Re: [avr-gcc-list] measuring uC supply voltage

[JAMIE MORKEN]

----- Original Message -----
From: Andy Warner <address@hidden>
Date: Saturday, October 23, 2004 12:11 pm
Subject: Re: [avr-gcc-list] measuring uC supply voltage

> JAMIE MORKEN wrote:
> > I am trying to figure out the best way to measure the 5V supply 
> voltage for an atmega128.  I am using the 5V supply as the ADC 
> reference voltage.  The problem I am seeing is when the 5V supply 
> droops, the reference voltage will also droop.  For other sensors 
> I need to use the 5V reference voltage.  Any ideas on how to solve 
> this?  Is it possible to switch to the internal 2.5V reference 
> while the 5V external reference is still hooked up?  Thanks,
> 
> I can't vouch for the quality of the internal reference(s) on
> AVRs, but measuring the absolute level of a variable supply,
> when it is also your reference is sub-optimal.
> 
> I think you're on the right track using a fixed reference
> that is stable well below the minimum value of V+. Following
> your example of a 2.5V reference, you could use a voltage
> divider to reduce V+ by 50% or more. You probably want to
> arrange things so you can measure 5.1V as well as 4.9V.
> For example, a 3V reference with a  / 2 votage divider
> would let you measure up to nearly 6V on the V+ line.
> I would also recommend taking multiple readings and
> averaging the result.
> 
> Do the other sensors need 5V, or are you simply going to
> be correcting their output values to accomodate the lower
> supply voltage ?

I have some potentiometer joysticks that are run by the 5V supply, so I may 
have to correct these if the supply droops below 5V, but under normal operating 
conditions the supply should be 5V and if it is not there will be more problems 
than just innacurate sensor readings.  Thanks,

cheers,
Jamie



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> Andy Warner           Voice: (612) 801-8549   Fax: (208) 575-5634
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