I say "MUST" because the spec sheet requires it. That implies that Atmel will not guarantee long-term chip functionality (to the extent that Atmel "guarantees" anything).

 

Jim

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

Last Edited: Wed. Oct 15, 2014 - 05:34 PM
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Is that all?

Aren't you violating Absolute Maximum Ratings, as well as the datasheet "should"?

 

It should be externally connected to VCC,
even if the ADC is not used.

 

Perhaps if you told us the reason for this snipe hunt we would be more interested.  Do you get a bonus at work if you connect fewer pins of the microcontroller to the target board?

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.

Last Edited: Wed. Oct 15, 2014 - 05:44 PM
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theusch wrote:

  Do you get a bonus at work if you connect fewer pins of the microcontroller to the target board?

 

Not really a bonus but mulling about how to make use of PCBs which are printed with AVcc floating :). Thank you and Jim for emphasizing the obvious.

Cheers,
Vignesh

If everything seems to be coming onto your way, then you are probably driving on a wrong lane..

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Is that all?

Oh, yeah--how do you expect your app to work, when the floating AVcc value is tripping the Brown-Out Detector (BOD) randomly?  (You ARE using the BOD in your applications, aren't you?)

 

 

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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mulling about how to make use of PCBs which are printed with AVcc floating

Add jumper wire(s) as needed.

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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theusch wrote:

mulling about how to make use of PCBs which are printed with AVcc floating

Add jumper wire(s) as needed.

And charge them to the person who laid out the board.

 

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theusch wrote:

Oh, yeah--how do you expect your app to work, when the floating AVcc value is tripping the Brown-Out Detector (BOD) randomly?  (You ARE using the BOD in your applications, aren't you?)

 

So you mean BOD compares internal bandgap against AVcc pin? In which case, it would certainly be disastrous with a floating AVcc line. Thanks for this insight, that's why I wrote "Is that all" in my original post!

Cheers,
Vignesh

If everything seems to be coming onto your way, then you are probably driving on a wrong lane..

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So you mean BOD compares internal bandgap against AVcc pin?

Well, I'm not a sparkie but how could you make a BOD to check rail voltage using a "bandgap" generated from -- rail voltage?

 

IME the BOD trips when either Vcc or AVcc drops below the trigger level.

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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This post can be a new thread but I want to retain the history.

1. Now AVcc is firmly shorted with Vcc
2. BOD is set at 1V8
3. Vcc is 3V3
4. Its a mega644PA, with external xtal 11.0592 MHz

 

AVR is controlling another circuit with a series P-MOS. When the P-MOS is turned on, 3V3 dips down to 2V5. This was making AVR to reset which I believed because AVcc was floating. 

Now that AVcc and Vcc are shorted, it still happens. How could I explain this?

Cheers,
Vignesh

If everything seems to be coming onto your way, then you are probably driving on a wrong lane..

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Quote:
When the P-MOS is turned on, 3V3 dips down to 2V5.
How have you determined this?  Multimeter?  DSO?  Voltage need only dip below BOD threshold for 2 µs in order to trigger a reset.  Are you capturing the voltage on VCC/AVCC with this kind of resolution?

 

EDIT:

 

You're running at 11.0592 MHz.  At that speed the part is spec'd for a VCC of 2.9V, so evening dipping down to 2.5V is out of spec.  It's not far out, so it might work, but it might not.  'Not working' could include apparent device resets.

 

Have you confirmed that the reset source is in fact BORF?

 

I assume with the 11.0592 MHz crystal that you're using one or both of the USARTs?  I've never tested a 644p (or a 1284p for that matter), but the 1284p has a known errata whereby the low-power crystal oscillator becomes unstable when a high slew-rate signal is applied to RXD0.  It is at least possible that the 644p suffers the same problem.  The fix is to switch to the full-swing oscillator.  Which are you using?

 

 

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

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"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

Last Edited: Thu. Feb 12, 2015 - 02:38 PM
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The "dip" may be much lower when measured with a fast 'scope right at the AVR's Vcc pins.

 

Surely you have enough power-supply capacitance to sustain a momentary dip?

 

The dip itself indicates design problems to me.

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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It would be easier to analyze the problem if you posted a schematic.

 

JC

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joeymorin: How have you determined this?  Multimeter?  DSO? 

500 MHz probe on 4 GHz DSO, at 20 us/div time base. Checked with faster resolution also.

 

joeymorin: Have you confirmed that the reset source is in fact BORF? 

Yes it is BOD; AVR doesn't get reset if I disable BOD fuse. Also verified by reading the reset source at start up.

joeymorin: I assume with the 11.0592 MHz crystal that you're using one or both of the USARTs? ...The fix is to switch to the full-swing oscillator.  Which are you using?

Indeed. I'll check the errata. Thank you.

theusch: The "dip" may be much lower when measured with a fast 'scope right at the AVR's Vcc pins.

 

Surely you have enough power-supply capacitance to sustain a momentary dip?

 

The dip itself indicates design problems to me.

DocJC: It would be easier to analyze the problem if you posted a schematic.

 

'Left' side of the P-MOS is a DC-DC with 3V3 output, with 65 uF of total capacitance; Right side is an RF module with 25 uF of total capacitance. P-MOS has 0.5 Ohms Rds. So the dip is due to sudden addition of 25 uF and the RF module in parallel with 65 uF. DC-DC takes time to handle this transient. However the dip stays above 2.5 V still causing AVR to brown out; MLF-44 has 100 nF each on its three Vcc pins; AVcc is jumpered to nearest Vcc. Adding 47 uF more to output of DC-DC reduces the dip and solves (masks) the problem but its like sweeping the dust under the carpet.

 

Cheers,
Vignesh

If everything seems to be coming onto your way, then you are probably driving on a wrong lane..

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Are you measuring directly on the AVR's VCC (and AVCC) pin?  And with the DSO probe's ground clip directly on the AVR's GND pin?

 

Are you certain you've correctly set BOD for 1.8V?  Read back your fuse values and post them here.

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

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[Solved]
Sorry for the delay in reply. There was a soldering issue in the jumper connecting AVcc and Vcc, hence it was essentially floating.

 

However, I realized that there exist back to back diodes between Vcc and AVcc pins of AVR. I confirmed this by diode test in both polarities. Vf of this diode is 0.71 V, at 100 uA test current.

When Vcc was at 3.3 V, AVcc was at 2.5 V which was enough for internal bandgap and rest of the analog circuitry. Hence with BOD is set at 1.8 V, when Vcc drops to 2.5 V, AVcc pin will drop to 1.8 V (due to internal diode drop). Hence what appeared like AVcc mistakenly triggering at 2.5 V is indeed a proper operation. 

 

So summary is, obey the datasheet for AVcc to Vcc short!

Cheers,
Vignesh

If everything seems to be coming onto your way, then you are probably driving on a wrong lane..