I have seen a couple of discussions on how to get the actual power supply voltage. There were a number of steps that seemed to me to be to complicated. Why can not I just connect PC5 or any of the ADC pins to the + voltage rail?

For the Temperture Sensor project 5000mv is assumed:

double sampleToFahrenheit(uint16_t sample) 
{
  // conversion ratio in DEGREES/STEP:
  // (5000 mV / 1024 steps) * (1 degree / 10mV)
  //    ^^^^^^^^^^^      ^^^^^^^^^^
  //     from ADC         from LM34
  return sample * (5000.0 / 1024.0 / 10.0);  
}

Why could I not make the 5000.0 a variable referencing a ADC pin? I assume have the true power supply voltage would make for more accurate calculations and using a variable it would allways be accurate even if the power supply was drainnig down as from a battery.

Thanks for the help, again.

Ralph

The ADC doesn't measure the input voltage, it measures the ratio between the input voltage and the reference voltage, Vin/Vref. If AREF and PC5 are both connected to VCC, the ratio will always be 1.0 and the ADC will always return 1023 (maybe minus noise).

I was thinking of using the Temperature Sensor code.

With the ADC pin connected to the + rail I currently get 499.51⁰F. Which has decreased with battery usage. It had been at 500 with a fresh battery.

I just measured the rail voltage and got 4.95v so they are not in sync or my meter is not sensitive enough. Seems like I could figure out a scale to be able to use this.

Of course my imediate project does not require great accuracy, so it was just a thought.

Thanks for the knowledgable reply it all ways helps. Ralph

Something maybe worth looking into...

Back in the day, I used a chip similar, (maybe even the same) from Analog Devices. It was the AD580 2.5V precision reference.

If you use it for VREF (not ground), it shifts your scale to a point where negative temps can be read without going with a negative power source. Of course, it means changing a few numbers in the program but that should not be too hard...

I think there is another thread on this but I can't find it.

Hi all,

Check out this thread. There's a voltage reference built into the chip that you can use, along with an external resistor divider (of a known ratio), to measure the power supply voltage.

Let me note that 499.51 degrees F (for an LM34) represents a voltage or 4.9951 volts... and that 5.0*(1023/1024) = 4.9951! Coincidence? I think not :-) As bretm points out, the ADC is reporting the highest reading it's capable of (binary 1023).

Mike