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measure battery voltage with microcontroller

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joskem

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I wish to measure the voltage of a 1.5V battery using a microcontroller (supply voltage of 3V).
I thought of connecting the battery on the ADC4 output of the microcontroller, and wrote a program which reads the ADC4.
Initially, I programmed the microcontroller without connecting the battery to ADC4. I was expecting to get a value of 0V (nothing connected to ADC4), but I didn't. Instead, I am getting a higher value which is also fluctuating.. I tried to measure the voltage on ADC4 with a voltmeter, but I can't get a reading since it is obviously fluctuating. Why is that happening?
 

I have set up the reference voltage to be AVcc / AVss. I measured these voltages using a voltmeter as 2.91V and 0V respectively.
 

No, all I have done is to measure the voltages on the pins. I was expecting ADC4 to read 0V, however the voltmeter does not indicate any value, meaning that it is not dc, but rather a fluctuating waveform.. I was wondering why this is happening.
 

may be you have a problem with your power supply, or you are in a terrible EMF "soupy" environment. try the pull down resistor.
what kind of mcu are you using in this project?
 

It is an MSP430F1611.. Since I am all new to microcontrollers, how should I use the pull down resisotr? And would it be the same if I used a regular 10K resistor?
 

Thanks! So, I will just connect a 10K resistor between the ADC and the ground.. Should I do something similar when I try to measure the voltage of the battery? (i.e. ADC4 - battery - 10K - ground)?
 

you have to drop the voltage across a resistance to measure it. that is what i meant with the pull down resistance. ADC4 - battery - 10K - ground is the thing. If you remove the battery, the adc will get 0v and then a 0 v reading will be given. reference voltage is also another criteria, connect it with a capacitor i.e. like a .01 uf (if you still have the fluctuations) : adc ref pin <---> cap <-----> ground. and then give the reference voltage.
 
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    joskem

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thank you very much!
 

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