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

[Keith Gudger]

If your pots are run by the same 5V as your reference, then they *might*
track together (as the other email said).  You can get good results this
way if you have enough decoupling on your 5V so that the signal at the
VREF pin matches the 5V at the pots.  If the 5V at the pots has an RC
effect associated with it, then you can get inaccurate data.  The RC
effect comes from distributed R and C along the wire between the VREF line
and the pots.  Are they close by?  Do they have decouping Caps at the
pots?  These can create an RC between the where you use the voltage (the
pots) and where you measure the voltage (VREF pin).

Keith

On Sat, 23 Oct 2004, JAMIE MORKEN wrote:

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