Measuring battery voltage with SAM3S

Discussion around product based on ARM Cortex M3 core.

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crwper
Posts: 19
Joined: Thu Oct 25, 2012 3:55 am

Measuring battery voltage with SAM3S

Tue Dec 16, 2014 10:26 pm

I have a battery powered application and would like to monitor the battery voltage to warn the user when it's getting low. I thought I might be able to do this by dividing VBAT and using the ADC with ADVREF tied to the 3.3 V regulated supply. However, this measurement only works as long as the battery voltage is higher than 3.3 V.

Ideally, I'd like to compare the divided voltage against an internal voltage reference. However, I haven't been able to find such a reference in the SAM3S.

Is there an internal voltage reference that I've missed, or a way to monitor the battery voltage generally without adding an external reference?

Thanks!

Michael
nutsnbolts
Posts: 31
Joined: Mon Aug 25, 2014 11:20 pm

Re: Measuring battery voltage with SAM3S

Sun Jan 04, 2015 12:46 am

VDDCORE is supplied from VDDOUT at 1V8.. pick up a feed through a resistor (say, 10K), and decouple (100n)?

Mike.
crwper
Posts: 19
Joined: Thu Oct 25, 2012 3:55 am

Re: Measuring battery voltage with SAM3S

Sun Jan 04, 2015 1:24 am

nutsnbolts wrote:VDDCORE is supplied from VDDOUT at 1V8.. pick up a feed through a resistor (say, 10K), and decouple (100n)?
That's an interesting idea. I sent a message to Atmel support, who confirmed that there is no internal voltage reference in the SAM3S.

One issue with your plan... Looking at the SAM3S Schematic Checklist entry for ADVREF, I see this:
2.0V to VDDIO(*)
Decoupling capacitor(s).
(*)2.0V is used for 10-bit ADC resolution
only. In other case the minimum ADVREF
value is 2.4V.
Looks like 1.8 V is probably too low for ADVREF.

Michael

Edit: On the other hand, one thing you could do is to divide VBAT and apply that to ADVREF, then apply VDDOUT to one of the ADC pins.
nutsnbolts
Posts: 31
Joined: Mon Aug 25, 2014 11:20 pm

Re: Measuring battery voltage with SAM3S

Sun Jan 04, 2015 1:40 am

Now you're cooking! 

Mike.

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