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

[Andy Warner]

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