ADC inaccuracy - new prototype

[Praneethsvch]

commit - 905cb07
Using ADC channel - single shot single channel. No DMA. Seeing 0.05 to 0.1 Volts inaccuracy in ADC readings. Measurements are compared using true RMS Multimeter - mfg.no UT61E.
ST states factory read-only calibration value is available. ADC Calibration function is called but currently seems to be inaccurate by 0.05-0.1 Volts. Need further digging on this issue and verification
Todo:

• Check whether recalibration value is readable and present
• Verify upon recalibrating using this constant, reading matches with actual battery level
• Re-Re-calibrate the recalibration function to give correct readings