- Mon Mar 16, 2020 3:03 pm
What great timing for you to have provided your code! I was just getting ready to create a new post regarding what I believed to be errors in the calculation of battery voltage and temperature, in the Ambiq SDK 2.2.0 (and probably all versions) examples such as "(SDK)\boards\apollo3_evb\examples\adc_vbat", when I downloaded and looked at what you did. And I found that you have made the same corrections that I believe were needed. The first is that in the examples the FIFO sample value gets scaled by dividing by 65,636, which would be correct for a 16 bit value, but the ADC registers are only 14 bit, which should use 16,384 as the scaling factor as you have done. For VBATT I get a sample value of 12,206 (using a reference voltage of 1.5V) which results in VCC = 3.35 V using the 14 bit scale value and 0.84 V using the 16 bit scale factor. Obviously, the 3.35 V result is reasonable, whereas the 0.84 V result is not. The other error, which I haven't figured out yet, is that the temperature FIFO sample value is about 43,840, which is impossible given a 14 bit register. I think the problem is related to the use of a 10 bit, instead of 14 bit, resolution for temperature in the example problem. Oddly enough, the resulting temperature, using the incorrect 16 bit scaling factor as done in the example, is about 72 degF as compared to my room thermostat reading of 69 degF. I'll have to study your code at length to know what you've done, but it appears at a quick glance that you have taken the default values for resolution which are 14 bit. Since your results are in close agreement with an independent temperature measurement your code appears to be working correctly. Another error that I think is present in the example, although it does not affect the results, are the expected temperature and voltage ranges given respectively as -40 F to 225 F and 0.825 V to 1.283 V. The given temperature range results in a maximum difference of 265 F or 147 C. For a specified sensitivity of 3.8 mV/C, for the temperature sensor, the resulting voltage difference would be 559 mV as compared to the given range of 458 mV. I believe that a more credible expected temperature range would be the maximum operating range for the chip, which is given in the Apollo3 Blue data sheet as -40 C to 85 C (-40 F to 185 F) for which the expected voltage difference would be 475 mV. I don't find anywhere in the data sheet document where a temperature range or voltage reference point is given for the temperature sensor.
One additional error that I have found is in the Arduino "Example4_analogRead" problem for which the equations are correct but the result gives a temperature of 40 C (104 F) instead of the expected 21 C (70 F). I believe the problem relates to the internal pad number of 102 that is assigned to the temperature sensor. I have not found any documentation to validate the correct values for these pads. The battery voltage divider, which is assigned pad number 103, provides a correct result of about 3.32 V.
I'm looking forward to further exploring and executing Paul's work.