ARef > Vdd - Battery sensing

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Hi,

I'm starting to think about a low consumption embedded application which will have a nominal 7.4V battery supply plus a 5V low quesicent regulator at an ATMega32xxxx.

If I want to measure the battery voltage can I feed the raw 7.4V into the ARef level? Is that allowed - ie. > Vdd? I couldn't see any specific min/max in the spec sheet.

The other option that I can think of is a dividing resistor and sense from there - but that is constant load which I would like to avoid.

Thanks in advance,

Carl

PS: Been a while since I posted here - the 'big project' is now flying - huge buzz.. http://www.youtube.com/zkvii :D

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zkvii wrote:
Been a while since I posted here - the 'big project' is now flying - huge buzz.. http://www.youtube.com/zkvii :D

I can see why!

Oh! So Sweet!!!

You definitely have some skill and real talent!

I'm flabbergasted!!!

I only dream about that level of skill.

Carl, that is simply fantastic!

You can avoid reality, for a while.  But you can't avoid the consequences of reality! - C.W. Livingston

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For the ATmega32 (doc2503 rev L) on page 287 -

Quote:
Absolute Maximum Ratings*
---------------------------------------------
Voltage on any Pin except RESET
with respect to Ground ................................-0.5V to VCC+0.5V

Stan

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sbennett wrote:
For the ATmega32 (doc2503 rev L) on page 287 -

Quote:
Absolute Maximum Ratings*
---------------------------------------------
Voltage on any Pin except RESET
with respect to Ground ................................-0.5V to VCC+0.5V

Stan

Thank you Stan.... Wood and trees - I must have been past it 3 times.

Is there a 'better' way than a divider for low consumption perspective?

Best,

Carl

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Carl,

Wow! What an incredible project. How many hours (years) did it take to build? Man, what a panel, too! See you at Oshkosh???

Regarding the AVR question...

Perhaps you could use a pin to switch your resistance divider on/off to ground. Turn it on when you wish to make a measurement, off the rest of the time?

Wow!

I'm still impressed!

JC

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DocJC wrote:
Carl,
Perhaps you could use a pin to switch your resistance divider on/off to ground. Turn it on when you wish to make a measurement, off the rest of the time?
JC

If you disconnect the lower leg of the divider from ground, it won't produce the voltage drop any more and the full voltage will go to the uC pin, frying the device. :( Only high side disconnection will do - PNP transistor and a couple of resistors.

If you think education is expensive, try ignorance.

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:oops:

Oh Man, I hate it when I give bad advice.

My first thought was to use a PNP. Then I thought the OP will ask how..., and then I'd have to figure out how to bias it, and use 'backwards' current flow and 'backwards' logic, and, and, and,...

And then I gave bad advice.

Thanks for spotting it, saving the OP's chip, and keeping me on track!

JC

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My plan was the AVR controls a FET which provides power to the rest of the board when it wakes up from it's slumber.

I think I can put the divider on that side of the system. I don't need the ADC system to wake up, so I'm assuming as long as I give it time to stabilse after waking and enabling that will be fine.

Cheers,

Carl (OP :-) )

PS: wrt the plane - thank you for the comments / compliments. IMO finishing it is more about attitude and drive than skills. In hour terms more than I'll admit to, but it has been close to full time, 7 days a week for 2+ years......

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Quote:

Only high side disconnection will do

emuler likes to use the superlatives--another post today referred to the "best" way to do an app. ;)

Sorry, high-side disconnection may not be the >>only<< way. A series of diodes to drop down the voltage to below Vcc, then a resistive divider with the tail to a port pin would do it, right?

A recent thread on the same topic had a recommendation to drop voltage using a 2-terminal regulator.

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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Quote:
emuler likes to use the superlatives

:lol: Yes, I'll plead guilty to that. :blush:

A zener diode/ 2 terminal regulator will do the trick, but both need bias current, and you have to subtract the drop from the divider calculations. If by some mishap the voltage increases further than designed for (eg. 110V transformer plugged into 220V mains), the same end result is produced. :(

If the ADC needs a low impedance input (very likely), you can either put a capacitor after the high resistance divider, or a low power op-amp/buffer (subject to availability of course - many designs have a op-amp left over).

If you think education is expensive, try ignorance.

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Will this device be in a controlled temp environment? If not then you might have to look at the TC of the devices and compensate if necessary.